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SubjectRE: touch_cache() only touches two thirds
On Fri, 17 Nov 2006, dean gaudet wrote:

> another pointer chase arranged to fill the L1 (or L2) using many many
> pages. i.e. suppose i wanted to traverse 32KiB L1 with 64B cache lines
> then i'd allocate 512 pages and put one line on each page (pages ordered
> randomly), but colour them so they fill the L1. this conveniently happens
> to fit in a 2MiB huge page on x86, so you could even ameliorate the TLB
> pressure from the microbenchmark.

btw, for L2-sized measurements you don't need to be so clever... you can
get away with a random traversal of the L2 on 128B boundaries. (need to
avoid the "next-line prefetch" issues on p-m/core/core2, p4 model 3 and
later.) there's just so many more pages required to map the L2 than any
reasonable prefetcher is going to have any time soon.

-dean


> the benchmark i was considering would be like so:
>
> switch to cpu m
> scrub the cache
> switch to cpu n
> scrub the cache
> traverse the coloured list and modify each cache line as we go
> switch to cpu m
> start timing
> traverse the coloured list without modification
> stop timing
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