Re: [PATCH RFC 2/2] [x86] Optimize copy_page by re-arranging instruction sequence and saving register

[Andi Kleen]

ling.ma@xxxxxxxxx writes:

> From: Ma Ling <ling.ma@xxxxxxxxx>
>
> Load and write operation occupy about 35% and 10% respectively
> for most industry benchmarks. Fetched 16-aligned bytes code include 
> about 4 instructions, implying 1.34(0.35 * 4) load, 0.4 write.  
> Modern CPU support 2 load and 1 write per cycle, so throughput from write is
> bottleneck for memcpy or copy_page, and some slight CPU only support one mem
> operation per cycle. So it is enough to issue one read and write instruction
> per cycle, and we can save registers. 

I don't think "saving registers" is a useful goal here.

>
> In this patch we also re-arrange instruction sequence to improve performance
> The performance on atom is improved about 11%, 9% on hot/cold-cache
> case respectively.

That's great, but the question is what happened to the older CPUs that
also this sequence. It may be safer to add a new variant for Atom,
unless you can benchmark those too.

-Andi


-- 
ak@xxxxxxxxxxxxxxx -- Speaking for myself only
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