Patent Application
20160188529
This disclosure pertains to computing system, and in particular (but not exclusively) to a system and method for guaranteed real-time quality of service in system-on-a-chip uncore fabric.
In many computer systems with multiple devices, an arbitration is performed to provide access to a shared resource such as a shared memory. Different types of arbitration mechanisms are provided to enable arbitration between the different agents or requestors. Some systems use a fixed priority arbitration system in which different agents are allocated a particular priority. However, this can lead to unfairness in usage and starvation of one or more agent's ability to obtain access to the shared resource. Other arbitration systems provide for a round robin-based approach to allocating access to the shared resource,
In certain embodiments, the arbitration does not account for shared resource factors such as power state. Thus, in one example, a request is granted access to the shared resource and causes the resource to exit a low power state, although the device does not require immediate access to the shared resource.
In the following description, numerous specific details are set forth, such as examples of specific types of processors and system configurations, specific hardware structures, specific architectural and micro architectural details, specific register configurations, specific instruction types, specific system components, specific measurements/heights, specific processor pipeline stages and operation etc. in order to provide a thorough understanding of the present invention. It will be apparent, however, to one skilled in the art that these specific details need not be employed to practice the present invention. In other instances, well known components or methods, such as specific and alternative processor architectures, specific logic circuits/code for described algorithms, specific firmware code, specific interconnect operation, specific logic configurations, specific manufacturing techniques and materials, specific compiler implementations, specific expression of algorithms in code, specific power down and gating techniques/logic and other specific operational details of computer system haven't been described in detail in order to avoid unnecessarily obscuring the present invention.
In an example, a control system may include a system-on-a-chip (SoC), including one processor for real-time operation to manage devices in the control system, and another processor configured to execute auxiliary functions such as a user interface for the control system. The first core and second core may share memory such as dynamic random access memory (DRAM), and may also share an uncore fabric configured to communicatively couple the processors to one or more peripheral devices. The first core may require a guaranteed quality of service (QoS) to memory and/or peripherals. The uncore fabric may be divided into a first “real-time” virtual channel designated for traffic from the first processor, and a second “auxiliary” virtual channel designated for traffic from the second processor. The uncore fabric may apply a suitable selection or weighting algorithm to the virtual channels to guarantee the QoS.
In various embodiments, a shared fabric couples multiple independent devices, also referred to herein as “agents,” to a shared memory and to input/output (I/O) devices. In one embodiments, the shared fabric comprises an interconnect structure of a single die semiconductor device that includes intellectual property (IP) logic blocks of different types. At a high level, the shared fabric may include a shared memory fabric for accessing system memory, shared I/O interconnect fabric, and a system agent that provides intelligence to the overall shared fabric. The shared fabric may be configured to enable compliance with quality of service (QoS) requirements for time-critical isochronous devices while also providing memory bandwidth proportioning for non-isochronous devices, also referred to herein as “best effort” devices.
This fabric may be referred to as “uncore,” indicating that the interconnects, registers, and other resources provided in the fabric are not part of the processing cores that comprise the “agents.” The uncore fabric may include the memory fabric described above, portions that connect “agents” to memory, and also portions that connect agents to each other and in particular, processor cores to other “agents.”
Reliable and predictable allocation and scheduling of memory bandwidth occurs to support multiple devices and device types connected to the shared memory fabric. By including QoS functionality in a common shared memory fabric (rather than a memory controller or other non-fabric circuitry), the design may be more easily reused across multiple semiconductor devices such as systems-on-a-chip (SOCs) since the design is independent of memory technology.
Embodiments thus perform resource allocation, bandwidth apportioning and time-aware QoS properties in a shared memory fabric to provide predictable and reliable memory bandwidth and latencies to meet the requirements of devices connected to the fabric.
A class of service category is assigned to each device coupled to the shared memory fabric. In an embodiment, this assignment can be identified using configuration registers of the fabric. Multiple classes of service may be supported by the fabric. In one non-limiting example, devices of two classes of service categories may be present, including an isochronous class of service category used for latency sensitive devices and a best effort class of service category used for devices that can tolerate longer latencies to service their requests to memory. In some embodiments, latency sensitive devices include content rendering devices such as, by way of non-limiting example, audio or video players, camera devices, and so forth, while lower priority devices include processor cores, graphics processing units, and so forth.
Time, in the form of a request deadline, is communicated from the isochronous devices to the fabric to indicate to the fabric the required latency to complete a request to memory. To enable synchronization, the fabric broadcasts a global timer to all isochronous requesting agents. This global timer is continuously driven on outputs from the fabric so it is available for sampling by the isochronous devices. Responsive to this time value, the agents determine a latency requirement for completion of a request and add this latency value to the global timer value to form a deadline for the request. As an example, the latency for a read can be determined by the amount of data in the agent's data buffer and the drain rate of the buffer by the agent. If the agent consumes 1 cache line of data every 250 nanoseconds (ns) and has 8 cache lines of data in the buffer, the required deadline for a new request would 8×250 ns or 2 microseconds (μs) before the buffer is empty. Based on this communicated latency or deadline value, the fabric may make better scheduling decisions based on knowledge of the current power state of the memories and the required latencies for other unscheduled memory requests pending in the fabric. This deadline communication may improve memory bandwidth and also save system power.
The use of request deadlines provides the fabric with latency information for each request from an isochronous device. Configuration registers programmed within the fabric provide the fabric with information about the memory configuration such as the latency required for the memories to exit a low power, e.g., self-refresh and state. The fabric also controls when the memory controller causes the attached memory to enter and exit the self-refresh state by sending an indication to the memory controller, e.g., in the form of a status channel. The fabric determines when the memories should enter and exit self-refresh by evaluating the latency requirements for all pending memory requests. Because the fabric has knowledge of the required latency for all pending memory requests and required latency to exit self-refresh, greater management of power state transitions of the memories may results in additional power savings.
Embodiments may also provide for efficiency in memory bandwidth by allowing memory requests to be scheduled out of order; however this may result in long scheduling latencies for some requests. To resolve such concern, the fabric assigns a priority level to each isochronous memory request, e.g., a high or low priority. When scheduling high priority isochronous requests, the amount of out-of-order scheduling allowed is less than what is acceptable when scheduling best effort or low priority isochronous requests. Limiting the amount of out-of-order scheduling for high priority requests ensures that the request latency requirement is met. Because request priority is determined from the deadline of the request, the fabric can determine immediately after a request is scheduled what the priority levels of other pending requests are for an isochronous device. Using the deadline method the priority level of all pending requests change only when the global timer increments.
Embodiments may also improve portability and reuse of the sophisticated QoS memory scheduling algorithms across multiple SoC implementations, in that intelligent memory scheduling logic is incorporated in the fabric, while technology specific memory controller logic may be implemented within the memory controller.
Embodiments may also incorporate anti-starvation algorithms into multiple arbitration points of the fabric. In one embodiment, these anti-starvation algorithms include a weighted age-based arbitration method used by an admit arbiter and an oldest of available scheduling queues used in a memory scheduler and request tracker. In addition, request weights may be used to switch between different priority levels at the arbitration points in the fabric and for switching from scheduling read requests to write requests, in contrast to fixed-priority arbitration in which requests from high priority isochronous devices always win.
In an embodiment, the shared memory fabric includes two arbitration points that are used for scheduling requests being sent to the memory controller. The first arbitration point is used to admit requests from the devices into the shared memory fabric and is referred to as an “admit arbiter.” The second arbitration point is used to schedule the requests sent to the memory controller from the shared memory fabric and is referred to as a “scheduler arbiter.”
Each device connected to the shared memory fabric has a request interface that is connected between the device and fabric. The request interface supplies information about the request that can be used for QoS memory scheduling. In an embodiment, this information includes a memory address, order ID field and an opcode field. For isochronous devices an additional field called a request deadline field is provided to indicate the required latency needed to complete the request. Note that in some implementations of SoCs the memory fabric interface may be connected to other fabrics or switches which allows multiple devices to share a common request interface.
In one embodiment of an SoC, one or more requesting agents are designated as “real-time” cores, while one or more requesting agents are designated as “auxiliary” cores, which are provided on a best-effort basis. As an example, an SoC may be provided to control industrial machinery, provide life support functions, control an autonomous or semi-autonomous vehicle (such as a crash-avoidance system), or otherwise perform mission-critical or safety-critical functionality. In this example, the SoC includes a dual-core processor, with a first core acting as the real-time agent, and the second core acting as the auxiliary (best-effort) agent. Design considerations may drive a requirement that the real-time agent perform with a guaranteed QoS so that it does not fail in its control function. The auxiliary agent may perform other tasks that do not require a guaranteed QoS, such as providing a user interface, reporting function, or other support function that does not have critical timing.
In certain applications, the auxiliary agent may provide excess traffic that temporarily “floods” or otherwise overwhelms the uncore fabric so that the real-time agent is temporarily unable to operate on its required guaranteed QoS. For example, if the auxiliary agent provides a user interface, a user's interaction with the interface may provide a stream of data that overwhelms the uncore fabric so that real-time traffic is temporarily bottle necked. This may result in a situation where the real-time agent encounters a failure condition.
To avoid such bottle necks, the uncore fabric may be divided into two or more separate virtual channels. Separation into virtual channels may be accomplished by appending to each data packet flowing through the uncore fabric header information, including the agent that either originated or is to receive the data, and in some cases the device or peripheral that either originated or is to receive the data. For example, header information may designate a particular packet as originating from “core 0” (in this example, the real-time agent) and routed to DRAM. Another packet may be designated as originating from “core 1” (in this example, the auxiliary agent) and routed to a user display. In this case, the arbiters described herein may provide preemptive priority to the first packet over the second packet. In another example, traffic from core 0 to a certain peripheral may be given increased priority, but may not be so critical as to warrant preemptive scheduling. Advantageously, dividing traffic into multiple virtual channels enables of any suitable priority scheme according to the requirements and design parameters of a particular application.
In the FIGURES below,
In the embodiment shown in
In the embodiment of
The shared memory fabric may include certain finite resources that are first allocated before a request from a requesting agent can be granted by the admit arbiter. These resources include available entries in the internal data buffer and address tag storage. Other finite resources include available entries in the memory scheduler and request tracker scoreboards. There is a one-to-one correspondence in resources for the fabric's internal data buffer, tag array and memory scheduler scoreboard. In an embodiment, these resources are allocated to a predetermined region (e.g., a cache line width such as 64 bytes) of memory. Each active request is also allocated its own entry in the request and coherency tracker, but multiple requests to the same region in memory share the same entry in the data butler, tag array and memory scheduler scoreboard. Although it is possible for more than one request to be allocated to the same data buffer, tag array, and scheduler scoreboard entry, only one read request is scheduled to the memory controller for all outstanding read requests in the request and coherency tracker.
The request interface for all devices connects to the admit arbiter of the fabric. Isochronous devices use the deadline field of the request bus to indicate to the fabric the required latency to complete the request. The fabric sends a global timer value to all isochronous devices that are attached to the fabric. For each request to be sent to the fabric, the isochronous device, e.g., in a deadline logic, determines the required latency needed for the request to complete and adds the value to the current value of the global timer in order to create the request deadline. Different methods may be used by different isochronous devices to determine the required latency for the request, but in one embodiment, all isochronous devices indicate to the fabric the request latency using a deadline field of the request interface. In other embodiments, the uncore fabric may assign a deadline on behalf of agents that require real-time QoS. Specific methods may vary, but one example may be that by default, the uncore fabric assigns a fixed deadline of x microseconds from current time for all requests from the real-time core. In this example, x could be a value derived from a configuration register.
In an embodiment, the admit arbiter has two levels of priority. There is a high priority path in the arbiter that is used for urgent isochronous requests. A request is considered urgent if the requesting agent is configured as an isochronous agent and the deadline field of the request is less than a value stored in a configuration register specifying a threshold value, referred to as an “urgency threshold value.” The admit arbiter also has a low priority path used for best effort requests and for isochronous requests that are not considered urgent. The final level of arbitration is done using a priority selector that selects between the winner of the high priority arbitration and the winner of the low priority arbitration.
In one embodiment, the admit arbiter final selector has two modes that can he selected using a configuration register. The first mode is a fixed priority mode in which, assuming at least one high priority request is present at the input of the admit arbiter, the selector chooses the winner of the high priority arbitration path before choosing the winner of the low priority arbitration path. The second mode of the final selector is a weighted round robin mode in which the final selector switches between granting the high priority path to granting the low priority path after N number of high priority requests are granted. The selector then grants M number of low priority requests from the winner of the low priority path before switching back to granting requests from the high priority path. In an embodiment, the values for N and M may be referred to as “grant counts,” and are specified using configuration registers.
In the embodiment shown, admit arbiter 120 includes a first age-based arbiter 122 and a second age-based arbiter 124, which correspond to low and high priority age-based arbiters, respectively. Thus as seen, requests from all agents 115 are provided to first arbiter 122, while only requests from isochronous agents 115-2 and 115-3 are provided to second arbiter 124. To determine whether a particular request from one of the isochronous agents is of an urgent status, a pair of deadline checker logics 120-1 and 120-n, are each coupled to receive requests from a corresponding one of the isochronous agents, as well as global timing information from global timer 150. Based on a comparison of the deadline information provided by the agent and the global timing information, an indication of an urgent status for a corresponding request can be provided to second arbiter 124.
In operation, arbiters 122 and 124 operate to select an arbitration winner from a set of incoming requests. In the embodiment shown, this determination is based in part on information from an age storage 126 that stores an age value for each of the agents. The corresponding winners from each of the arbiters may be coupled to a priority arbiter selector 125 that selects based on mode of operation a corresponding request to provide to scheduler arbiter 130 (
Weighted Age-Based Arbitration Details
The age-based algorithm implemented by the admit arbiter is such that the requesting agent which has waited the longest since last being granted by the arbiter will be given the highest priority level. Once an agent has received the highest priority level, the priority level for that agent will not change unless that agent has been granted by the arbiter. In this way, starvation issues that may occur in certain embodiments of round robin arbitration may be avoided by ensuring that the priority level for a requesting agent can only increase in priority level until that requesting agent has been granted by the arbiter.
The admit arbiter also allows for agent weights to be assigned to all requesting agents. Weights are used to allocate a percentage of the request bandwidth for each requesting agent. In an embodiment, a weight value is specified for each agent via a value stored in an agent weight configuration register. In one non-limiting example, the percentage of request bandwidth that is allocated to an agent is equal to the agent weight value divided by the sum of weights for all agents. In another example, weight for real-time core can be higher than other cores to give better QoS for a real-time core.
Weighted Age-Based Algorithm
The admit arbiter weighted age-based algorithm is based on the relative age of when a requesting agent was last granted by the arbiter. For each requesting agent that connects to the admit arbiter, there is one age counter instantiated and one weight counter instantiated.
Both the high priority and low priority arbitration paths in the admit arbiter share common age and weight counters for the agents connected to the admit arbiter. The updating of the requesting agent's age and weight registers is determined by the final selector (namely the priority arbiter selector 125) after choosing the final arbitration winner.
In an example, the age registers (e.g., of age storage 126) for all requesting agents are first initialized responsive to receiving a reset input to the admit arbiter. When reset asserts, the age registers are initialized to unique values in a range starting at 0 and ending at a value of N−1, where the value of N equals the number of request interfaces connected to the admit arbiter.
Prior to any requests being asserted by the requesting agents, the agent weight counters (e.g., of weight storage 128) are initialized from programmed values in the agent weight configuration registers of the fabric. Once the weight counters initialize, the counter for an agent decrements by one for each request granted for that agent. Once an agent's weight counter reaches zero and if the agent is granted again by the admit arbiter, the counter is reloaded with the value programmed in the configuration register for that agent's weight.
In one embodiment, the age-based arbitration method performed in first and second arbiters 122 and 124 uses a request bit vector (each arbiter having its own vector) to determine the winner of the arbitration. When a request is asserted for an agent the arbiter uses the age value for the requesting agent as the priority level of the request. The priority levels for the arbiter and thus the range of the bit vector width is from 0 to N−1. The age-based algorithm guarantees that the age values for all requesting agents are always unique and therefore there is always only one winner per arbitration.
The arbiter updates the age registers for all agents when the weight counter for the winner of the request arbitration has reached zero. In one embodiment, the age registers for all agents are updated according to the following rules that guarantee the age values for the agents are always a unique value:
Similarly at decrement agent weight state 280, a winner arbitration weight counter is decremented. But here no weight counter reloads are performed. It should be noted that the state machine block diagram of
The states and descriptions of the state machine of
If it is determined that the deadline value is greater than the latency threshold, control passes to block 320b, where the memory request is forwarded to a low-priority arbiter. Otherwise control passes to block 320a, where the memory request is forwarded to a high-priority arbiter.
Note the presence of parallel paths such that at block 325 (blocks 325a and 325b), an arbitration is performed in the corresponding arbiter that is based on a bit vector associated with the age values for the devices that provide requests to the corresponding arbiter. Next at block 330 (blocks 330a and 330b), the winning memory requests are forwarded to a final arbiter. At block 335, a final arbitration is performed to select the winner memory request.
Depending upon a mode of configuration for this final arbiter, the winner request can be selected from the high priority arbiter only, or a weighting between high priority and low priority paths may occur. Thus at this point the winning memory request is forwarded to a memory scheduler scoreboard where it can be stored in an entry to thus enable arbitration in the memory scheduler arbiter to consider this memory request.
Various updating operations may further be performed responsive to selection of a winner by the final arbiter. Specifically, at decision block 340 it can be determined whether the weight value of the winner agent equals zero. If so, control passes to block 345 where this weight value can be updated to its configured value, e.g., stored in a configuration register of the shared memory fabric. Control next passes to block 350 where the age values for all agents can be updated (block 350). To this end all non-winning agents may have their age value incremented, while the winning agent may have its age value set to a lowest priority value. e.g., zero. If instead at decision block 340 it is determined that the weight value of the winner agent is not zero, control passes to block 355 where the weight value of the winner agent is decremented. It should be noted that the flow chart of
Shared Memory Fabric Shared Resource Allocation
The memory fabric includes logic to allow for fair allocation of the shared resources within the fabric, e.g., the resource allocation logic 148 of
A portion of the memory scheduling algorithm deals with minimizing the performance impact of read-to-write turnaround times for memory technologies. In order minimize the number of times the memory scheduler switches between scheduling read requests to scheduling write requests, a flush pool is used for queuing write requests. The flush pool allows write requests targeting memory to be accumulated in the memory fabric until enough write requests have been received to allow the fabric's memory scheduler to send the write requests to the memory controller as a burst of back-to-back requests. In order to prevent all available resource in the fabric to be used up by the flush pool, a flush limit can be specified. When specified, the flush limit causes the fabric to block new write requests from all agents at the admit arbiter until the number of entries in the flush pool is less than the value programmed for the flush pool.
Memory Fabric Flush Pool for Write Requests
When a write request is received from a requesting agent, the fabric transfers the write data from the requesting agent to an internal data buffer. Once the new data is written to the fabric's internal data buffer and the request is retired from the agent's point of view, the buffer entry is considered to be in the “flush pool”. For coherent memory traffic the fabric may receive snooped requests from the requesting agents. Snooped requests can be either read or write requests to memory. When the fabric receives a snooped read or write request from a requesting agent, it sends a snoop request to all caching agents coupled to the fabric. The caching agents will respond to a snooped request that hits in their cache and will return the write back (WB) data for a cache line that has been modified by the caching agent. The WB data is then written into the fabric's internal data buffer and is then considered to be included in the flush pool of write requests targeting memory. When the number of entries in the flush pool reaches the value programmed for the flush limit, new write requests, e.g., as determined by decoding of the request opcode field, are blocked at the admit arbiter.
Memory Fabric Reservations and Limits
The memory fabric allows reservations to be specified for any agent using agent reservation configuration registers. Using these configuration registers the user can specify the number of entries in the memory fabric to reserve for each agent. The reserved entries for an agent are the first entries allocated to the agent and the last entries to be retired for the agent. In order to determine if an agent's reserved entries are being allocated or retired, each agent has a request counter that is compared against the value specified in the configuration register. If the value in the request counter is less than or equal to the value in the configuration register, the agent's reserved entries are being used.
The mechanism used to provide agents with reserved entries varies over the full threshold limit as reserved entries are allocated or freed for requesting agents. Initially, the full threshold for all agents is calculated by subtracting the total number of reserved entries for all agents (e.g., as specified by configuration registers) from the total number of entries in the scoreboards. As reserved entries are allocated to an agent, an accumulator is used to adjust the full threshold based on the total number of reserved entries that have been used. Agents that have used their reserved entries or do not have reserved entries specified are blocked when the total number of pending requests in the memory fabric reaches this adjusted full threshold. Agents that have not used their reserved entries are not blocked by the admit arbiter until they have used all their reserved entries and the total number of pending requests reaches the adjusted full threshold limit.
Agent limits may also be specified in configuration registers of the memory fabric. These agent limits may be disabled by setting the request limit for an agent to zero, in an embodiment. When agent limits are disabled any agent may be allocated all existing entries of the request tracker. In order to prevent a single agent from using all request tracker entries, a request limit can be specified for the agent. When the agent's request counter reaches the request limit specified for the agent the request input to the admit arbiter for that agent is disabled. When the request tracker retires requests for the agent and the agent's request counter becomes less than the agent's request limit, the request input to the admit arbiter for that agent is enabled.
As such, the sum generated by adder 375 corresponds to an adjusted full threshold value that is provided to one input of a comparator 382 that further receives a number of allocated tag entries from flip-flop 376. If it is determined that the adjusted full threshold value is less than or equal to this number of allocated tag entries, a full flag is generated and used to mask requests of agents that have no reserve entries or have used their reserve entries.
As further seen, another comparator 380 is configured to receive a given requestor's reserve configuration value and a request counter value for that requestor (from flip-flop 378). The comparator thus generates an indication as to whether that requester has any free reserved entries, which is provided as an input to a pair of AND gates 384 and 385 that further receive indications of a channel grant and a retirement of an entry for that channel. As such, these AND gates thus generate, respectively the increment and decrement values for the corresponding requestor. Similar logic and operations are performed for the other requestors, with all increment and decrement reserve values being provided to corresponding OR gates 386 and 387 that respectively generate the increment reserve count value and the decrement reserve count value.
Finally, the request counter value for a requestor is provided to another comparator 390 along with a configured limit value for that requestor to thus determine whether this requestor has reached its limit. If so, an indication of this limit is used to mask off the requests from this agent for further arbitration. It should be noted that the block diagram of
Shared Memory Fabric Scheduler Arbitration Details
Embodiments may incorporate multiple scheduling algorithms to enhance reuse across multiple SoCs that support different memory technologies. The fabric's memory scheduler logic contains advanced QoS scheduling algorithms, and is also optimized to minimize performance bottlenecks that are commonly found in most memory technologies. The typical performance bottlenecks that occur using, e.g., DRAM memories include entering and exiting of low power memory states, read-write turnaround times, consecutive memory accesses to the same DRAM bank but to different rows of memory, and consecutive memory accesses to different DRAM memory ranks By including complex out-of-order scheduling algorithms in the shared memory fabrics scheduling logic, the fabric can be adapted to many different SoCs by attaching simplified technology-specific constraint solvers to the fabric to support their unique requirements for memory technologies or configurations.
In other embodiments, the methods described in
In addition to improving the portability of the memory scheduling logic, embodiments also provide predictability of memory request latency in that the combination of advanced out-of-order scheduling algorithm with QoS scheduling logic results in improved predictability of the maximum request latency, in that the memory controller has much less flexibility to reorder memory requests.
Once a request is granted by the admit arbiter, it is enqueued into the scheduler scoreboard. The scheduler scoreboard stores information about the request that it uses to forward the request to the memory controller in order to perform a read or write to memory. In one embodiment, the information includes request address, request length, command type (read or write), class of service category, memory channel, memory bank, memory rank, and page hit/miss status.
Memory Scheduler Oldest of Available Queue
Embodiments provide for out-of-order page aware scheduling that is based on a history of requests sent to the memory controller, although the fabric has no direct knowledge of the true state of the memory bank. More specifically, the fabric's memory scheduler uses the scheduler scoreboard as a history buffer of requests that have been sent to memory. Because the scheduler scoreboard is used to reflect the history of requests, it seeks to retain the status information for a request in the scoreboard as long as possible. The memory scheduler uses a structure called the oldest of available queue to determine the oldest scoreboard entry that is available to be reallocated.
The oldest of available queue is also used by the memory scheduler to avoid starvation issues that can arise due to the out-of-order scheduling of the requests to memory. The fabric's memory scheduler uses the oldest of available queue to determine how many requests of the same class of service category and type, read or write, have bypassed the oldest pending request to memory. Once the number of requests that have bypassed the oldest request reaches a preprogrammed limit (e.g., set by software) the fabric's memory scheduler disables out-of-order scheduling of requests and grants the oldest pending request.
As mentioned above, the scheduler keeps track of the relative age of all requests in its scoreboard using the oldest of available queue. When a request targeting a new memory address is granted by the admit arbiter an index pointer into the scheduler scoreboard is enqueued into the tail entry of the oldest of available queue which is then considered to be the newest request. When all pending requests have completed transferring data to/from the requesting agents and to/from the memory controllers, a scoreboard entry is available to be reallocated and can be reallocated for a new request granted by the admit arbiter. Due to the out-of-order scheduling, the oldest entry in the oldest of available queue may not always be available for reallocation.
To select the scoreboard entry to be re-allocated to a new request, the scheduler detects whether all outstanding requests to a scoreboard entry have completed. In one embodiment, the scheduler uses a request bit vector having a length equal to the number of scoreboard entries to indicate which entries are available for reallocation. A bit set to 1 in the request bit vector indicates the entry corresponding to that bit position is available for reallocation. The request bit vector is then sent to the oldest of available queue. The oldest of available queue uses the indexes stored in the queue to select the bit in the request vector corresponding to the request for that entry of the queue. Each entry of the queue is associated with a unique bit in the request vector and a “find first” function is performed starting from the oldest entry in the queue to determine the oldest available request to be reallocated. After determining the oldest available entry to be reallocated, the scoreboard index for that entry is output from the oldest of available queue.
As seen, an output of each flip-flop 410 is coupled to one of a corresponding plurality of multiplexer 420-0-420-n, each of which is further configured to receive a bit of a scoreboard request vector. As such, this bit vector provides an indication. e.g., via a set bit to indicate that a corresponding scoreboard entry is available for reallocation. Using the outputs from multiplexers 420, a grant signal can be generated either directly from the comparator output (as from comparator 420-n) or via a corresponding one of logic gates 430-0-430-n (which in the embodiment shown are configured as AND gates having a first input received from a corresponding multiplexer 420 and a second input corresponding to an inverted output of a corresponding OR gate 425-0-425-(n−2)). In this way only a single one of the grant signals may be active at a time.
As further seen in
Shared Memory Fabric Memory Scheduling Details
In an example, the fabric memory scheduler contains three state machines that work together to schedule requests sent to the memory controller.
As seen in
Self-Refresh State Machine
Embodiments may control when the memories are allowed to enter and exit the low power memory state, also referred to as the self-refresh state. The self-refresh state machine is responsible for controlling when to send an indication to the memory controller to enter or exit self-refresh. For best effort read requests, the self-refresh state machine transitions immediately to the exit self-refresh state. For isochronous read requests, the memory scheduler checks the request deadline to determine if it is to exit self-refresh in order to satisfy the required read latency for the request. To determine if exiting self-refresh is required for meeting the isochronous read requirement, the memory scheduler subtracts the deadline of the request from the current value of the global timer. The result of the subtraction is checked against a configuration register in the fabric that is programmed to reflect the worst case latency needed for the memory controller to exit self-refresh and the fabric to return data to the request agent.
For write requests, the fabric counts the number of dirty entries in the flush pool and checks the result against a programmable threshold value, termed the flush high water mark. If the number of dirty entries exceeds the value of the flush high water mark, the self-refresh state machine passes control to the exit self-refresh state. In addition, the fabric checks for read/write conflicts to the same tag address in which the request is blocked by the admit arbiter. When the fabric determines that a request is blocked by an address conflict, agent limit or if the request tracker or memory scheduler scoreboards are full, control passes from the self-refresh state machine to the exit self-refresh state. The fabric also contains a configuration register that can be programmed to disable entering self-refresh, in an embodiment.
When the memory scheduler sends an indication to the memory controller to exit self-refresh, requests may begin to be sent to the memory controller. The memory scheduler continues to send an indication to the memory controller to remain out of self-refresh while it is actively sending memory requests to the memory controller. When the memory scheduler completes sending all read requests to the memory controller and the number of write requests in the flush pool is below the casual high water mark limit, the memory scheduler transitions to the request self-refresh slate.
In the request self-refresh state if no new requests are granted by the admit arbiter the state machine transitions to the “enter self-refresh” state after a programmable delay value called the “enter self-refresh delay” is met. In an embodiment, this delay is programmed in configuration registers in the fabric. If new requests are granted by the admit arbiter, the self-refresh state machine may transition to the “exit self-refresh” state under certain conditions. If a new best effort read request is received or if a write request is received that results in the number of entries in the flush pool exceeding the number programmed in the flush high water mark configuration register, the self-refresh state machine transitions from the request self-refresh state back to the exit self-refresh state. If an isochronous read request is received when the state machine is in the request self-refresh state, the deadline value of the request is checked against a programmed value called the “enter self-refresh” threshold. If the deadline latency is greater than the enter-self-refresh threshold, the state machine continues in request sell-refresh state if the deadline latency for a request is below the enter self-refresh threshold, the state machine will transition to the exit self-refresh state.
The self-refresh state machine drives status to the memory controller to remain out of self-refresh until the state machine transitions to the enter self-refresh state. Once in the enter self-refresh state, the state machine sends an indication to the memory controller to enter self-refresh.
Table 2 below is a description of a self-refresh state machine in accordance with an embodiment of the present Specification.
Read/Write Grant State Machine
In an embodiment, the memory scheduler uses configurable threshold values to specify when to start and stop transferring a burst of write requests to the memory controller. The memory scheduler may perform different types of transfers of write data to memory. e.g., a high priority transfer and a low priority transfer, also termed herein as a high priority flush of write requests and casual flush of write requests to memory, respectively. When the number of entries in the flush pool reaches or exceeds a threshold value (the flush high water mark), the memory scheduler begins scheduling a high priority write flush to memory and begins sending write requests to the memory controller. The memory scheduler continues to schedule write requests using the high priority flush mechanism until the number of entries in the flush pool reaches or is less than a threshold value (the flush low water mark).
A casual flush may also be performed by the fabric memory scheduler. A casual flush is triggered when the memory scheduler has completed sending all read requests to the memory controller and the number of entries in the flush pool exceeds a threshold value (the casual flush limit). In an embodiment, the casual flush limit can be typically set lower than the high water mark, but greater than or equal to the low water mark, for performance reasons. In some cases this casual flush limit can be set to 0 to flush all write data to memory. Once the last read request is sent to the memory controller, if the number of entries in the flush pool is above the casual flush limit, a counter called the casual flush timer starts incrementing every′ clock cycle. If no new read requests to memory are received by the fabric and the casual flush timer reaches the value specified by the casual flush delay, which is a threshold stored in a configuration register, the memory scheduler begins sending write requests to the memory controller. This casual flush continues until the number of entries in the flush pool is less than the casual flush limit or until a new read request is received by the fabric.
The read/write grant state machine is responsible for switching from granting read requests to granting write requests. In an embodiment, the memory scheduler is configurable to allow write requests to have priority over read requests or to use weights when switching between read requests and write requests (in order to prevent starvation of reads when the system is saturated by write requests). When weights are enabled, the memory fabric uses configuration registers to specify the read and write weights independently.
Table 3 below is a description of a read/write grant state machine in accordance with an embodiment of the present Specification.
Read State Machine
The read state machine is responsible for switching between high priority isochronous read requests, best effort read requests and low priority isochronous read requests. The read state machine can be configured to operate in one of multiple modes. In one embodiment, two such modes are provided. A first mode is a fixed priority mode where the read state machine gives high priority isochronous reads highest priority, best effort read requests medium priority, and low priority isochronous read requests receive the lowest priority. A second mode is to enable the use of weights for switching between high priority isochronous reads and best effort read requests. In this mode, low priority isochronous requests are only granted when there are no longer any high priority isochronous or best effort read requests.
Table 4 is a description of a read state machine according to the present Specification.
Scheduler Agent Weights
The memory scheduler uses agent weights for proportioning memory bandwidth between agents within the same class of service category. In an embodiment, configuration registers specify the weight value for each requesting agent, and a weight counter is provided for each agent. The agent weight configuration registers are common between the admit arbiter and the memory scheduler.
When there are no requests pending in the memory scheduler for any of the agents connected to the fabric, the agent weight counters are loaded with values specified in the agent weight configuration registers. When requests are granted by the admit arbiter and enqueued into the memory scheduler scoreboard, an agent ID field is stored in the memory scheduler scoreboard along with the request information. When the memory scheduler grants a request in its scoreboard, the agent ID field is used to determine the source of the request and the weight counter for that agent is decremented by one. Once an agent's weight counter has reached zero, the remaining requests for that agent are masked and no longer take part in the scheduler arbitration. When an agent is masked from arbitration due to its weight counter reaching zero, the memory scheduler continues to schedule requests from the remaining agents. Once the weight counters for all agents have reached zero or if an agent's weight counter is non-zero but there are no remaining requests for that agent, all agent weight counters are reloaded with the values from agent weight configuration registers.
Still referring to
From both of decision blocks 630 and 640, control passes to decision block 650 where it can be determined whether the weight value of all agents equals zero. If so, control passes to block 660 where the weight values for all the agents can be updated to their configured values, e.g., obtained from a configuration register of the fabric. Otherwise, control passes from decision block 650 to decision block 670 to determine whether there are any remaining requests in the memory scheduler for agents having a non-zero weight value. If so, those requests can be handled. e.g., via another iteration of method 600. Otherwise if no additional requests remain, control passes to block 660 where the weight values can be updated as described. It should be noted that the flow diagram of
Table 5 below provides example operation of memory scheduling for plurality of clock cycles, based on initial weight values for three agents as follows:
Out of Order Page Aware Scheduling
The memory scheduler reorders requests sent to the memory controller and seeks to optimize the stream of requests for the maximum memory bandwidth possible. The memory scheduler contains configuration registers programmed to provide the scheduler with information about the memory controller to which it is attached. In one embodiment, these configuration registers include information about what address bits are used for the memory channel, bank, rank and row addresses. Using the memory configuration information programmed in the configuration registers the memory scheduler determines the bank, rank, row, and channel of each request in the scheduler scoreboard. The memory scheduler scoreboard also contains a page hit status bit for each request that is used to optimize requests sent to the memory controller so that requests to the same page in memory are sent to the memory controller before sending request to a different page.
After initialization and before any requests are sent to the memory controller, the memory scheduler clears all page hit status bits in its scoreboard. As requests are sent to the memory controller the memory scheduler updates the page hit status bits in the scoreboard to indicate whether other requests are to the same page or to a different page in memory. Although the scheduler is not aware of the actual state of the page in a given memory bank, these page hit status bits may be used as a hint as to which requests are the best candidates to send to the memory controller for optimal memory bandwidth.
When a request is sent to the memory controller, the memory scheduler compares the channel, rank and bank information for all other requests pending in the scoreboard. If the channel, rank and bank information of a scoreboard entry matches a request that is sent to the memory controller the row address of the entry is compared against the row address of the request sent to the memory controller. If the row address of a scoreboard entry matches for the request the page hit status bit is set to 1; if the row address does not match the request the page hit status bit is set to 0 indicating a page miss. For scoreboard entries where the channel, rank or bank bits are different than the request sent to the memory controller, no update of the page hit status occurs.
As new requests are granted by the admit arbiter and enqueued into the scheduler scoreboard, the row address information is compared against all entries currently in the scoreboard. If the row address of the new request matches one or more entries in the scheduler scoreboard and the page hit status bit of any matching entries is set, the page hit status for the new request is also set. If the row address does not match any entries in the scoreboard or all entries it matches have the page hit status set to zero, the page hit status for the new request is also set to zero.
Using the page hit and rank status information stored in the scheduler scoreboard, the memory scheduler reorders requests sent to the memory controller based on a priority encoded scheduling scheme that has been determined to provide optimal bandwidth for most DRAM-based memory technologies. The memory scheduler grants higher priority requests before granting requests with lower priority levels.
Table 6 below shows the different priority levels used by a memory scheduler in accordance with one embodiment of the present Specification.
Memory Scheduler Page Aware Scheduling Priority
Age Based Memory Scheduling and Starvation Prevention
In order to prevent starvation of requests due to the out-of-order page aware scheduling algorithm, the concept of age is used at least in part to schedule requests. For each class of service (COS) category, the memory scheduler contains a configuration register to specify an out-of-order (OOO) scheduling limit. To provide a shorter maximum read latency for the isochronous COS category, the OOO scheduling limit is typically set to a smaller value than the OOO scheduling limit of the best effort COS category. The memory scheduler creates a request hit vector for all pending requests in its scoreboard for the best effort and isochronous COS categories. These request bit vectors are sent to the oldest of available queue, which determines the oldest request that is still pending. The oldest of available queue outputs a one hot encoded bit vector with the bit set to 1 to indicate the oldest request. As the memory scheduler grants requests OOO based on its page aware scheduling algorithm, the memory scheduler counts how many requests were granted that were not the oldest pending request for each COS category. Once the counter reaches the OOO scheduling limit for the COS category, which may be determined by performance analysis done for worst case acceptable latency for a COS category, the page aware scheduling logic is disabled and the oldest request for the COS category is granted by the memory scheduler. Any time the oldest request for a COS category is granted, the counter for that COS category is reset to zero. To provide the lowest possible latency for a COS category the OOO scheduling limit can be programmed to zero, essentially disabling the page aware scheduling logic for that COS category. When the OOO scheduling limit is set to zero for a COS category, requests to memory may be scheduled using request age, which is determined by the oldest of available queue.
Best Effort Maximum Latency Starvation Prevention
For best effort read requests, the fabric utilizes the deadline storage information in the scheduler scoreboard to store a value that is used to specify a maximum latency value for scheduling best effort requests. The scoreboard is a pool of entries and a request stored in the scoreboard may be either a best effort or isochronous request determined by the request's class of service category, also stored in the scoreboard for each request. In the case a request in the scoreboard is a best effort read request, a maximum allowable latency. e.g., a preprogrammed value stored in a configuration register, is used to schedule the request. When the request is enqueued in the scoreboard and is a best effort read request the maximum latency value is added to the current value of the global timer. Once the global timer reaches the value stored for the best effort requests' maximum latency, page aware scheduling is ignored for the request and results in the request being scheduled when it is the oldest request pending. e.g., as determined by the oldest of available queue.
Request Tracker Write Priority and Weights
The request tracker is responsible for the transfer of data from the requesting agents to the internal memory butler of the fabric. The write protocol used by the shared memory fabric causes all write data to be transferred in request order from the requesting agent to the internal memory buffer in the fabric. In one embodiment, the request tracker uses separate linked lists per agent to preserve the ordering of the write requests. The request tracker may perform coherency checks for a write request prior to transferring data from the requesting agent to the internal data buffer.
For write requests, the request tracker may be configured to support one or more priority levels. When a request is granted by the admit arbiter the deadline information for the request is stored in an array having a length corresponding to the number of entries in the request tracker. The fabric uses a threshold value, e.g., stored in a configuration register, to specify when a request deadline value is considered to be high priority. Each deadline value for a request is compared against the threshold value programmed in the configuration register. When the deadline latency is less than the value in the configuration register, a bit is set in the tracker's scoreboard entry for the request indicating the request is a high priority request.
When enabled for two priority level operation, if a write request for an agent reaches the head of the linked list and the high priority bit is set for the request the write request is considered to be high priority. If any write requests at the head of any of the agent linked lists indicate the write request is a high priority request, all low priority write requests at the head of the other linked list for other agents are masked before being input to the write request arbiter. If multiple requests of the same priority level are present at the head of the agent linked lists, an arbitration is performed to select which agent to choose to transfer the write data.
Request Tracker Write Request Arbiter
The write request arbiter uses a weighted priority based fair arbiter to select which agent to transfer write data. The weights for the write request arbiter are programmed in configuration registers in the request tracker. The write arbiter assigns each agent a unique priority at reset. On each cycle, the arbiter only considers request candidates with data that is ready to transfer, and grants to the requester with the highest priority. When granted, a request candidate's weight is decremented by one. If the granted candidate already had a weight of zero, then the arbiter also updates request candidate priorities as follows: the granted candidate's priority is set to the lowest priority (e.g., zero): all candidates with priorities lower than the granted candidate increment their priority, and all candidates with priorities higher than the granted candidate leave their priority unchanged.
Request Tracker Read Data Return
Requesting agents either support in order data return or out-of-order data return. To support out-of-order data return, an order ID field is used. An order ID is sent from the agent with each request and is stored in the request tracker scoreboard. Requests from the same agent that have the same order ID are returned in request order. Data for requests from the same agent having different order IDs do not need to be returned in request order. In an embodiment, the request tracker uses linked lists for ensuring read data is properly ordered when it is returned to the requesting agent.
The entry of the internal data buffer where data is to be written is chosen prior to a request being granted by the admit arbiter. When a request is granted by the admit arbiter, request information including the index into the internal data buffer is forwarded to the request tracker. As data is returned from the memory controller, the memory scheduler forwards a read completion indication to the request tracker, which includes the index field into the internal data buffer where the data is being written and an indication of which chunks of the memory address have completed a read of memory. When the request tracker receives a read completion, it compares the index field with the index fields for all requests Stored in the request tracker scoreboard. If a scoreboard entries' index field matches a read completion for a request and all chunk bits for the request are set for the read completion, a bit is set in the request tracker scoreboard indicating the read request has completed.
If a read request has reached the head of the linked list and the read completion status bit in the request tracker is set and all coherency checks for the request have completed, the request is available to return read data to the agent. Similar to write requests, the request tracker uses the request deadline information for a scoreboard entry to indicate request priority. In one embodiment, the request tracker creates two request bit vectors for scoreboard entries that have data ready to return to the requesting agents. One bit vector is for low priority read requests and the other bit vector is for high priority read requests. The request bit vectors are input to the request tracker oldest of available queue. The oldest of available queue determines which request is the oldest for both request hit vectors. The request tracker has a configuration mode which when enabled will cause a return of data from the oldest high priority request selected by the oldest of available queue before returning data for any low priority requests. When support of the high priority data return is not enabled, the request tracker treats all scoreboard entries that are ready to return read data as having the same priority level. In this mode, only the low priority bit vector is used as an input to the oldest of available queue that in turn determines the oldest read request in the scoreboard. Read data for the scoreboard entry determined to be the oldest is then returned to the requesting agent.
Embodiments may be used in many different SoCs or other semiconductor devices that integrate various IPs onto a single die to connect these IPs to memory via a memory fabric. Still further a memory fabric in accordance with an embodiment of the present Specification may be used to provide a QOS level for meeting isochronous requirements of at least some of these IPs.
A system agent 730 is coupled to these cores and graphics engines via corresponding in-die interconnects 728 and 729. As seen, system agent 730 includes a shared memory fabric 735 which may be configured as described herein. Various other logic, controllers and other units such as a power management unit may also be present within system agent 730. As seen, shared memory fabric 735 communicates with a memory controller 740 that in turn couples to an off-chip memory such as a system memory configured as DRAM. In addition, system agent 730 is coupled via a set of interconnects 744 to one or more internal agents 750 such as various peripheral devices. In an embodiment, interconnect 744 may include a priority channel interconnect, a sideband channel interconnect, and a memory channel interconnect. A similarly configured interconnect 74 provides for communication between system agent 730 and one or more off-chip agents (not shown for ease of illustration in the embodiment of
As seen in
Processor 810 may communicate with a system memory 815, which in an embodiment can be implemented via multiple memory devices to provide for a given amount of system memory. To provide for persistent storage of information such as data, applications, one or more operating systems and so forth, a mass storage 820 may also couple to processor 810. Also shown in
Various input/output (to) devices may be present within system 800. Specifically shown in the embodiment of
For perceptual computing and other purposes, various sensors may be present within the system and can be coupled to processor 810 in different manners. Certain inertial and environmental sensors may couple to processor 810 through a sensor hub 840, e.g., via an I2C interconnect. In the embodiment shown in
Also seen in
System 800 can communicate with external devices in a variety of manners, including wirelessly. In the embodiment shown in
As further seen in
In addition, wireless wide area communications. e.g., according to a cellular or other wireless wide area protocol, can occur via a wireless wide area network (WWAN) unit 856 which in turn may couple to a subscriber identity module (SIM) 857. In addition, to enable receipt and use of location information, a GPS module 855 may also be present. Note that in the embodiment shown in
To provide for audio inputs and outputs, an audio processor can be implemented via a digital signal processor (DSP) 860, which may couple to processor 810 via a high definition audio (HDA) link. Similarly. DSP 860 may communicate with an integrated coder/decoder CODEC) and amplifier 862 that in turn may couple to output speakers 863 which may be implemented within the chassis. Similarly, amplifier and CODEC 862 can be coupled to receive audio inputs from a microphone 865 which in an embodiment can be implemented via dual array microphones to provide for high quality audio inputs to enable voice-activated control of various operations within the system. Note also that audio outputs can be provided from amplifier/CODEC 862 to a headphone jack 864.
In the example of
RT agent 115-0 and auxiliary agent 115-1 share memory controller 170-0 and memory controller 170-1, which control memory bank 1220-0 and 1220-1 respectively. In certain examples, memory bank 1220-0 and memory bank 1220-1 are completely independent of one another, and may be interleaved such that even-numbered memory addresses go through memory controller 170-0 to bank 1220-0, while odd-numbered memory locations are routed through memory controller 170-1 to memory bank 1220-1. This is provided by way of example only, and other memory configurations and interleaving methods are available. It should also be noted that in this example, memory controllers 170 and memory banks 1220 are shown on a separate memory bus. This is also disclosed by way of non-limiting example. In other examples, other memory architectures may be used, such as a shared bus, or a network-on-a-chip.
RT agent 115-0 may be configured to interface with a control subsystem 1290 for controlling a device under control 1292. In one embodiment, device under control 1292 may be a mission-critical or safety-critical device such as a manufacturing robot, life support system, environmental control system, traffic control system, or drive-by-wire system by way of non-limiting example. Control subsystem 1290 provides to RT agent 115-0 all of the software, firmware, and hardware necessary to control device under control 1292. The requirements of controlled system 1290 may be such that a guaranteed QoS is necessary to maintain real-time operation.
However, it may also be desirable to provide auxiliary functions, such as a user interface so that a user can provide necessary inputs. Auxiliary agent 115-1 may also provide functions such as monitoring and user feedback. Thus, it is desirable to design SoC 1200 so that RT agent 115-0 is guaranteed its necessary QoS for its real-time functions, but doesn't completely monopolize system resources so that auxiliary agent 115-1 is unable to perform its function, or vice versa. To this end, a shared uncore fabric 1230 with multiple virtual channels to separate out real-time and auxiliary traffic, and an associated priority scheme, may be provided to grant higher priority to real-time traffic, while leaving sufficient bandwidth for auxiliary agent 115-1 to function properly.
In this example, RT agent 115-0 communicatively couples to controlled system 1290 via suitable means, such as a network interface, dedicated bus, or other connection. In this drawing, RT agent 115-0 also communicatively couples to RT peripheral device 1210-0 via shared uncore fabric 1230. In certain embodiments, shared uncore fabric 1230 may be provided as single or multiple modular IP blocks for simplicity of design.
For simplicity of the drawing, and to illustrate that many different styles of interconnect are possible, no physical or logical connection is illustrated here between RT peripheral device 1210-0 and control subsystem 1290. But this is not intended to exclude such a connection. In some examples, RT peripheral device 1210-0 may be a control interface that forms a part of control subsystem 1290, or a physical and logical interface to device under control 1292, in which case a logical and/or physical connection may be provided. In other embodiments, RT peripheral device 1210-0 may provide other real-time functionality that may or may not be directly logically related to device under control 1292.
Similarly, auxiliary agent 115-1 communicatively couples to user interface 1270 by way of example, or to any other suitable auxiliary system or subsystem. User interface 1270 may, similar to control subsystem 1290, provide any suitable set of software, firmware, and hardware for provisioning a user interface.
Auxiliary agent 1150-1 also communicatively couples to auxiliary peripheral device 1210-1 via shared uncore fabric 1230. As with real-time peripheral device 1210-0, auxiliary peripheral device 1210-1 may or may not communicatively couple to user interface 1270. For simplicity of the drawing, and to illustrate that many different connection options are possible, no physical or logical connection is shown in this figure between auxiliary peripheral device 1210-1 and user interface 1270, but in some embodiments, such a connection may be provided.
In a non-limiting example, selected elements of shared uncore fabric 1230 include a shared I/O fabric 1232, shared memory fabric 100, and a system agent 730. Shared I/O fabric 1232 provides interconnects, scheduling, and other communication services to peripheral devices 1210. In one example, shared I/O fabric 1232 is substantially similar to shared memory fabric 100, including implementing similar priority schemes to those described in this Specification. Shared memory fabric 100 was described in more detail in connection with
In one example, shared uncore fabric 1230 includes only one set of physical buses, interconnects, registers, and other resources that real-time agent 115-0 and auxiliary agent 115-1 (along with any other agents) may use to communicatively couple to peripheral devices 1210, and to memory controllers 170. Thus, to ensure a guaranteed QoS for real-time agent 115-0, shared interconnect resources 1230 may need to provide a priority scheme between agents 115, peripherals 1210, and memory controllers 170.
Certain embodiments of a shared uncore fabric may employ only one virtual channel that is shared between all agents. However, the present Specification also describes a method of providing a plurality of virtual channels so that shared uncore fabric 1230 can discriminate, segregate, and prioritize between traffic for real-time agent 115-0 and traffic for auxiliary agent 115-1. This segregation may be desirable so that in cases where it is necessary, traffic from real-time agent 115-0 may receive priority, including preemptive priority over traffic from auxiliary agent 115-1.
In one example, two virtual channels are defined: namely virtual channel VC_AUX 1240, and virtual channel VC_RT 1242. VC_AUX 1240 may be provided for best-effort transactions, while VC_RT 1242 is dedicated to real-time or isochronous transactions. Traffic patterns may include agent-to-peripheral (via shared I/O fabric 1232), agent-to-memory (via shared memory fabric 100), peripheral-to-agent (via shared I/O fabric 1232), peripheral to memory (via shared I/O fabric 1232 and shared memory fabric 100), and memory-to-peripheral (via shared I/O fabric 1232 and shared memory fabric 100).
Division into virtual channels may be accomplished in one example by decoding the source agent for a packet originating from an agent 115. It should be noted that in certain known embodiments, the destination of each packet is decoded for routing purposes, and may be based on attributes of the packet such as memory address and/or opcode. In this example, destination decoding may still be provided, and may be in addition to decoding of the source agent. Once the source agent is decoded, the packet may be assigned to a “traffic class,” which may have a one-to-one correspondence to a virtual channel. For example, traffic class 0 may correspond to VC_AUX, while traffic class 1 may correspond to VC_RT. Advantageously, the traffic class may be encoded as a field in the header or metadata for the packet so that endpoints such as peripherals 1210 need not be aware of the virtual channel architecture to preserve end-to-end virtual channel functionality. This may preserve legacy interoperability. In one example, system agent 730 may prepend header data to each packet, identifying the virtual channel or traffic class on which the packet is to be carried. Certain virtual channels may be given certain priority weights according to the QoS scheme described herein. Priority schemes may include providing a high “grant count” number for high-priority traffic and/or assigning traffic on VC_RT an expired deadline to expedite that traffic.
In the case of a packet, such as a response packet, originating from a peripheral 1210, the peripheral 1210 may not be aware of the virtual channel architecture. However, a well-configured peripheral 1210 should echo back the traffic class field that was attached to the packet it is responding to. Thus, a legacy peripheral 1210 may be able to successfully direct the packet to its appropriate virtual channel despite being agnostic of the existence of multiple virtual channels within shared uncore fabric 1230.
In the case of a packet originating from a peripheral 1210 and directed to memory 1220 (rather than a response to an agent 115), a traffic class may be assigned based on the nature of the peripheral 1210 itself. For example, if RT peripheral 1210 is known to be generally used for real-time transactions, then packets from RT peripheral 1210 to memory 1220 may be directed to VC_RT. Similarly, packets originating from auxiliary peripheral device 1210-1 and directed to memory 1220 may be directed to VC_AUX.
In one example, virtual channels may also be further subdivided, for example according to the destination of each packet. Thus, for example, traffic from real-time agent 115-0 to any memory controller 170 may be given very high or even preemptive priority to guarantee a QoS. However, traffic from real-time agent 115-0 to real-time peripheral device 1210-0 may be less time critical. Thus, this traffic may be assigned a somewhat lower (though possibly still expedited) priority. In one example, VC_RT and VC_AUX may be prioritized differently based on which shared resource is being considered. For example, a VC_RT and VC_AUX path from peripherals to memory may use deadlines for different prioritization, while a VC_RT and VC_AUX path between cores and peripherals may use grant counts. These configurations are, of course, provided by way of non-limiting example only. A person having skill in the art will select an appropriate priority scheme according to the design constraints of a particular embodiment.
In block 1310, shared uncore fabric 100 receives incoming traffic from a requesting agent 115.
In block 1320, system agent 730 of shared uncore fabric 100 decodes the source agent for the traffic. The source agent in this case refers to the core that originates the transaction. This operation may also include (or a precedent operation may include) designating one core as the “real-time” core, for example by writing a value to a special-purpose register. For example, in a four-core system, a register may be provided in which the value “00” may be written to indicate that core 0 is the real-time core. Other configurations are also contemplated herein.
In block 1330, system agent 730 assigns the incoming packet to a traffic class, which maps to a particular channel, based on the source agent identified in block 1320. Assigning a traffic class and virtual channel may also include other factors, such as the opcode of the instruction or the destination address, thus permitting finer-grained control over traffic classification. In one example, the traffic class is prepended to a header or other metadata for the packet, so that it can be routed to the correct virtual channel throughout uncore fabric 100, and so that return traffic can also be correctly classified.
In decision block 1350, system agent 730 determines whether the assigned virtual channel is VC_RT.
If the assigned virtual channel is VC_RT, then a high-priority scheme may be applied. In some cases, preemptive priority may be necessary for certain sub-channels of VC_RT. However, preemptive priority may be overkill for other sub-channels of VC_RT, and thus a non-preemptive high-priority scheme may be used instead. For example, in block 1360, shared uncore fabric 100 may assign an expired deadline or higher-than-normal grant count to ensure that the packet receives higher priority. In one embodiment, this may be applicable only for traffic from cores and peripheral devices targeting DRAM. Other paths originating from cores to peripherals and peripherals to cores may simply rely on different grant counts to give preferential treatment for VC_RT.
In block 1370, if the packet is not on VC_RT, then it may be assigned a standard deadline.
In block 1390, the method is done.
In block 1410, shared uncore fabric 1230 receives incoming traffic from an agent 115.
In decision block 1420, system agent 730 determines whether the shared uncore fabric is currently configured for real-time operation. In one embodiment, real-time operation is not enabled immediately after device power up. Rather, device enumeration occurs strictly in auxiliary mode. Only after all devices have been enumerated and appropriate drivers loaded does real-time mode become enabled.
If RT mode is not currently enabled, then in block 1450, the traffic is automatically assigned to VC_AUX, and to the appropriate traffic class, which in this example is TC0. In certain embodiments, each traffic class maps to exactly one virtual channel, so that TC0==VC_AUX.
Returning to block 1420, if RT mode is enabled, then in block 1430, system agent 730 decodes the source agent for the packet to determine which agent 115 originated the packet. Decoding may also include decoding the destination, or at least whether the traffic is to be routed through interconnect fabric 1232 or memory fabric 100. This may affect classification into a particular VC or subchannel.
In block 1440, system agent 730 assigns a traffic class and corresponding virtual channel (or subchannel, as appropriate) to the packet. The TC may be prepended to the packet, such as within a header or other metadata, so that the VC can be tracked even through portions of the system that are not VC aware, such as peripherals 1210 and memory 1220.
In block 1460, system agent 730 sends the packet to shared interconnect fabric 1232 via the designated VC.
In block 1480, system agent 730 may prioritize the packet according to the VC, for example by assigning higher priority to RT packets.
In block 1490, the method is done.
In block 1510, shared uncore fabric 1230 receives incoming traffic from an agent 115.
In block 1520, system agent 730 (or another decoder, as appropriate) decodes the traffic class, which may be included, for example, in a header or other metadata attached to the packet.
In block 1530, system agent 730 or another decoder maps the TC to its corresponding VC. In some embodiments, each TC maps to exactly one VC.
In block 1540, system agent 730 or another decoder sends the packet to the assigned VC on shared memory fabric 100.
In block 1560, depending on the type of memory operation, memory 1220 may provide a response. The response may include the appropriate TC, in which case, blocks 1520, 1530, 1540, and 1550 may be repeated for the response.
In block 1570, system agent 730 or another decoder forwards the response as a return to the device 1220 that initiated the transaction.
In block 1590, the method is done.
In block 1610, shared uncore fabric 1230 receives incoming traffic from an agent 115.
In block 1620, system agent 730 (or another decoder, as appropriate) decodes the traffic class, which may be included, for example, in a header or other metadata attached to the packet.
In block 1630, system agent 730 or another decoder maps the TC to its corresponding VC. In some embodiments, each TC maps to exactly one VC.
In block 1640, system agent 730 or another decoder sends the packet to the assigned VC on shared memory fabric 100.
In block 1650, the packet is forwarded to the agent 115 according to the attributes that were decoded.
In block 1590, the method is done.
In this example, a dual-core processor 1710 includes RT CPU 115-0 and AUX CPU 115-1, each of which is an independent processing core configured to act as a requesting agent. Dual-core processor 1710 is provided strictly by way of example, and is intended to be non-limiting. Furthermore, the hierarchical structure for shared interconnect fabric 1232 as disclosed in
Dual-core CPU 1710 communicatively couples to shared fabric element SF01720, which is a top layer of shared interconnect fabric 1232. SF01720 in this example includes system agent 730, which more generally may be treated as a species of decoder. In one example system agent 730 is configured to sufficiently decode a packet to determine whether it should be directed to display 1740 (a species of auxiliary peripheral device 1210-1 of
Because display 1740 is inherently an auxiliary device, any traffic directed toward display 1740 is assigned to VC_AUX/TC0.
Traffic assigned to SF11720 can be either VC_AUX traffic or VC_RT traffic. Thus, to assign an appropriate TC, system agent 730 may also need to sufficiently decode the packet to determine which port on SF01720 the traffic is assigned to.
SF11720 also includes a decoder 1732, which is one example includes less functionality than system agent 730. For example, decoder 1732 need not make VC or TC assignments in certain embodiments. Decoder 1732 may sufficiently decode a packet to direct it to either “other device” 1750, which may be, for example, any suitable auxiliary peripheral device 1210-1, or to root port (RP) 1760. Traffic directed to other device 1750 may be delivered via VC_AUX, and prioritized appropriately. Traffic to RP 1760 may be delivered via VC_RT when RT mode is enabled, or via VC_AUX during enumeration.
Finally, RP 1760 forwards the packet to RT device 1770, which may be, for example, any suitable RT peripheral device 1210-0.
In an embodiment with two-level shared interconnect fabric hierarchy, downstream transactions may flow as follows:
In one embodiment, channel map registers may be used in shared uncore fabric 1230 to steer traffic correctly.
In some cases, transactions from the RT devices may be statically mapped to VC_RT (TC1).
A PCIe IP block may be configured to include TC preservation and propagation from request to response to aid in routing.
Upstream packets may also be handled in a similar, but reversed fashion, for example according to method 1600 of
Embodiments of the present Specification provide advantages over certain embodiments that provide only a single VC for downstream traffic, some of which do not provide end-to-end QoS for real-time applications.
Advantages may be maximized by judicious use of preemptive priority schemes. In particular, overuse of preemption may result in conditions where best effort devices such as auxiliary agent 115-1 are completely swamped so that they are unable to perform any useful work at all. Thus, in certain embodiments, preemptive priority is reserved only for traffic that has a strict guaranteed QoS requirement that cannot otherwise be satisfied. In designing a particular embodiment of the present specification, a practitioner in the art will exercise reasonable and judicious skill to assign a priority scheme that works for the particular embodiment.
While the present invention has been described with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this present invention.
A design may go through various stages, from creation to simulation to fabrication. Data representing a design may represent the design in a number of manners. First, as is useful in simulations, the hardware may be represented using a hardware description language (HDL) or another functional description language. Additionally, a circuit level model with logic and/or transistor gates may be produced at some stages of the design process. Furthermore, most designs, at some stage, reach a level of data representing the physical placement of various devices in the hardware model. In the case where conventional semiconductor fabrication techniques are used, the data representing the hardware model may be the data specifying the presence or absence of various features on different mask layers for masks used to produce the integrated circuit.
In some implementations, software based hardware models, and HDL and other functional description language objects can include register transfer language (RTL) files, among other examples. Such objects can be machine-parsable such that a design tool can accept the HDL object (or model), parse the HDL object for attributes of the described hardware, and determine a physical circuit and/or on-chip layout from the object. The output of the design tool can be used to manufacture the physical device. For instance, a design tool can determine configurations of various hardware and/or firmware elements from the HDL object, such as bus widths, registers (including sizes and types), memory blocks, physical link paths, fabric topologies, among other attributes that would be implemented in order to realize the system modeled in the HDL object. Design tools can include tools for determining the topology and fabric configurations of system on chip (SoC) and other hardware device. In some instances, the HDL object can be used as the basis for developing models and design files that can be used by manufacturing equipment to manufacture the described hardware. Indeed, an HDL object itself can be provided as an input to manufacturing system software to cause the described hardware.
In any representation of the design, the data may be stored in any form of a machine readable medium. A memory or a magnetic or optical storage such as a disc may be the machine readable medium to store information transmitted via optical or electrical wave modulated or otherwise generated to transmit such information. When an electrical carrier wave indicating or carrying the code or design is transmitted, to the extent that copying, buffering, or re-transmission of the electrical signal is performed, a new copy is made. Thus, a communication provider or a network provider may store on a tangible, machine-readable medium, at least temporarily, an article, such as information encoded into a carrier wave, embodying techniques of embodiments of the present disclosure.
A module as used herein refers to any combination of hardware, software, and/or firmware. As an example, a module includes hardware, such as a micro-controller, associated with a non-transitory medium to store code adapted to be executed by the micro-controller. Therefore, reference to a module, in one embodiment, refers to the hardware, which is specifically configured to recognize and/or execute the code to be held on a non-transitory medium. Furthermore, in another embodiment, use of a module refers to the non-transitory medium including the code, which is specifically adapted to be executed by the microcontroller to perform predetermined operations. And as can be inferred, in yet another embodiment, the term module (in this example) may refer to the combination of the microcontroller and the non-transitory medium. Often module boundaries that are illustrated as separate commonly vary and potentially overlap. For example, a first and a second module may share hardware, software, firmware, or a combination thereof, while potentially retaining some independent hardware, software, or firmware. In one embodiment, use of the term logic includes hardware, such as transistors, registers, or other hardware, such as programmable logic devices.
Use of the phrase ‘to’ or ‘configured to,’ in one embodiment, refers to arranging, putting together, manufacturing, offering to sell, importing and/or designing an apparatus, hardware, logic, or element to perform a designated or determined task. In this example, an apparatus or element thereof that is not operating is still ‘configured to’ perform a designated task if it is designed, coupled, and/or interconnected to perform said designated task. As a purely illustrative example, a logic gate may provide a 0 or a 1 during operation. But a logic gate ‘configured to’ provide an enable signal to a clock does not include every potential logic gate that may provide a 1 or 0. Instead, the logic gate is one coupled in some manner that during operation the 1 or 0 output is to enable the clock. Note once again that use of the term ‘configured to’ does not require operation, but instead focus on the latent state of an apparatus, hardware, and/or element, where in the latent state the apparatus, hardware, and/or element is designed to perform a particular task when the apparatus, hardware, and/or element is operating.
Furthermore, use of the phrases ‘capable of/to,’ and or ‘operable to,’ in one embodiment, refers to some apparatus, logic, hardware, and/or element designed in such a way to enable use of the apparatus, logic, hardware, and/or element in a specified manner. Note as above that use of to, capable to, or operable to, in one embodiment, refers to the latent state of an apparatus, logic, hardware, and/or element, where the apparatus, logic, hardware, and/or element is not operating but is designed in such a manner to enable use of an apparatus in a specified manner.
A value, as used herein, includes any known representation of a number, a state, a logical state, or a binary logical state. Often, the use of logic levels, logic values, or logical values is also referred to as 1's and 0's, which simply represents binary logic states. For example, a 1 refers to a high logic level and 0 refers to a low logic level. In one embodiment, a storage cell, such as a transistor or flash cell, may be capable of holding a single logical value or multiple logical values. However, other representations of values in computer systems have been used. For example the decimal number ten may also be represented as a binary value of 1010 and a hexadecimal letter A. Therefore, a value includes any representation of information capable of being held in a computer system.
Moreover, states may be represented by values or portions of values. As an example, a first value, such as a logical one, may represent a default or initial state, while a second value, such as a logical zero, may represent a non-default state. In addition, the terms reset and set, in one embodiment, refer to a default and an updated value or state, respectively. For example, a default value potentially includes a high logical value, i.e. reset, while an updated value potentially includes a low logical value, i.e. set. Note that any combination of values may be utilized to represent any number of states.
The embodiments of methods, hardware, software, firmware or code set forth above may be implemented via instructions or code stored on a machine-accessible, machine readable, computer accessible, or computer readable medium which are executable by a processing element. A non-transitory machine-accessible/readable medium includes any mechanism that provides (i.e., stores and/or transmits) information in a form readable by a machine, such as a computer or electronic system. For example, a non-transitory machine-accessible medium includes random-access memory (RAM), such as static RAM (SRAM) or dynamic RAM (DRAM); ROM; magnetic or optical storage medium; flash memory devices; electrical storage devices; optical storage devices; acoustical storage devices; other form of storage devices for holding information received from transitory (propagated) signals (e.g., carrier waves, infrared signals, digital signals); etc, which are to be distinguished from the non-transitory mediums that may receive information there from.
Instructions used to program logic to perform embodiments of the invention may be stored within a memory in the system, such as DRAM, cache, flash memory, or other storage. Furthermore, the instructions can be distributed via a network or by way of other computer readable media. Thus a machine-readable medium may include any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computer), but is not limited to, floppy diskettes, optical disks, Compact Disc, Read-Only Memory (CD-ROMs), and magneto-optical disks, Read-Only Memory (ROMs), Random Access Memory (RAM), Erasable Programmable Read-Only Memory (EPROM), Electrically Erasable Programmable Read-Only Memory (EEPROM), magnetic or optical cards, flash memory, or a tangible, machine-readable storage used in the transmission of information over the Internet via electrical, optical, acoustical or other forms of propagated signals (e.g., carrier waves, infrared signals, digital signals, etc.). Accordingly, the computer-readable medium includes any type of tangible machine-readable medium suitable for storing or transmitting electronic instructions or information in a form readable by a machine (e.g., a computer)
The following examples pertain to embodiments in accordance with this Specification. One or more embodiments may provide an apparatus, a system, a machine readable storage, a machine readable medium, hardware- and/or software-based logic, and a method to receive a first packet from a first agent over a shared fabric that communicatively couples at least the first agent and a second agent to one or more terminals, assign the first packet to a first virtual channel within the shared fabric, receive a second packet from the second agent directed to one of the one or more terminals, and assign the second packet to a second virtual channel within the shared fabric.
In at least one example, the first virtual channel is a real-time virtual channel (VC_RT) and the second virtual channel is an auxiliary virtual channel (VC_AUX).
In at least one example, a prioritizer assigns VC_RT a first priority and VC_AUX a second priority.
In at least one example, the prioritizer is to assign VC_RT an expedited priority.
In at least one example, assigning VC_RT an expedited priority includes assigning an expired deadline or a higher-than-normal grant count.
In at least one example, a decoder is to assign the packet to a traffic class, where the traffic class corresponds exactly to a virtual channel.
In at least one example, the decoder is further to receive a packet from a data terminal directed to an agent, decode the traffic class from the packet, and assign the packet to the virtual channel that corresponds exactly to the traffic class.
In at least one example, the one or more data terminals include a peripheral device, and where the shared fabric includes a shared interconnect fabric to communicatively couple the one or more agents to the peripheral device.
In at least one example, the one or more data terminals include a memory, and where the shared fabric includes a shared memory fabric to communicatively couple the one or more agents to the memory.
One or more embodiments may provide a system on chip that includes a first agent including a first processing core, a second agent including a second processing core, at least one data terminal, and a shared fabric to communicatively couple the first agent and the second agent to the at least one data terminal. The shared fabric can include a decoder to receive a first packet from the first agent directed to the at least one data terminal, assign the first packet to a first virtual channels within the shared fabric, receive a second packet from the second agent directed to the same or a different data terminal, and assign the second packet to a second virtual channel within the shared fabric.
Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.
In the foregoing specification, a detailed description has been given with reference to specific exemplary embodiments. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention as set forth in the appended claims. The specification and drawings are, accordingly, to be regarded in an illustrative sense rather than a restrictive sense. Furthermore, the foregoing use of embodiment and other exemplarily language does not necessarily refer to the same embodiment or the same example, but may refer to different and distinct embodiments, as well as potentially the same embodiment.
