A processor typically comprises a number of registers and where the processor is a multi-threaded processor, the registers may be shared between threads (global registers) or dedicated to a particular thread (local registers). Where the processor executes DSP (Digital Signal Processing) instructions, the processor includes additional registers which are dedicated for use by DSP instructions.
A processor's registers 100 form part of a memory hierarchy 10 which is provided in order to reduce the latency associated with accessing main memory 108, as shown in
The embodiments described below are not limited to implementations which solve any or all of the disadvantages of known processors.
This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter.
Methods of increasing the efficiency of memory resources within a processor are described. In an embodiment, instead of including dedicated DSP indirect register resource for storing data associated with DSP instructions, this data is stored in an allocated and locked region within the cache. The state of any cache lines which are used to store DSP data is then set to prevent the data from being written to memory. The size of the allocated region within the cache may vary according to the amount of DSP data that needs to be stored and when no DSP instructions are being run, no cache resources are allocated for storage of DSP data.
A first aspect provides a method of managing memory resources within a processor comprising: dynamically using a locked portion of a cache for storing data associated with DSP instructions; and setting a state associated with any cache lines in the portion of the cache allocated to and used by a DSP instruction, the state being configured to prevent the data stored in the cache line from being written to memory.
A second aspect provides a processor comprising: a cache; a load-store pipeline; and two or more channels connecting the load-store pipeline and the cache; and wherein a portion of the cache is dynamically allocated for storing data associated with DSP instructions when DSP instructions are executed by the processor and lines within the portion of the cache are locked.
Further aspects provide a method substantially as described with reference to any of
The methods described herein may be performed by a computer configured with software in machine readable form stored on a tangible storage medium e.g. in the form of a computer program comprising computer readable program code for configuring a computer to perform the constituent portions of described methods or in the form of a computer program comprising computer program code means adapted to perform all the steps of any of the methods described herein when the program is run on a computer and where the computer program may be embodied on a computer readable storage medium. Examples of tangible (or non-transitory) storage media include disks, thumb drives, memory cards etc and do not include propagated signals. The software can be suitable for execution on a parallel processor or a serial processor such that the method steps may be carried out in any suitable order, or simultaneously.
The hardware components described herein may be generated by a non-transitory computer readable storage medium having encoded thereon computer readable program code.
This acknowledges that firmware and software can be separately used and valuable. It is intended to encompass software, which runs on or controls “dumb” or standard hardware, to carry out the desired functions. It is also intended to encompass software which “describes” or defines the configuration of hardware, such as HDL (hardware description language) software, as is used for designing silicon chips, or for configuring universal programmable chips, to carry out desired functions.
The preferred features may be combined as appropriate, as would be apparent to a skilled person, and may be combined with any of the aspects of the invention.
Embodiments of the invention will be described, by way of example, with reference to the following drawings, in which:
Common reference numerals are used throughout the figures to indicate similar features.
Embodiments of the present invention are described below by way of example only. These examples represent the best ways of putting the invention into practice that are currently known to the Applicant although they are not the only ways in which this could be achieved. The description sets forth the functions of the example and the sequence of steps for constructing and operating the example. However, the same or equivalent functions and sequences may be accomplished by different examples.
As described above, a processor which can execute DSP instructions typically includes an additional register resource which is dedicated for use by those DSP instructions.
As shown in
The following paragraphs describe a processor, which may be a single or multi-threaded processor and may comprise one or more cores, in which the DSP indirect register resource is not provided as a dedicated register resource but is instead absorbed into the cache state (e.g. the L1 cache). Also the functionality of the DSP access pipeline is absorbed into that of the Load-Store pipeline such that it is only the address range used to hold DSP indirect registers state within the L1 cache that identifies the special accesses to the cache. The L1 cache address range used is reserved for accesses to the DSP indirect register resource of each thread preventing any data contamination. Through use of dynamic allocation of the cache resources to DSP instructions, the register overhead is eliminated (i.e. there does not need to be any dedicated DSP indirect registers within the processor) along with the power overhead and the utilization of the overall memory hierarchy is more efficient (i.e. when no DSP instructions have been run, all cache resources are available for use in the standard way). As described in more detail below, in some examples, the size of the portion of the cache which is allocated to the DSP instructions can grow and shrink dynamically according to the amount of data that the DSP instructions need to store.
The parts of the cache (i.e. the cache lines) which are used to store data by related DSP instructions are not used in the same way that the cache is traditionally used because these values are only ever filled from inside the processor and they are not initially loaded from another level in the memory hierarchy or written back to any memory (except upon a context switch, as described in more detail below). Consequently, as shown in
The state (‘write never’) and the locking of the cache lines used instead of DSP indirect register resource may be set using existing bits which indicate the state of a cache line. Allocation control information, which sets the bits (and hence performs the locking and sets the state), may be sent alongside each L1 cache transaction created by the Load-Store pipeline. This state is read and interpreted by the internal state machine of the cache such that when implementing an eviction algorithm, the algorithm determines that it cannot evict data from a locked cache line and instead has to select an alternative (non-locked) cache line to evict.
In an example, the setting of the state may be implemented by the Load-Store pipeline (e.g. by hardware logic within the Load-Store pipeline), for example the Load-Store pipeline may have access to a register which controls the state or the setting of the state may be controlled via address page tables as read by the MMU.
The method may comprise a configuration step (block 306) which sets up a register to indicate that a thread can use a portion of the cache for DSP data. This is a static set-up process in contrast to the actual allocation of lines within the cache (in block 302) which is performed dynamically. In some examples, all the threads in a multi-threaded processor may be enabled to use a portion of the cache for storing DSP data, or alternatively, only some of the threads may be enabled to use a portion of the cache in this way.
The registers which indicate that a thread can use a portion of the cache for DSP data may be located within the L1 cache or within the MMU. In an example, the L1 cache may include local state settings that indicate DSP-type lines within the cache and this information may be passed from the MMU to the L1 cache.
In order that the portion of the cache may be used instead of DSP indirect registers to store the DSP data, the cache architecture is modified so that the required amount of information can be accessed from the portion of the cache by the DSP instructions. In particular, to enable two reads or one read and one write to be performed at the same time (i.e. simultaneously) the number of semi-independent data accesses to the cache is increased, for example by providing two channels to the cache and the cache is partitioned (e.g. the cache architecture is split into two storage elements) to provide two sets of locations for the two channels. In an example implementation, the access ports to the cache may be expanded to present two load ports and one store port (where the store port can access either of the two storage elements).
The term ‘semi-independent’ is used in relation to the data accesses to the cache because each DSP operation may use a number of DSP data items, but there are set relations between those that are used together. The cache therefore can arrange storage of sets of items, knowing that only particular sets will be accessed together.
The standard non-DSP-related cache accesses can make use of the multiple ports provided to the structures/banks, and may also opportunistically combine individual cache accesses to perform multiple accesses within a single clock cycle. The individual accesses are not required to be related beyond the independent structure in which they are each accessing (which allows them to be operated together), i.e. the individual accesses are not related and only need to access different storage elements.
Further division of the storage elements by data width may also be performed to allow a greater range of data alignment accesses to be performed. This does not affect the operations described above, but also enables the possibility of operating on multiple data within the same set. In one example this would allow operations to access to an additional element within a cached line to an alternate offset from the first.
The example flow diagram in
In an example implementation of block 318, an address indexed data lookup within the MMU may determine the DSP property of accesses through its address range and this could be used in conjunction with a modified cache maintenance operation (which searches the cache for other reasons) to search and update the cache line state back to the locked DSP state.
The controls which are used to unlock and lock lines (in blocks 310 and 318) and the control which is used to lock the lines originally (in block 304) may be stored within the cache itself, e.g. within the tag RAM, or in hardware logic associated with the cache. Existing control parameters within the cache provide locked in cache lines and new additional instructions or modifications to existing instructions are provided to enable these control parameters to be readable and updateable such that the DSP data contents can be saved and restored. This may be implemented purely in hardware or in a combination of hardware and software.
In the second example, as soon as a DSP instruction has some data to store (block 502), a portion of the cache is allocated which is large enough to store that data (block 505) and the allocation is then increased (in block 510) when more data needs to be stored, up to a maximum allocation size. This option is more efficient than the first example, because the amount of cache which is unavailable for normal use (because it is allocated to DSP and locked against use by anything else) is dependent upon the amount of DSP data that needs to be stored; however this second example may add a delay where the size of the allocated portion is increased (in block 510). It will be appreciated that there are a number of different ways in which the increase in allocation (in block 510) may be managed. In one example, the allocated portion may be increased in size when it is not possible to store the new data in the existing allocated portion and in another example, the allocated portion may be increased in size when the remaining free space falls below a predefined amount. It will further be appreciated that the amount allocated initially (in block 505) may be only of a sufficient size to store the required data (from block 502) or may be larger than this, such that the size of the allocated portion does not need to be increased with each new DSP instruction that has data to store but only occurs periodically.
In some implementations of the second example, the allocation may be reduced in size (in block 518) in a reverse operation to that which occurs in block 510, e.g. when there is available space in the allocated portion (block 516). Where this is implemented, the allocated portion grows and shrinks its footprint within the cache which increases efficiency in the use of cache resources.
The allocation (in block 504 or 505) may, for example, be provoked by the DSP instruction accessing a location within a page marked as DSP and finding that it does not have permission to read or write. This would cause an exception and software would prepare the cache with a DSP area (in block 504 or 505).
In a third example, the cache may be pre-prepared such that a portion of the cache is pre-allocated to DSP data (block 507). This means that exception handling would not be caused (as may be the case in the first two examples and trigger the allocation process); however this may require a DSP area to be reserved in the cache earlier than is necessary.
In any of the examples in
The methods described above may also be implemented in a single-threaded processor and an example processor 700 is shown in
Where the methods are implemented in a multi-threaded processor, the method shown in
As described above (e.g. with reference to
In some implementations, the methods shown in
As described above, the allocation of cache resource for use as if it was DSP indirect register resource (i.e. for use in storing DSP data) is performed dynamically. In an example, the hardware logic may periodically perform the allocation of cache resource to threads for use to store DSP data, and the size of any allocation may be fixed or may vary (e.g. as shown in
Although the above description relates to use of the cache to store DSP data, the modified cache architecture described above and shown in
The methods and apparatus described above enable an array of indirectly accessed DSP registers (which is typically large compared to other register resource) to be moved into the L1 cache as a locked resource.
Using the methods described above, the overhead associated with provision of dedicated DSP indirect registers is eliminated and through re-use of existing logic (e.g. the load-store pipeline) additional logic to write the DSP data to the cache is not required. Furthermore, where dedicated DSP indirect registers are used (e.g. as shown in
A particular reference to “logic” refers to structure that performs a function or functions. An example of logic includes circuitry that is arranged to perform those function(s). For example, such circuitry may include transistors and/or other hardware elements available in a manufacturing process. Such transistors and/or other elements may be used to form circuitry or structures that implement and/or contain memory, such as registers, flip flops, or latches, logical operators, such as Boolean operations, mathematical operators, such as adders, multipliers, or shifters, and interconnect, by way of example. Such elements may be provided as custom circuits or standard cell libraries, macros, or at other levels of abstraction. Such elements may be interconnected in a specific arrangement. Logic may include circuitry that is fixed function and circuitry can be programmed to perform a function or functions; such programming may be provided from a firmware or software update or control mechanism. Logic identified to perform one function may also include logic that implements a constituent function or sub-process. In an example, hardware logic has circuitry that implements a fixed function operation, or operations, state machine or process.
Any range or device value given herein may be extended or altered without losing the effect sought, as will be apparent to the skilled person.
It will be understood that the benefits and advantages described above may relate to one embodiment or may relate to several embodiments. The embodiments are not limited to those that solve any or all of the stated problems or those that have any or all of the stated benefits and advantages.
Any reference to an item refers to one or more of those items. The term ‘comprising’ is used herein to mean including the method blocks or elements identified, but that such blocks or elements do not comprise an exclusive list and an apparatus may contain additional blocks or elements and a method may contain additional operations or elements. Furthermore, the blocks, elements and operations are themselves not impliedly closed.
The steps of the methods described herein may be carried out in any suitable order, or simultaneously where appropriate. The arrows between boxes in the figures show one example sequence of method steps but are not intended to exclude other sequences or the performance of multiple steps in parallel. Additionally, individual blocks may be deleted from any of the methods without departing from the spirit and scope of the subject matter described herein. Aspects of any of the examples described above may be combined with aspects of any of the other examples described to form further examples without losing the effect sought. Where elements of the figures are shown connected by arrows, it will be appreciated that these arrows show just one example flow of communications (including data and control messages) between elements. The flow between elements may be in either direction or in both directions.
It will be understood that the above description of a preferred embodiment is given by way of example only and that various modifications may be made by those skilled in the art. Although various embodiments have been described above with a certain degree of particularity, or with reference to one or more individual embodiments, those skilled in the art could make numerous alterations to the disclosed embodiments without departing from the spirit or scope of this invention.
Number | Date | Country | Kind |
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1314891.1 | Aug 2013 | GB | national |