1.6 Improving memory performance  (Page 4/5)

In addition to the bank stall hazard, single-word references made directly to a multibanked memory system carry a greater latency than those of (successfully) cached memory accesses. This is because references are going out to memory that is slower than cache, and there may be additional address translation steps as well. However, banked memory references are pipelined. As long as references are started well enough in advance, several pipelined, multibanked references can be in flight at one time, giving you good throughput.

The CDC-205 system performed vector operations in a memory-to-memory fashion using a set of explicit memory pipelines. This system had superior performance for very long unit-stride vector computations. A single instruction could perform 65,000 computations using three memory pipes.

Software managed caches

Here’s an interesting thought: if a vector processor can plan far enough in advance to start a memory pipe, why can’t a RISC processor start a cache-fill before it really needs the data in those same situations? In a way, this is priming the cache to hide the latency of the cache-fill. If this could be done far enough in advance, it would appear that all memory references would operate at the speed of the cache.

This concept is called prefetching and it is supported using a special prefetch instruction available on many RISC processors. A prefetch instruction operates just like a standard load instruction, except that the processor doesn’t wait for the cache to fill before the instruction completes. The idea is to prefetch far enough ahead of the computation to have the data ready in cache by the time the actual computation occurs. The following is an example of how this might be used:

This is not the actual FORTRAN. Prefetching is usually done in the assembly code generated by the compiler when it detects that you are stepping through the array using a fixed stride. The compiler typically estimate how far ahead you should be prefetching. In the above example, if the cache-fills were particularly slow, the value 8 in I+8 could be changed to 16 or 32 while the other values changed accordingly.

In a processor that could only issue one instruction per cycle, there might be no payback to a prefetch instruction; it would take up valuable time in the instruction stream in exchange for an uncertain benefit. On a superscalar processor, however, a cache hint could be mixed in with the rest of the instruction stream and issued alongside other, real instructions. If it saved your program from suffering extra cache misses, it would be worth having.

Post-risc effects on memory references

Memory operations typically access the memory during the execute phase of the pipeline on a RISC processor. On the post-RISC processor, things are no different than on a RISC processor except that many loads can be half finished at any given moment. On some current processors, up to 28 memory operations may be active with 10 waiting for off-chip memory to arrive. This is an excellent way to compensate for slow memory latency compared to the CPU speed. Consider the following loop: