On Wednesday 21 January 2015 12:27:38 Anton Blanchard wrote:
> I noticed ksm spending quite a lot of time in memcmp on a large
> KVM box. The current memcmp loop is very unoptimised - byte at a
> time compares with no loop unrolling. We can do much much better.
>
> Optimise the loop in a few ways:
>
> - Unroll the byte at a time loop
>
> - For large (at least 32 byte) comparisons that are also 8 byte
> aligned, use an unrolled modulo scheduled loop using 8 byte
> loads. This is similar to our glibc memcmp.
>
> A simple microbenchmark testing 10000000 iterations of an 8192 byte
> memcmp was used to measure the performance:
>
> baseline: 29.93 s
>
> modified: 1.70 s
>
> Just over 17x faster.
>
> v2: Incorporated some suggestions from Segher:
>
> - Use andi. instead of rdlicl.
>
> - Convert bdnzt eq, to bdnz. It's just duplicating the earlier compare
> and was a relic from a previous version.
>
> - Don't use cr5, we have plans to use that CR field for fast local
> atomics.
>
> Signed-off-by: Anton Blanchard <an...@samba.org>
Would it help to also add a way for an architecture to override
memcmp_pages() with its own implementation? That way you could
skip the unaligned part, hardcode the loop counter and avoid the
preempt_disable() in kmap_atomic().
Arnd
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