Patent Application
20150254182
Significant advances have been achieved in microprocessor technology. Such advances have been driven by a consistently increasing demand for processing power and speed in communications networks, computer devices, handheld devices, and other electronic devices. The achieved advances have resulted in substantial increase in processing speed, or power, and on-chip memory capacity of processor devices existing in the market. Other results of the achieved advances include reduction in the size and power consumption of microprocessor chips.
Increase in processing power has been achieved by increasing the number of transistors in a microprocessor chip, adopting multi-core structure, as well as other improvements in processor architecture. The increase in processing power has been an important factor contributing to improved performance of communication networks, as well as to the huge burst in smart handheld devices and related applications.
According to at least one example embodiment, a chip device architecture includes an inter-chip interconnect interface configured to enable efficient and reliable cross-chip communications in a multi-chip system. The inter-chip interconnect interface, together with processes and protocols employed by the chip devices in the multi-chip, or multi-node, system, allow resources' sharing between the chip devices within the multi-node system.
According to at least one example embodiment, a method of data coherence is employed within the multi-chip system, enforcing cache coherence between chip devices of the multi-node system. According to at least one example embodiment, a method of data coherence is employed within a multi-chip system to enforce cache coherence between chip devices of the multi-node system. According at least one example embodiment, a message is received by a first chip device of the multiple chip devices from a second chip device of the multiple chip devices. The message triggers invalidation of one or more copies, if any, of a data block. The data block stored in a memory attached to, or residing in, the first chip device. Upon determining that one or more remote copies of the data block are stored in one or more other chip devices, other than the first chip device, the first chip device sends one or more invalidation requests to the one or more other chip devices for invalidating the one or more remote copies of the data block.
The foregoing will be apparent from the following more particular description of example embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating embodiments of the present invention.
A description of example embodiments of the invention follows.
Many existing networking processor devices, such as OCTEON devices by Cavium Inc., include multiple central processing unit (CPU) cores, e.g., up to 32 cores. The underlying architecture enables each core processor in a corresponding multi-core chip to access all dynamic random-access memory (DRAM) directly attached to the multi-core chip. Also, each core processor is enabled to initiate transactions on any input/output (I/O) device in the multi-core chip. As such, each multi-core chip may be viewed as a standalone system whose scale is limited only by the capabilities of the single multi-core chip.
Multi-core chips usually provide higher performance with relatively lower power consumption compared to multiple single-core chips. In parallelizable applications, the use of a multi-core chip instead of a single-core chip leads to significant gain in performance. In particular, speedup factors may range from one to the number of cores in the multi-core chip depending on how parallelizable the applications are. In communications networks, many of the typical processing tasks performed at a network node are executable in parallel, which makes the use of multi-core chips in network devices suitable and advantageous.
The complexity and bandwidth of many communication networks have been continuously increasing with increasing demand for data connectivity, network-based applications, and access to Internet. Since increasing processor frequency has run its course, the number of cores in multi-core networking chips has been increasing in recent years to accommodate demand for more processing power within network elements such as routers, switches, servers, and/or the like. However, as the number of cores increases within a chip, managing access to corresponding on-chip memory as well as corresponding attached memory becomes more and more challenging. For example, when multiple cores attempt to access a memory component simultaneously, the speed of processing the corresponding access operations is constrained by the capacity and speed of the bus through which memory access is handled. Furthermore, implementing memory coherency within the chip gets more challenging as the number of cores increases.
According to at least one example embodiment, a new processor architecture, for a new generation of processors, allows a group of chip devices to operate as a single chip device. Each chip device includes an inter-chip interconnect interface configured to couple the chip device to other chip devices forming a multi-chip system. Memory coherence methods are employed in each chip device to enforce memory coherence between memory components associated with different chip devices in the multi-chip system. Also, methods for assigning processing tasks to different core processors in the multi-chip system, and methods for allocating cache blocks to chip devices within the multi-chip system, are employed within the chip devices enabling the multi-chip system to operate like a single chip. Furthermore, methods for synchronizing access, by cores in the multi-chip system, to input/output (I/O) devices are used to enforce efficient and conflict-free access to (I/O) devices in the multi-chip system.
Chip Architecture
According to at least one example embodiment, the chip device 100 includes an intra-chip interconnect interface 120 configured to couple the core processors and the shared cache memory 110, or the TADs, to each other through a plurality of communications buses. The intra-chip interconnect interface 120 is used as a communications interface to implement memory coherence within the chip device 100. As such, the intra-chip interconnect interface 120 may also be referred to as a memory coherence interconnect interface. According to at least one aspect, the intra-chip interconnect interface 120 has a cross-bar (xbar) structure.
According to at least one example embodiment, the chip device 100 further includes one or more coprocessors 150. A coprocessor 150 includes an I/O device, a compression/decompression processor, a hardware accelerator, a peripheral component interconnect express (PCIe), or the like. The core processors 150 are coupled to the intra-chip interconnect interface 120 through I/O bridges (IOBs) 140. As such, the coprocessors 150 are coupled to the core processors and the shared memory cache 110, or TADs, through the IOBs 140 and the intra-chip interconnect interface 110. According to at least one aspect, coprocessors 150 are configured to store data in, or load data from, the shared cache memory 110, or the TADs. The coprocessors 150 are also configured to send, or assign, processing tasks to core processors in the chip device 100, or receive data or processing tasks from other components of the chip device 100.
According to at least one example embodiment, the chip device 100 includes an inter-chip interconnect interface 130 configured to couple the chip device 100 to other chip devices. In other words, the chip device 100 is configured to exchange data and processing tasks/jobs with other chip devices through the inter-chip interconnect interface 130. According to at least one aspect, the inter-chip interconnect interface 130 is coupled to the core processors and the shared cache memory 110, or the TADs, in the chip device 100 through the intra-chip interconnect interface 120. The coprocessors 150 are coupled to the inter-chip interconnect interface 130 through the IOBs 140 and the intra-chip interconnect interface 120. The inter-chip interconnect interface 130 enables the core processors and the coprocessors 150 of the chip device 100 to communicate with other core processors or other coprocessors in other chip devices as if they were in the same chip device 100. Also, the core processors and the coprocessors 150 in the chip device 100 are enabled to access memory in, or attached to, other chip devices as if the memory was in, or attached to the chip device 100.
Intra-Chip Interconnect Interface
According to at least one aspect, the communications bus 210 includes five different channels; an invalidation channel 211, add channel 212, store channel 213, commit channel 214, and fill channel 215. The invalidation channel 211 is configured to carry invalidation requests, for invalidating cache blocks, from the shared cache memory controller 115 to one or more of the core processors 201 in the cluster 105. For example, the invalidation channel is configured to carry broad-cast and/or multi-cast data invalidation messages/instructions from the TADs to the core processors 201 of the cluster 105. The add channel 212 is configured to carry address and control information, from the core processors 201 to other components of the chip device 100, for initiating or executing memory and/or I/O transactions. The store channel 213 is configured to carry data associated with write operations. That is, in storing data in the shared cache memory 110 or an external memory, e.g., DRAM, a core processor 201 sends the data to the shared cache memory 110, or the corresponding controller 115, over the store channel 213. The fill channel 215 is configured to carry response data to the core processors 201 of the cluster 105 from other components of the chip device 100. The commit channel 214 is configured to carry response control information to the core processors 201 of the cluster 105. According to at least one aspect, the store channel 213 has a capacity of transferring a memory line, e.g., 128 bits, per clock cycle and the fill channel 215 has a capacity of 256 bits per clock cycle.
According to at least one example embodiment, the intra-chip interconnect interface 120 includes a separate communications bus 210, e.g., with the invalidation 211, add 212, store 213, commit 214, and fill 215 channels, for each cluster 105 of core processors 201. Considering the example architecture in
According to at least one example embodiment, each communications bus 320 includes multiple communications channels. The multiple channels are coupled to the coprocessors 150 through the corresponding IOBs 140, and are configured to carry data between the coprocessors 150 and shared cache memory 110 and/or the corresponding controller 115. The multiple communications channels of the communications bus 320 include an add channel 322, store channel 323, commit channel 324, and a fill channel 325 similar to those in the communications bus 210. For example, the add channel 322 is configured to carry address and control information, from the coprocessors 150 to the shared cache memory controller 115, for initiating or executing operations. The store channel 323 is configured to carry data associated with write operations from the coprocessors 150 to the shared cache memory 110 and/or the corresponding controller 115. The fill channel 325 is configured to carry response data to the coprocessors 150 from the shared cache memory 110, e.g., TADs, or the corresponding controller 115. The commit channel 324 is configured to carry response control information to the coprocessors 150. According to at least one aspect, the store channel 323 has a capacity of transferring a memory line, e.g., 128 bits, per clock cycle and the fill channel 325 has a capacity of 256 bits per clock cycle.
According to at least one aspect, the communications bus 320 further includes an input/output command (IOC) channel 326 configured to transfer I/O data and store requests from core processors 201 in the chip device 100, and/or other core processors in one or more other chip devices coupled to the chip device 100 through the inter-chip interconnect interface 130, to the coprocessors 150 through corresponding IOB(s) 140. The communications bus 320 also includes an input/output response (IOR) channel 327 to transfer I/O response data, from the coprocessors 150 through corresponding IOB(s) 140, to core processors 201 in the chip device 100, and/or other core processors in one or more other chip devices coupled to the chip device 100 through the inter-chip interconnect interface 130. As such, the IOC channel 326 and the IOR channel 327 provide communication media between the coprocessors 150 in the chip device 100 and core processors in the chip device 100 as well as other core processors in other chip device(s) coupled to the chip device 100. Also, the communications bus 320 includes a multi-chip input coprocessor MIC channel 328 and a multi-chip output coprocessor (MOC) channel configured to provide an inter-chip coprocessor-to-coprocessor communication media. In particular, the MIC channel 328 is configured to carry data, from coprocessors in other chip device(s) coupled to the chip device 100 through the inter-chip interconnect interface 130, to the coprocessors 150 in the chip device 100. The MOC channel 329 is configured to carry data from coprocessors 150 in the chip device 100 to coprocessors in other chip device(s) coupled to the chip device 100 through the inter-chip interconnect interface 130.
Inter-Chip Interconnect Interface
According to at least one example embodiment, the inter-chip interconnect interface 130 provides a one-to-one communication media between each pair of chip devices in a multi-chip system. According to at least one aspect, each chip device includes a corresponding inter-chip interconnect interface 130 configured to manage flow of communication data and instructions between the chip device and other chip devices.
According to at least one example embodiment, the inter-chip interconnect interface 130 is also coupled to the intra-chip interconnect interface 120 through a number of multi-chip input buses (MIBs), e.g., 410a-410d, and a number of multi-chip output buses (MOBs), e.g., 420a-420b. According to at least one aspect, the MIBs, e.g., 410a-410d, and MOBs, e.g., 420a-420d, are configured to carry communication data and instructions other than those carried by the MIC and MOC channels, 328 and 329. According to at least one aspect, the MIBs, e.g., 410a-410d, carry instructions and data, other than instructions and data between the coprocessors 150 and coprocessors on other chip devices, received from another chip device and destined to the core processors 201, the shared cache memory 110 or the corresponding controller 115, and/or the IOBs 140. The MOBs carry instructions and data, other than instructions and data between the coprocessors on other chip devices and the coprocessors 150, sent from the core processors 201, the shared cache memory 110 or the corresponding controller 115, and/or the IOBs 140 and destined to the other chip device(s). The MIC and MOC channels, 328 and 329, however, carry commands and data related to forwarding processing tasks or memory allocation between coprocessors in different chip devices. According to at least one aspect, the transmission capacity of each MIB, e.g., 410a-410d, or MOB, e.g., 420a-420d, is a memory data line, e.g., 128 bits, per clock cycle. A person skilled in the art should appreciate that the capacity of the MIB, e.g., 410a-410d, MOB, e.g., 420a-420d, MIC 328, MOC 329, or any other communication channel or bus may be designed differently and that any transmission capacity values provided herein are for illustration purposes and re not to be interpreted as limiting features.
According to at least one example embodiment, the inter-chip Interconnect interface 130 is configured to forward instructions and data received over the MOBs, e.g., 420a-420d, and the MOC channel 329 to appropriate other chip device(s), and to route instructions and data received from other chip devices through the MIBs, e.g., 410a-410d, and the MIC channel 328 to destination components in the chip device 100. According to at least one aspect, the inter-chip interconnect interface 130 includes a controller 435, a buffer 437, and a plurality of serializer/deserializer (SerDes) units 439. For example, with 24 SerDes units 439, the inter-chip interconnect interface 130 has a bandwidth of up to 300 Giga symbols per second (Gbaud). According to at least one aspect, the inter-chip interconnect interface bandwidth, or the SerDes units 439, is/are flexibly distributed among separate links coupling the chip device 100 to other chip devices. Each links is associated with one or more I/O ports. For example, in a case where the chip device 100 is part of a multi-chip system having four chip devices, the inter-chip interconnect interface 130 has three full-duplex links—one per each of the three other chip devices—each with bandwidth of 100 Gbaud. Alternatively, the bandwidth may not be distributed equally between the three links. In another case where the chip device 100 is part of a multi-chip system having two chip devices, the inter-chip interconnect interface 130 has one full-duplex link with bandwidth equal to 300 Gbaud.
The controller 435 is configured to exchange messages with the core processors 201 and the shared cache memory controller 115. The controller 435 is also configured to classify outgoing data messages by channels, form data blocks comprising such data messages, and transmit the data blocks via the output ports. The controller 435 is also configured to communicate with similar controller(s) in other chip devices of a multi-chip system. Transmitted data blocks may also be stored in the retry buffer 437 until receipt of the data block is acknowledged by the receiving chip device. The controller 435 is also configured to classify incoming data messages, forms blocks of such incoming messages, and route the formed blocks to proper communication buses or channels.
TAD Structure
Each quad unit 520 includes a number of fill buffers 521, number of store buffers 523, data array 525, and number of victim buffers 527. According to at least one aspect, the fill buffers 521 are configured to store response data, associated with corresponding requests, for sending to one or more core processors 201 over a fill channel 215 coupled to the TAD 500. The fill buffers 521 are also configured to receive data through a store channel 213 or MIB 410, coupled to the TAD 500. Data is received through a MIB 410 at the fill buffers 521, for example, if response data to a request resides in another chip device. The fill buffers 521 also receive data from the data array 525 or from the main memory, e.g., DRAM, attached to the chip device 100 through a corresponding LM 117. According to at least one aspect, the victim buffers 527 are configured to store cache blocks that are replaced with other cache blocks in the data array 525.
The store buffers 523 are configured to maintain data for storing in the data array 525. The store buffers 523 are also configured to receive data from the store channel 213 or the MIB 410 coupled to the TAD 500. Data is received over MIB 410 if the data to be stored is sent from a remote chip device. The data arrays 525 in the different quad units 520 are the basic memory components of the shared cache memory 110. For example, the data arrays 525 associated with a quad group 501 have a cumulative storage capacity of 1 Mega Byte (MB). As such, each TAD has a storage capacity of 2 MB while the shared cache memory 110 has storage capacity of 16 MB.
A person skilled in the art should appreciate that in terms of the architecture of the chip device 100, the number of the core processors 201, the number of clusters 105, the number of TADs, the storage capacity of the shared cache memory 110, and the bandwidth of the inter-chip interconnect interface 130 are to be viewed as design parameters that may be set, for example, by a manufacturer or buyer of the chip device 100.
Multi-Chip Architecture
The architecture of the chip device 100 in general and the inter-chip interconnect interface 130 in particular allow multiple chip devices to be coupled to each other and to operate as a single system with computational and memory capacities much larger than that of the single chip device 100. Specifically, the inter-chip interconnect interface 130 together with a corresponding inter-chip interconnect interface protocol, defining a set of messages for use in communications between different nodes, allow transparent sharing of resources among chip devices, also referred to as nodes, within a multi-chip, or multi-node, system.
Besides the inter-chip interconnect interface 130 and the point-to-point connection between pairs of nodes in a multi-node system, an inter-chip interconnect interface protocol defines a set of messages configured to enable inter-node memory coherence, inter-node resource sharing, and cross-node access of hardware components associated with the nodes. According to at least one aspect, memory coherence methods, methods for queuing and synchronizing work items, and methods of accessing node components are implemented within chip devices to enhance operations within a corresponding multi-node system. In particular, methods and techniques described below are designed to enhance processing speed of operations and avoid conflict situations between hardware components in the multi-node system. As such, techniques and procedures that are typically implemented within a single chip device, as part of carrying out processing operations, are extended in hardware to multiple chip devices or nodes.
A person skilled in the art should appreciate that the chip device architecture described above provides new system scalability options via the inter-chip interconnect interface 130. To a large extent, the inter-chip interconnect interface 130 allows multiple chip devices to act as one coherent system. For example, forming a four-node system using chip devices having 48 core processors 201, up to 256 GB of DRAM, SerDes-based I/O capability of up to 400 Gbaud full duplex, and various coprocessors, the corresponding four-node system scales up to 192 core processors, one Tera Byte (TB) of DRAM, 1.6 Tera baud (Tbaud) I/O capability, and four times the coprocessors. The core processors, within the four-node system, are configured to access all DRAM, I/O devices, coprocessors, etc., therefore, the four-node system operates like a single node system with four times the capabilities of a single chip device.
Work Scheduling and Memory Allocation
The hardware capabilities of the multi-node system 600 are multiple times the hardware capabilities of each chip device in the multi-node system 600. However, in order for the increase in hardware capacities, in the multi-node system 600 compared to single chip devices, to reflect positively on the performance of the multi-node system 600, methods and techniques for handling processing operations in a way that takes into account the multi-node architecture are employed in chip devices within the multi-node system 600. In particular, methods for queuing, scheduling, synchronization, and ordering of work items that allow distribution of work load among core processors in different chip devices of the multi-node system 600 are employed.
According to at least one example embodiment, the chip device 100 includes hardware features that enable support of work queuing, scheduling, synchronization, and ordering. Such hardware features include a schedule/synchronize/order (SSO) unit, free pool allocator (FPA) unit, packet input (PKI) unit, and packet output (PKO) unit, which provide together a framework enabling efficient work items' distribution and scheduling. Generally, a work item is a software routine or handler to be performed on some data.
A work item may be created by either hardware units, e.g., PKI unit 710, PKO unit 740, PCIe, etc., or a software running on a core processor 201. For example, upon receiving a data packet (1), the PKI unit 710a scans the data packet received and determines a processing operation, or work item, to be performed on the data packet. Specifically, the PKI unit 710a creates a work-queue entry (WQE) representing the work item to be performed. According to at least one aspect, the WQE includes a work-queue pointer (WQP), indication of a group, or queue, a tag type, and a tag. Alternatively, the WQE may be created by a software, for example, running in one of the core processors 201 in the multi-chip system 600, and a corresponding pointer, WQP, is passed to a coprocessor 150 acting as a work source.
The WQP points to a memory location where the WQE is stored. Specifically, at (2), the PKI unit 710a requests a free-buffer pointer from the FPA unit 720a, and stores (3) the WQE in the buffer indicated by the pointer returned by the FPA unit 720a. The buffer may be a memory location in the shared cache memory 110a or the external memory 790a. According to at least one aspect, every FPA unit 720 is configured to maintain a number, e.g., K, of pools of free-buffer pointers. As such, core processors 201 and coprocessors 150 may allocate a buffer by requesting a pointer from the FPA unit 720 or free a buffer by returning a pointer to the FPA unit 720. Upon requesting and receiving a pointer from the FPA unit 720a, the PKI unit 710a stores (3) the WQE created in the buffer indicated by the received pointer. The pointer received from the FPA unit 720a is the WQP used to point to the buffer, or memory location, where the WQE is stored. The WQE is then (4) designated by the PKI unit 710a to an SSO unit, e.g., 730a, within the multi-node system 600. Specifically, the WQP is submitted to a group, or queue, among multiple groups, or queues, of the SSO unit 730a.
According to at least one example embodiment, each SSO 730 in the multi-node system 600 schedules work items using multiple groups, e.g., L groups, with work on one group flows independently from work on all other groups. Groups, or queues, provide a means to execute different functions on different core processors 201 and provide quality of service (QoS) even though multiple core processors share the same SSO unit 730a. For example, packet processing may be pipelined from a first group of core processors to a second group of core processors, with the first group performing a first stage of work and the second group performing a next stage of work. According to at least one aspect, the SSO unit 730 is configured to implement static priorities and group-affinity arbitration between these groups. The use of multiple groups in a SSO unit 730 allows the SSO 730 to schedule work item in parallel whenever possible. According to at least one aspect, each work source, e.g., PKI unit 710, core processors 201, PCIe, etc., enabled to create work items is configured to maintain a list of the groups, or queues, available in all SSO units of the multi-node system 600. As such, each work source makes use of the maintained list to designate work items to groups in the SSO units 730.
According to at least one example embodiment, each group in a SSO unit 730 is identified through a corresponding identifier. Assume that there are n SSO units 730 in the multi-node system 600, with, for example, one SSO unit 730 in each node 100, and L groups in each SSO unit 730. In order to uniquely identify all the groups, or queues, within all the SSO units 730, each group identifier includes at least log2 (n) bits to identify the SSO unit 730 associated with group and at least log2 (L) bits to identify the group within the corresponding SSO unit 730. For example, if there are four nodes each with a single SSO unit 730 having 254 groups, each group may be identified using a 10-bit identifier with two bits identifying the SSO unit 730 associated with the group and eight other bits to distinguish between groups within the same SSO unit 730.
After receiving the WQP at (4), the SSO unit 730a is configured to assign the work item to a core processor 201 for handling. In particular, core processors 201 request work from the SSO unit 730a and the SSO unit 730a responds by assigning the work item to one of the core processors 201. In particular, the SSO unit 730 is configured to respond back with a WQP pointing to the WQE associated with the work item. The SSO unit 730a may assign the work item to a processor core 201a in the same node 100a as illustrated by (5). Alternatively, the SSO unit 730a may assign the work item to a core processor, e.g., 201b, in a remote node, e.g., 100b, as illustrated in (5″). According to at least one aspect, each SSO unit 730 is configured to assign a work item to any core processor 201 in the multi-node system 600. According to yet another aspect, each SSO unit 730 is configured to assign work items only to core processors 201 on the same node 100 as the SSO unit 730.
A person skilled in the art should appreciate that a single SSO unit 730 may be used to schedule work in the multi-node system 600. In such case, all work items are sent the single SSO unit 730 and all core processors 201 in the multi-node system 600 request and get assigned work from the same single SSO unit 730. Alternatively, multiple SSO units 730 are employed in the multi-node system 600, e.g., one SSO unit 730 in each node 100 or only a subset of nodes 100 having one SSO unit 730 per node 100. In such case, the multiple SSO units 730 are configured to operate independently and no synchronization is performed between the different SSO units 730. Also, different groups, or queues, of the SSO units 730 operate independent of each other. In the case where each node 100 includes a corresponding SSO unit 730, each SSO unit may be configured to assign work items only to core processors 201 in the same node 100. Alternatively, each SSO unit 730 may assign work items to any core processor in the multi-node system 600.
According to at least one aspect, the SSO unit 730 is configured to assign work items associated with the same work flow, e.g., same communication session, same user, same destination point, or the like, to core processors in the same node. The SSO unit 730 may be further configured to assign work items associated with the same work flow to a subset of core processors 201 in the same node 100. That is, even within a given node 100, the SSO unit 730 may designate work items associated with a given work flow, and/or a given processing stage, to a first subset of core processors 201, while work items associated with a different work flow, or a different processing stage of the same work flow, to a second subset of core processors 201 in the same node 100. According to yet another aspect, the first subset of core processors and the second subset of core processors are associated with different nodes 100 of the multi-node system 600.
Assuming multi-stage processing operations are associated with the data packet, once a core processor 201 is selected to handle a first-stage work item, as shown in (5) or (5″), the selected processor processes the first-stage work item and then creates a new work item, e.g., a second-stage work item, and the corresponding pointer is sent to a second group, or queue, different than the first group, or queue, to which the first-stage work item was submitted. The second group, or queue, may be associated with the same SSO unit 730 as indicated by (5). Alternatively, the core processor 201 handling the first-stage work item may schedule the second-stage work item on a different SSO unit 730 than the one used to schedule the first-stage work item. The use of multiple groups, or queues, that handle corresponding working items independent of each other enables work ordering with no synchronization performed between distinct groups or SSO units 730.
At (6), the second-stage work item is assigned to a second core processor 201a in node 100a. The second core processor 201a processes the work item and then submits it to the PKO unit 740a, as indicated by (7), for example, if all work items associated with the data packet are performed. The PKO unit, e.g., 740a or 740b, is configured to read the data packet from memory and send it off the chip device (see (8) and (8′)). Specifically, the PKO unit, e.g., 740a or 740b, receives a pointer to the data packet from a core processor 201, and use the pointer to retrieve the data packet from memory. The PKO unit, e.g., 740a or 740b, may also free the buffer where the data packet was stored in memory by returning the pointer to the FPA unit, e.g., 720a or 720b.
A person skilled in the art should appreciate that memory allocation and work scheduling may be viewed as two separate processes. Memory allocation may be performed by, for example, a PKI unit 710, core processor 201, or another hardware component of the multi-node system 600. A component performing memory allocation is referred to as a memory allocator. According to at least one aspect, each memory allocator maintains a list of the pools of free-buffer pointers available in all FPA units 720 of the multi-node system 600. Assume there are m FPA units 720 in the multi-node system 600, each having K pools of free-buffer pointers. In order to uniquely identify all the pools within all the FPA units 720, each pool identifier includes at least log2 (m) bits to identify the FPA unit 720 associated with the pool and at least log2 (K) bits to identify pools within a given corresponding FPA unit 720. For example, if there are four nodes each with a single FPA unit 720 having 64 pools, each pool may be identified using an eight-bit identifier with two bits identifying the FPA unit 720 associated with the pool and six other bits to distinguish between pools within the same FPA unit 720.
According to at least one example embodiment, the memory allocator sends a request for a free-buffer pointer to a FPA unit 720 and receives a free-buffer pointer in response, as indicated by (2). According to at least one aspect, the request includes an indication of a pool from which the free-buffer pointer is to be selected. The memory allocator is aware of associations between pools of free-buffer pointers and corresponding FPA units 720. By receiving a free-buffer pointer from the FPA unit 720, the corresponding buffer, or memory location, pointed to by the pointer is not free anymore, but is rather allocated. That is, memory allocation may be considered completed upon receipt of the pointer by the memory allocator. The same buffer, or memory location, is freed later, by the memory allocator or another component such as the PKO unit 740, when the pointer is returned back to the FPA unit 720.
When scheduling a work item, a work source, e.g., a PKI unit 710, core processor 201, PCIe, etc., may be configured to schedule work items only through a local SSO unit 730, e.g., a SSO unit residing in the same node 100 as the work source. In such case, if the group, or queue, selected by the work source does not belong to the local SSO unit 720, the pointer is forwarded to a remote SSO unit, e.g., not residing in the same node 100 as the work source, associated with the selected group and the work item is then assigned by the remote SSO unit 720, as indicated by (4′). Once the forwarding of the WQE pointer is done in (4′), the operations indicated by (5)-(9) may be replaced with similar operations in the remote node indicated by (5′)-(9′).
A person skilled in the art should appreciate that memory allocation within the multi-node system may be implemented according to different embodiments. First, the free-buffer pools associated with each FPA unit 720 may be configured in way that each FPA unit 720 maintains a list of pools corresponding to buffers, or memory locations, associated with same node 100 as the FPA unit 720. That is, the pointers in pools associated with a given FPA unit 720 point to buffers, or memory locations, in the shared cache memory 110 residing in the same node 100 as the FPA unit 720, or in the external memory 790 attached to same node 100 where the FPA unit 720 resides. Alternatively, the list of pools maintained by a given FPA unit 720 includes pointers pointing to buffers, or memory locations, associated with remote nodes 100, e.g., nodes 100 different from the node 100 where the FPA unit 720 resides. That is, any FPA free list may hold a pointer to any buffer from any node 100 of the multi-node system 600.
Second, a single FPA unit 720 may be employed within the multi-node system 600, in which case, all requests for free-buffer pointers are directed to the single FPA unit when allocating memory, and all pointers are returned to the single FPA unit 720 when freeing memory. Alternatively, multiple FPA units 720 are employed within the multi-node system 600. In such a case, the multiple FPA units 720 operate independently of each other with little, or no, inter-FPA-units communication employed. According to at least one aspect, each node 100 of the multi-node system 600 includes a corresponding FPA unit 720. In such case, each memory allocator is configured to allocate memory through the local FPA unit 720, e.g., the FPA unit 720 residing on the same node 100 as the memory allocator. If the pool indicated in a free-buffer pointer request from the memory allocator to the local FPA unit 720 belongs to a remote FPA unit 720, e.g., not residing in the same node 100 as the memory allocator, the free-buffer pointer request is forwarded from the local FPA unit 720 to the remote FPA unit 720, as indicated by (2′), and a response is sent back to the memory allocator through the local FPA unit 720.
The forwarding of the free-buffer pointer request is made over the MIC and MOC channels, 328 and 329, given that the forwarding is based on communications between two coprocessors associated with two different nodes 100. The use of MIC and MOC channels, 328 and 329, to forward free-buffer pointer requests between FPA units 720 residing on different nodes 100 ensures that the forwarding transactions do not add cross-channel dependencies to existing channels. Alternatively, memory allocators may be configured to allocate memory through any FPA unit 720 in the multi-node system 600.
Third, when allocating memory for data associated a work item, the memory allocator may be configured to allocate memory in the same node 100 where the work item is assigned. That is the memory is allocated in the same node where the core processor 201 handling the work item resides, or in the same node 100 as the SSO unit 730 to which the work item is scheduled. A person skilled in the art should appreciate that the work scheduling may be performed prior to memory allocation, in which case memory allocated in the same node 100 to which the work item is assigned. However, if memory allocation is performed prior to work scheduling, then the work item is assigned to the same node 100 where memory is allocated for corresponding data. Alternatively, memory to store data corresponding to a work item may be allocated to different node 100 than the one to which the work item was assigned.
A person skilled in the art should appreciate that work scheduling and memory allocation with a multi-node system, e.g., 600, may be performed according to different combinations of the embodiments described herein. Also, a person skilled in the art should appreciate that all cross-node communications, shown in
Memory Coherence in Multi-Node Systems
A multi-node system, e.g., 600, includes more core processors 201 and memory components, e.g., shared cache memories 110 and external memories 790, than the corresponding nodes, or chip devices, 100 in the same multi-node system, e.g., 600. As such, implementing memory coherence procedures within a multi-node system, e.g., 600, is more challenging than implementing such procedures within a single chip device 100. Also, implementing memory coherence globally with the multi-node system, e.g., 600, would involve cross-node communications, which raise potential delay issues as well as issues associated with addressing the hardware resources in the multi-node system, e.g., 600. Considering such challenges, an efficient and reliable memory coherence approach for multi-node systems, e.g., 600, is a significant step towards configuring the multi-node system, e.g., 600, to operate as a single node, or chip device, 100 with significantly larger resources.
For simplicity, a single core processor 201 is shown in each of the nodes 100a and 100b in
According to at least one aspect, a data block associated with a memory location within an external memory 790 coupled to a corresponding node 100, may have multiple copies residing, simultaneously, within the multi-node system 600. The corresponding node 100 coupled to the external memory 790 storing the data block is defined as the home node for the data block. For the sake of simplicity, a data block stored in the external memory 790a is considered herein. As such, the node 100a is the home node for the data block, and any other nodes, e.g., 100b, of the multi-node system 600 are remote nodes. Copies of the data block, also referred to herein as cache blocks associated with the data block, may reside in the shared cache memory 110a, or local cache memories 820a within core processors 201a, of the home node 100a. Such cache blocks are referred to as home cache blocks. Cache block(s) associated with the data block may also reside in shared cache memory, e.g., 110b, or local cache memories, e.g., 820b, within core processors, e.g., 201b, of a remote node, e.g., 100b. Such cache blocks are referred to as remote cache blocks. Memory coherence, or data coherence, aims at enforcing such copies to be up-to-date. That is, if one copy is modified at a given point of time, the other copies are invalid
According to at least one example embodiment, a memory request associated with the data block, or any corresponding cache block, is initiated, for example, by a core processor 201 or an IOB 140 of the multi-node system 160. According to at least one aspect, the IOB 140 initiates memory requests on behalf of corresponding I/O devices, or agents, 150. Herein, a memory request is a message or command associated with a data block, or any corresponding cache blocks. Such request includes, for example, a read/load operation to request a copy of the data block by a requesting node from another node. The memory request also includes a store/write operation to store the cache block, or parts of the cache block, in memory. Other examples of the memory request are listed in the Tables 1-3.
According to a first scenario, the core processor, e.g., 201a, or the IOB, e.g., 140a, initiating the memory request resides in the home node 100a. In such case, the memory request is sent from the requesting agent, e.g., core processor 201a or IOB 140, directly to the shared cache memory controller 115a of the home node 100a. If the memory request is determined to be triggering invalidations of other cache blocks, associated with the data block, the shared cache memory controller 115a of the home node 100a determines if any other cache blocks, associated with the data block, are cached within the home node 100a. An example of a memory request triggering invalidation is a store/write operation where a modified copy of the data block is to be stored in memory. Another example of a memory request triggering invalidation is a request of an exclusive copy of the data block by a requesting node. The node receiving such request causes copies of the data block residing in other chip devices, other than the requesting node, to be invalidated, and provides the requesting node with an exclusive copy of the data block (See
According to at least one aspect, the shared cache memory controller 115a of the home node 100a first checks if any other cache blocks, associated with the data block, are cached within local cache blocks 820a associated with core processors 201a or IOBs 140, other than the requesting agent, of the home node 100a. If any such cache blocks are determined to exist in core processors 201a or IOBs 140, other than the requesting agent, of the home node 100a, the shared cache memory controller 115a of the home node sends invalidations requests to invalidate such cache blocks. The shared cache memory controller 115a of the home node 100a may update a local cache block, associated with the data block, stored in the shared cache memory 110 of the home node.
According to at least one example embodiment, the shared cache memory controller 115a of the home node 100a also checks if any other cache blocks, associated with the data block, are cached in remote nodes, e.g., 100b, other than the home node 100a. If any remote node is determined to include a cache block, associated with the data block, the shared cache memory controller 115a of the home node 100a sends invalidation request(s) to remote node(s) determined to include such cache blocks. Specifically, the shared cache memory controller 115a of the home node 100a is configured to send an invalidation request to the shared cache memory controller, e.g., 115b, of a remote node, e.g., 100b, determined to include a cache block associated with the data block through the inter-chip-interconnect interface link 610. The shared cache memory controller, e.g., 115b, of the remote node, e.g., 100b, then determines locally which local agents include cache blocks, associated with the data block, and sends invalidation requests to such agents. The shared cache memory controller, e.g., 115b, of the remote node, e.g., 100b, may also invalidate any cache block, associated with the data block, stored by its corresponding shared cache memory.
According to a first scenario, the requesting agent resides in a remote node, e.g., 100b, other than the home node 100a. In such case, the request is first sent to the local shared cache memory controller, e.g., 115b, residing in the same node, e.g., 100b, as the requesting agent. The local shared cache memory controller, e.g., 115b, is configured to forward the memory request to the shared cache memory controller 115a of the home node 100a. According to at least one aspect, the local shared cache memory controller, e.g., 115b, also checks for any cache blocks associated with data block that may be cached within other agents, other than the requesting agent, of the same local node, e.g., 100b, and sends invalidation requests to invalidate such potential cache blocks. The local shared cache memory controller, e.g., 115b, may also check for, and invalidate, any cache block, associated with the data block, stored by its corresponding shared cache memory.
Upon receiving the memory request, the shared cache memory controller 115a of the home node 100a checks locally within the home node 100a for any cache blocks, associated with the data block, and sends invalidation requests to agents of the home node 100 carrying such cache blocks, if any. The shared cache memory controller 115a of the home node 100a may also invalidate any cache block, associated with the data block, stored in its corresponding shared cache memory in the home node 100a. According to at least one example embodiment, the shared cache memory controller 115a of the home node 100a is configured to check if any other remote nodes, other than the node sending the memory request, includes a cache block, associated with the data block. If another remote node is determined to include a cache block, associated with the data block, the shared cache memory controller 115a of the home node 100a sends an invalidation request to the shared cache memory controller 115 of the other remote node 100. The shared cache memory controller 115 of the other remote node 100 proceeds with invalidating any local cache blocks, associated with the data, by sending invalidation requests to corresponding local agents or by invalidating a cache block stored in the corresponding local shared cache memory.
According to at least one example embodiment, the shared cache memory controller 115a of the home node 100a includes a remote tag (RTG) buffer, or data field. The RTG data field includes information indicative of nodes 100 of the multi-node system 600 carrying a cache block associated with the data block. According to at least one aspect, cross-node cache block invalidation is managed by the shared cache memory controller 115a of the home node 100a, which upon checking the RTG data field, sends invalidation requests, through the inter-chip interconnect interface request 610, to shared cache memory controller(s) 115 of remote node(s) 100 determined to include a cache block associated with the data block. The shared cache memory controller(s) 115 of the remote node(s) 100 determined to include a cache block, associated with the data block, then handle locally invalidation of any such cache block(s).
According to at least one example embodiment, invalidation of cache block(s) within each node 100 of the multi-node system 600 is handled locally by the local shared cache memory controller 115 of the same node. According to at least one aspect, each shared cache memory controller 115, of a corresponding node 100, includes a local data field, also referred to herein as BUSINFO, indicative of agents, e.g., core processors 201 or IOBs 140, in the same corresponding node carrying a cache block associated with the data block. According to at least one aspect, the local data field operates according two different modes. As such, a first subset of bits of the local data field is designated to indicate the mode of operation of the local data field. A second subset of bits of the local data field is indicative of one or more cache blocks, if any, associated with the data block being cached within the same node 100.
According to a first mode of the local data field, each bit in the second subset of bits corresponds to a cluster 105 of core processors in the same node 100, and is indicative of whether any core processor 201 in the cluster carries a cache block associated with the data block. When operating according to the first mode, invalidation requests are sent, by the local shared cache memory controller 115, to all core processors 201 within a cluster 105 determined to include cache block(s), associated with the data block. Each core processor 201 in the cluster 105 receives the invalidation request and checks whether its corresponding local cache memory 820 includes a cache block associated with the data block. If yes, such cache block is invalidated.
According to a second mode of the local data field, the second subset of bits is indicative of a core processor 201, within the same node, carrying a cache block associated with the data block. In such case, an invalidation request may be sent only to the core processor 201, or agent, identified by the second subset of bits, and the latter invalidates the cache block, associated with the data block, stored in its local cache memory 820.
For example, considering 48 core processors in each chip device, the BUSINFO field may have 48-bit size with one bit for each core processor. Such approach is memory consuming. Instead, a 9-bit BUSINFO field is employed. By using 9 bits, one bit is used per cluster 150 plus one extra bit is used to indicate the mode as discussed above. When the 9th bit is set, the other 8 bits select one CPU core whose cache memory holds a copy of the data block. When the 9th bit is clear, each of the other 8 bits represents one of the 8 clusters 105a-105h, and are set when any core processor in the cluster may hold a copy of the data block.
According to at least one aspect, memory requests triggering invalidation of cache blocks, associated with a data block, include a message, or command, indicating that a cache block, associated with the data block, was modified, for example, by the requesting agent, message, or command, indicating a request for an exclusive copy of the data block, or the like.
A person skilled in the art should appreciate that when implementing embodiments of data coherence, described herein, the order to process checking for, and/or invalidating, local cache block versus remote cache block at the home node may be set differently according to different implementations.
Managing Access of I/O Devices in a Multi-Node System
In a multi-node system, e.g., 600, designing and implementing reliable processes for sharing of hardware resources is more challenging than designing such processes in a single chip device for many reasons. In particular, enabling reliable access to I/O devices of the multi-node system, e.g., 600, by any agent, e.g., core processors 201 and/or coprocessor 150, of the multi-node system, e.g., 600, poses a lot of challenges. First, access of an I/O device by different agents residing in different nodes 100 of the multi-node system 600 may result in simultaneous attempts to access the I/O device by different agents resulting in conflicts which may stall access to the I/O device. Second, potential synchronization of access requests by agents residing in different nodes 100 of the multi-node system 600 may result in significant delays. In the following, embodiments of a process for efficient access to I/O devices in a multi-node system, e.g., 600, are described.
According to at least one example embodiment, each node 910 of the multi-node system 900 includes one or more queues, 909a, 909b, or 909 in general, configured to order access requests to I/O devices 905 in the multi-node system 900. In the following, the node, e.g., 910b, including an I/O device, e.g., 905, which is the subject of one or more access requests, is referred to as the I/O node, e.g., 910b. Any other node, e.g., 910 of the multi-node system 900 is referred to as a remote node, e.g., 910a.
As shown in
According to at least one example embodiment, one or more queues 909 designated to manage access to a given I/O device 905 are known to agents within the multi-node system 900. When an agent initiates a first access request destined toward the given I/O device 905, other agents in the multi-node system 900 are prevented from initiating new access requests toward the same I/O device 905 until the first access request is queued in the one or more queues 909 designated to manage access requests to the given I/O device 905.
Core Y is configured to check the flag at 1020. Since the flag is set on, Core Y keeps monitoring the flag at 1020. Once the first access request is queued in the one or more queues designated to manage access requests destined to the given I/O device, the flag is switched off at 1130. At 1140, Core Y detects modification to the flag. Consequently, Core Y initiates a second access request destined toward the same given I/O device 905. The core Y may start another SYNCW operation, which forces the second success request to be processed prior to any other following access request. The second success request may set the flag on again. The flag will be set on until the second access request is queued in the one or more queues designated to manage access requests destined to the given I/O device. While the flag is set on, no other agent initiates another access request destined toward the same given I/O device.
According to 1130 of
According to at least one example embodiment, in the case of a local-only ordering scenario, no acknowledgment is employed. That is, agents within the remote node 910a do not wait for, and do not receive, an acknowledgement when initiating an access request to the given I/O device 905. The agents simply assume that that an initiated access request is successfully queued in the local effective ordering point 9909a.
According at least one other example embodiment, local acknowledgement is employed in the local-only ordering scenario. According to at least one aspect, multiple versions of the SYNCW operation are employed—one version is employed in the case of a local-only ordering scenario, and another version is employed in the case of a global ordering scenario. As such, all inter-node I/O accesses involve queuing acknowledgment being sent. However, in the case of a local-only ordering scenario, the corresponding SYNCW version may be designed in way that agents do not wait for acknowledgment to be received before initiating a new access request.
According to yet another example embodiment, a data field is used by a software running on the multi-node system 900 to indicate a local-only ordering scenario and/or a global ordering scenario. For microprocessor without interlocked pipeline stages (MIPS) chip device, the cache coherence attribute (CCA) may be used as the data field to indicate the type of ordering scenario. When the data field is used, agents accessing the given I/O device 905 adjust their behavior based on the value of the data field. For example, for given operation, e.g., write operation, two corresponding commands—one with acknowledgement and another without—may be employed, and the data field indicates which command is to be used. Alternatively, instead of using the data field, two versions of the SYNCW are employed, with one version preventing any subsequent access operation from starting before an acknowledgement for a preceding access operation is received, and another version that does not enforce waiting for an acknowledgement for the preceding access operation. A person skilled in the art should appreciate that other implementations are possible.
According to at least one aspect, access requests include write requests, load requests, or the like. In order to further reduce the complexity of access operations in the multi-node system 900, inter-node I/O load operations, used in the multi-node system 900, are acknowledgement-free. That is, given that an inter-node queuing acknowledgement is already used, there is no need for another acknowledgement once the load operation is executed at the given I/O device.
Inter-Chip Interconnect Interface Protocol
Besides the chip device hardware architecture described above, an inter-chip interconnect interface protocol is employed by chip devices within a multi-node system. Considering a N-node system, the goal of the inter-chip interconnect interface protocol is to make the system appear as N-times larger, in terms of capacity, than individual chip devices. The inter-chip interconnect interface protocol runs over reliable point-to-point inter-chip interconnect interface links between nodes of the multi-node system.
According to at least one example embodiment, the inter-chip interconnect interface protocol includes two logical-layer protocols and a reliable link-layer protocol. The two logical layer protocols are a coherent memory protocol, for handling memory traffic, and an I/O, or configuration and status registers (CSR), protocol for handling I/O traffic. The logical protocols are implemented on top of the reliable link-layer protocol.
According to at least one aspect, the reliable link-layer protocol provides 16 reliable virtual channels, per pair of nodes, with credit-based flow control. The reliable link-layer protocol includes a largely standard retry-based acknowledgement/no-acknowledgement (ack/nak) protocol. According to at least one aspect, the reliable link-layer protocol supports 64-byte transfer blocks, each protected by a cyclic redundant check (CRC) code, e.g., CRC-24. According to at least one example embodiment, the hardware interleaves amongst virtual channels at a very fine-grained 64-bit level for minimal request latency, even when the inter-chip interconnect interface link is highly utilized. According to at least one aspect, the reliable link-layer protocol is very low-overhead enabling, for example, up to 250 Gbits/second effective reliable data transfer rate, in full duplex, over inter-chip interconnect interface links.
According to at least one example embodiment, the logical memory coherence protocol, also referred to as the memory space protocol, is configured to maintain cache coherence while enabling cross-node memory traffic. The memory traffic is configured to run over a number of independent virtual channels (VCs). According to at least one aspect, the memory traffic runs over a minimum of three VCs, which include a memory request (MemReq) channel, memory forward (MemFwd) channel, and memory response (MemRsp) channel. According to at least one aspect, no ordering is between VCs or within sub-channels of the same VC. In terms of memory addressing, a memory address includes a first subset of bits indicative of a node, within the multi-node system, and a second subset of nodes for addressing memory within a given node. For example, for a four-node system, 2 bits are used to indicate a node and 42 bits are used for memory addressing within a node, therefore resulting in a total of 44-bit physical memory addresses within the four-node system. According to at least one aspect, each node includes an on-chip sparse directory to keep track of cache blocks associated with a memory block, or line, corresponding to the node.
According to at least one example embodiment, the logical I/O protocol, also referred to as the I/O space protocol, is configured to handle access of I/O devices, or I/O traffic, across the multi-node system. According to at least one aspect, the I/O traffic is configured to run over two independent VCs including an I/O request (IOReq) channel and I/O response (IORsp) channel. According to at least one aspect, the IOReq VC is configured to maintain order between I/O access requests. Such order is described above with respect to
Memory Coherence Protocol
As illustrated in
According to at least one example embodiment, a home node tracks corresponding remotely held cache lines in its RTG. Information used to track remotely held cache blocks, or lines, includes states' information indicative of the states of the remotely held cache blocks in the corresponding remote nodes. The states used include an exclusive (E) state, owned (O) state, shared (S) state, invalid (I) state, and transient, or in-progress, (K) state. The E state indicates that there is only one cache block, associated with the data block in the external memory 790, exclusively held by the corresponding remote node, and that the cache block may or may not be modified compared to the data block in the external memory 790. According to at least one aspect, a sub-state of the E state, a modified (M) state, may also be used. The M state is similar to the E state, except that in the case of M state the corresponding cache block is known to be modified compared to the data block in the external memory 790.
According to at least one example embodiment, cache blocks are partitioned into multiple cache sub-blocks. Each node is configured to maintain, for example, in its shared memory cache 110, a set of bits, also referred to herein as dirty bits, on a sub-block basis for each cache block associated with the corresponding data block in the external memory attached to the home node. Such set of bits, or dirty bits, indicates which sub-blocks, if any, in the cache block are modified compared to the corresponding data block in the external memory 790 attached to the home node. Sub-blocks that indicated, based on the corresponding dirty bits, to be modified are transferred, if remote, to the home node through the inter-chip interconnect interface links 610, and written back in the external memory 790 attached to the home node. That is, a modified sub-block, in a given cache block, is used to update the data block corresponding to the cache block. According to at least one aspect, the use of partitioning of cache block provides efficiency in terms of usage of inter-chip interconnect interface bandwidth. Specifically, when a remote cache block is modified, instead of transferring the whole cache block, only modified sub-block(s) is/are transferred to other node(s).
According to at least one example embodiment, the O state is used when a corresponding flag, e.g., ROWNED_MODE, is set on. If a cache block is in O state in a corresponding node, then another node may have another copy, or cache block, of the corresponding data block. The cache block may or may not be modified compared to the data block in the external memory 790 attached to the home node.
The S state indicates that more than one node has a copy, or cache block, of the data block. The state I indicates that the corresponding node does not have a valid copy, or cache block, of the data block in the external memory attached to the home node. The K state is used by the home node to indicate that a state transition of a copy of the data block, in a corresponding remote node, is detected, and that the transition is still in progress, e.g., not completed. According to at least one example embodiment, the K state is used by the home node to make sure the detected transition is complete before any other operation associated with the same or other copies of the same data block is executed.
According to at least one aspect, state information is held in the RTG on a per remote node basis. That is, if one or more cache blocks, associated with the same data block, are in one or more remote node, the RTG will know which node has it, and the state of each cache block in each remote nodes. According to at least one aspect, when a node reads or writes a cache block that it does not own, e.g., corresponding state is not M, E, or O, it puts a copy of the cache block in its local shared cache memory 110. Such allocation of cache blocks in a local shared cache memory 110 may be avoided with special commands.
The logical coherent memory protocol includes messages for cores 201 and coprocessors 150 to access external memories 790 on any node 100 while maintaining full cache coherency across all nodes 100. Any memory space reference may access any memory on any node 100, in the multi-node system 600. According to at least one example embodiment, each memory protocol message falls into one of three classes, namely requests, forwards, and responses/write-backs, with each class being associated with a corresponding VC. The MemReq channel is configured to carry memory request messages. Memory request messages include memory requests, reads, writes, and atomic sequence operations. The memory forward (MemFwd) channel is configured to carry memory forward messages used to forward requests by home node to remote node(s), as part of an external or internal request processing. The memory response (MemRsp) channel is configured to carry memory response messages. Response messages include responses to memory request messages and memory forward messages. Also, response messages may include information indicative of status change associated with remote cache blocks.
Since the logical memory coherence protocol does not depend on any ordering within any of the corresponding virtual channels, each virtual channel may be further split into multiple independent virtual sub-channels. For example, the MemReq and MemRsp channels may be each split into two independent sub-channels.
According to at least one example embodiment, the memory coherence protocol is configured to operate according to out-of-order transmission in order to maximize transaction performance and minimize transaction latency. That is, home nodes of the multi-node system 600 are configured to receive memory coherence protocol messages in an out-of-order manner, and resolve discrepancy due to out-of-order reception of messages based on maintained states of remote cache blocks in information provided, or implied, by received messages.
According to at least one example embodiment, a home node for data block is involved in any communication regarding copies, or cache blocks, of the data block. When receiving such communications, or messages, the home node checks the maintained state information for the remote cache blocks versus any corresponding state information provided or implied by received message(s). In case of discrepancy, the home node concludes that messages were received out-of-order and that a state transition in a remote node is in progress. In such case the home node makes sure that the detected state transition is complete before any other operation associated with copies of the same data block are executed. The home node may use the K state to stall such operation.
According to at least one example embodiment, the inter-chip interconnect interface sparse directory is held on-chip in the shared cache memory controller 115 of each node. As such, the shared cache memory controller 115 is enabled to simultaneously probe both the inter-chip interconnect interface sparse directory and the shared cache memory, therefore, substantially reducing latency for both inter-chip interconnect interface intra-chip interconnect interface memory transactions. Such placement of the RTG, also referred to herein as the sparse directory, also reduces bandwidth consumption since RTG accesses never consume any external memory, or inter-chip interconnect interface, bandwidth. The RTG eliminates all bandwidth-wasting indiscriminate broadcasting. According to at least one aspect, the logical memory coherence protocol is configured to reduce consumption of the available inter-chip interconnect interface bandwidth in many other ways, including: by performing, whenever possible, operations in either local or remote nodes, such as, atomic operations, by optionally caching in either remote or local cache memories and by transferring, for example, only modified 32-byte sub-blocks of a 128-byte cache block.
Table 1 below provides a list of memory request messages of the logical memory coherence protocol, and corresponding descriptions.
Table 2 below provides a list of memory forward messages of the logical memory coherence protocol, and corresponding descriptions.
Table 3 below provides a list of example memory response messages of the logical memory coherence protocol and corresponding descriptions.
Table 4 below provides a list of example fields, associated with the memory coherence messages, and corresponding descriptions.
A person skilled in the art should appreciate that the lists in the tables below are provided for illustration purposes. The lists are not meant to represent complete sets of messages or message fields associated with the logical memory coherence protocol. A person skilled in the art should also appreciate that the messages and corresponding fields may have different names or different sizes than the ones listed in the tables above. Furthermore, some or all of the messages and field described above may be implemented differently.
After sending the forward message, e.g., FLDX—2H.E(h), the home node (node 1) receives the VICD message from node 0 and realizes that the cache block in node 0 was evicted. Consequently, the home node updates the maintained state for the cache block in node 0 from E to I. The home node (node 1) also changes a state of a corresponding cache block maintained in its shared cache memory 110 from state I to state S, upon receiving a response, e.g., HAKI(h), to its forward message. The change to state S indicates that now the home node stores a copy of the data block in its local shared cache memory 110. Once, the home node (node 1) receives the memory request message, RLDD, from node 1, it responds back, e.g., PEMD, with copy of the data block, changes the maintained state for node 0 from I to E, and changes its state from S to I. That is, the home node (node 1) grants an exclusive copy of the data block to node 0 and evicts the cache block in its shared cache memory 110. When receiving the PEMD message, node 0 may release the bits set when the forward message was received from the home node. The response, e.g., VICD.N, results in a change of the state of node 0 maintained at the home node from E to I.
When the PEMD message is received by node 0, node 0 first changes it local state to E from I. Then, node 0 responds, e.g., VICD.N, back to the previously received FLDX—2H.E message by sending the cache block it holds back to the home node (node 1), and changes its local state for the cache block from E to I. At this point, node 0 releases the bits set in its in-flight buffer 521. Upon receiving the VICD.N message, the home node (node 1) realizes that node 0 received the PEMD message and that the transaction is complete with receipt of the VICD. N message. The home node (node 1) changes the maintained state for node 0 from E to I.
The VICS message received indicates that the state, at node 0, of the same cache block is S, while the state maintained by the home node (node 1) is indicative of an O state. Such discrepancy implies that there was a state transition, at node 0, for the cache block, and that the corresponding message, e.g., VICC, indicative of such transition is not received yet by the home node (node 1). Upon receiving the VICS, the home node (node 1) changes the maintained state for node 0 from O to K to indicate that there is a state transition in progress for the cache block in node 0. The K state makes the home node (node 1) wait for such state transition to complete before allowing any operation associated with the same cache at node 0 or any corresponding cache blocks in other nodes to proceed.
Next, the home node (node 1) receives the RLDD message from node 0. Since the VICC message is not received yet by the home node (node 1)—the detected state transition at node 0 still in progress and not completed—the home node keeps the state K for node 0 and keeps waiting. When the VICC message is received by the home node (node 1), the home node changes the maintained state for node 0 from K to I. Note that the VICC and VICS messages together indicate state transitions from O to S, and then to I. The home node (node 1) then responds back, e.g., with PSHA message, to the RLDD message by sending a copy of the data block to node 0, and changing the maintained state for node 0 from I to S. At this point the transaction between the home node (node 1) and node 0 associated with the data block is complete.
Later, the home node receives the VICC message sent from node 2. In response, the home node (node 1) changes the maintained state for node 2 from K to I. The home node (node 1) then responds back to the RLDD message from node 0 by sending a copy of the data block to node 0, and changes the maintained state for node 0 from I to S. At this stage the transactions with both node 0 and node 2 are complete.
However, by the time the first and second forward messages are received by, respectively, node 2 and node 3, both node 2 and node 3 had already evicted their copies of the data block. Specifically, node 2 evicted its owned copy, changed its state from O to I, and sent a VICD message to the home node (node 1) to indicate the eviction of its owned copy. Also, node 3 evicted its shared copy, changed its state from S to I, and sent a VICS message to the home node (node 1) to indicate the eviction of its shared copy. The home node (node 1) receives the VICD message from node 2 after sending the first forward message, e.g., FLDX.O, to node 2. In response to receiving the VICD message from node 2, the home node updates the maintained state for node 2 from O to I. Later, the home node receives a response, e.g., HAKI, to the first forward message sent to node 2. The response, e.g., HAKI, indicates that node 2 received the first forward message but its state is I, and, as such, the response, e.g., HAKI, does not include a copy of the data block.
After receiving the response, e.g., HAKI, from node 2, the home node responds, e.g., PEMD, to node 0 by providing a copy of the data block. The copy of the data block is obtained from the memory attached to the home node. The home node, however, keeps the maintained state from node 0 as I even after providing the copy of the data block to the node 0. The reason for not changing the maintained state for node 0 to E is that the home node (node 1) is still waiting for a confirmation from node 3 indicating that the shared copy at node 3 is invalidated. Also, the response, e.g., PEMD, from the home node (node 1) to node 0 indicates the number of responses to be expected by the requesting node (node 0). In
Later the home node (node 1) receives a response, e.g., HAKV, to the second forward message acknowledging, by node 3, that it received the second forward message, e.g., SINV, but its state is I. At this point, the home node (node 1) still waits for a message, e.g., VICS, from node 3 indicating that the state at node 3 transitioned from S to I. Once the home node (node 1) receives the VICS message from node 3, the home node (node 1) changes the state maintained for node 3 from S to I, and changes the state maintained for node 0 from I to E since at this point the home node (node 1) knows that only node 0 has a copy of data block.
Node 3 also sends a message, e.g., PACK, acknowledging invalidation of the shared copy at node 3, to the requesting node (node 0). Upon receiving the acknowledgement of invalidation of the shared copy at node 3, node 0 changes its state from I to E.
While this invention has been particularly shown and described with references to example embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.
