Scratchpad RAM
Scratchpad memory (SPM), also known as scratchpad, scatchpad RAM or local store in computer terminology, is a high-speed internal memory used for temporary storage of calculations, data, and other work in progress. In reference to a microprocessor ("CPU"), scratchpad refers to a special high-speed memory circuit used to hold small items of data for rapid retrieval.
It can be considered as similar to an L1 cache in that it is the memory next closest to the ALU's after the internal registers, with explicit instructions to move data from and to main memory, often using DMA-based data transfer. In contrast with a system that uses caches, a system with scratchpads is a system with Non-Uniform Memory Access latencies, because the memory access latencies to the different scratchpads and the main memory vary. Another contrast with a system that employs caches is that a scratchpad does commonly not contain a copy of data that is also stored in the main memory.
Scratchpads are employed for simplification of caching logic, and to guarantee a unit can work without main memory contention in a system employing multiple processors, especially in multiprocessor system-on-chip for embedded systems. They are most suited to storing temporary results (such as would be found in the CPU stack for example) that typically wouldn't always need committing to main memory; however when fed by DMA, they can also be used in place of a cache for mirroring the state of slower main memory. The same issues of locality of reference apply relating to efficiency of use; although some systems allow strided DMA to access rectangular data sets. Another difference is that scratchpads are explicitly manipulated by applications.
Scratchpads are not used in mainstream desktop processors where generality is required for legacy software to run from generation to generation, in which the available on-chip memory size may change. They are suited to embedded systems, special-purpose processors and games consoles, where chips are often manufactured as MPSoC, and where software is often tuned to one hardware configuration.
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Examples of use
- SH2, SH4 used in Sega's consoles could lock cachelines to an address outside of main memory, for use as a Scratchpad.
- The Sony PS1's R3000 had a Scratchpad instead of an L1 cache. It was possible to place the CPU stack here, an example of the temporary workspace usage.
- Sony's PS2's customized R5000 employed a 16KiB Scratchpad, to and from which DMA transfers could be issued to its GS, and main memory.
- The Cell's SPEs are restricted purely to working in their "local-store", relying on DMA for transfers from/to main memory and between local stores, much like a Scratchpad. In this instance, additional benefit is derived from the lack of hardware to check and update coherence between multiple caches: the design takes advantage of the assumption that each processor's workspace is separate and private. It is expected this benefit will become more noticeable as the number of processors scales into the "many-core" future.
- Many other processors allow L1 cache lines to be locked.
- Most DSPs use a Scratchpad. Many past 3D accelerators and games machines (including the PS2) have used DSPs for vertex transformations. This contrasts with the stream based approach of modern GPUs which has more in common with a CPU cache in function.
- NVIDIA's 8800 GPU running under CUDA provides 16KiB of Scratchpad per thread-bundle when being used for gpgpu tasks.
- Ageia's PhysX chip utilizes Scratchpad RAM in a manner similar to the Cell; the theory being a cache hierarchy is of less use than software management for physics and collision calculations. These memories are also banked and a switch fabric manages transfers between them.
Alternatives
Cache control vs Scratchpads
Many architectures such as PowerPC attempt to avoid the need for cacheline locking or scratchpads through the use of cache control instructions. Marking an area of memory with "Data Cache Block Zero" (allocating a line but setting its contents to zero instead of loading from main memory) and discarding it after use ('Data Cache Block Invalidate', signaling that main memory needn't receive any updated data) the cache is made to behave as a scratchpad. Generality is maintained in that these are hints and the underlying hardware will function correctly regardless of actual cache size.
Shared L2 vs Cell local stores
Regarding interprocessor communication in a multicore setup, there are similarities between the Cell's inter-localstore DMA and a Shared L2 cache setup as in the Core2 Duo or the Xbox 360's custom powerPC: the L2 cache allows processors to share results without those results having to be committed to main memory. This can be an advantage where the working set for an algorithm encompasses the entirety of the L2. However when a program can be written to take advantage of inter-localstore DMA, the Cell has the benefit of each other Local Store serving the purpose of BOTH the private workspace for a single processor AND the point of sharing between processors i.e. the other Local Stores are on a similar footing viewed from one processor as the shared L2 in a conventional chip. The tradeoff is memory wasted in buffering and programming complexity for synchronization, though this would be similar to precached pages in a conventional chip. Domains where using this capability is effective include:
- pipeline processing (where one achieves the same effect as increased L1 size by splitting one job into smaller chunks).
- Extending the working set, e.g. a sweet spot for a merge sort where the data fits within 8x256KiB
- sharing code upload, e.g. load a piece of code to one SPU, then copy it from there to the others to avoid hitting main memory again.
It would be possible for a conventional processor to gain similar advantages with cache-control instructions, e.g. allowing prefetching to L1 bypassing L2, or an eviction hint that signaled a transfer from L1 to L2 but not committing to main memory; however, at present no systems offer this capability in a usable form and such instructions in effect should mirror explicit transfer of data among cache areas used by each core.
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