On Monday, 1 December 2014 at 20:23:00 UTC, Suliman wrote:
Could anybody explain why there is opinion that stack is fast and the heap is slow. All of them are located in the same memory. So the access time should be equal.
Yes, the problem is that if you load from a memory area (of 64 bytes) that has not been visisted in a while then the cpu will have to wait for approximately the same time it takes to execute 100 instructions. So if you touch a 64 byte block every 20 instructions that needs to be loaded from RAM you end up with the CPU being 80% underutilized…
The throughput is actually quite nice, but the latency is the problem.
It is possible to tell the cpu to prefetch new areas of memory ahead of time, but then you have to do it 100 instructions before you actually access it… Which is hard to get right and might be slower if the memory was already loaded into the caches anyway. On some CPUs the prefetch instructions can be counterproductive…
