force some data on L1 cache

Question

Apologies about this simple question. Still struggling with some of the memory concepts here. Question is: Suppose I have a pre-computed array A that I want to access repeatedly. Is there a way to tell a C program to keep this array as close as possible to the CPU cache for fastest access? Thanks.

Answer 1

There is no way to force an array to L1/L2 cache on most architectures; it is not needed usually, if you access it frequently it is unlikely to be evicted from cache.

On some architectures there is a set of instructions that allows you to give the processor a hint that the memory location will soon be needed, so that it can start loading it to L1/L2 cache early - this is called prefetching, see _mm_prefetch instruction for example ( http://msdn.microsoft.com/en-us/library/84szxsww(v=vs.80).aspx ). Still this is unlikely to be needed if you're accessing a small array.

The general advice is - make your data structures cache-efficient first (put related data together, pack data, etc.), try prefetching later if the profiler tells you that you're still spending time on cache misses and you can't improve the data layout any further.