RE: [patch 10/10] Scheduler profiling - Use immediate values

Li, Tong N
2007-07-06T20:57:17Z

> I found that memory latency is difficult to measure in modern x86
> CPUs because they have very clever prefetchers that can often
> outwit benchmarks.

A pointer-chasing program that accesses a random sequence of addresses
usually can produce a good estimate on memory latency. Also, prefetching
can be turned off in BIOS or by modifying the MSRs.

> Another trap on P4 is that RDTSC is actually quite slow and
synchronizes
> the CPU; that can add large measurement errors.
>
> -Andi

The cost can be amortized if the portion of memory accesses is long
enough.

tong