High Performance Computing - Charles Severance [15]
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:
DO I=1,1000000,8
PREFETCH(ARR(I+8))
DO J=0,7
SUM=SUM+ARR(I+J)
END DO
END DO
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:
LOADI R6,10000 Set the Iterations
LOADI R5,0 Set the index variable
LOOP: LOAD R1,R2(R5) Load a value from memory
INCR R1 Add one to R1
STORE R1,R3(R5) Store the incremented value back to memory
INCR R5 Add one to R5
COMPARE R5,R6 Check for loop termination
BLT LOOP Branch if R5 < R6 back to LOOP
In this example, assume that it take 50 cycles to access memory. When the fetch/ decode puts the first load into the instruction reorder buffer (IRB), the load starts on the next cycle and then is suspended in the execute phase. However, the rest of the instructions are in the IRB. The INCR R1 must wait for the load and the STORE must also wait. However, by using a rename register, the INCR R5, COMPARE, and BLT can all be computed, and the fetch/decode goes up to the top of the loop and sends another load into the IRB for the next memory location that will have to wait. This looping continues until about 10 iterations of the loop are in the IRB. Then the first load actually shows up from memory and the INCR R1 and STORE from the first iteration begins executing. Of course the store takes a while, but about that time the second load finishes, so there is more work to do and so on…
Like many aspects of computing, the post-RISC architecture, with its out-of-order and speculative execution, optimizes memory