Re: [PATCH 1/2] powerpc: Add 64bit optimised memcmp

[Arnd Bergmann <arnd@arndb.de>]

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 <anton@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