Re: x86 memcpy performance

From: Andi Kleen
Date: Fri Aug 12 2011 - 14:33:38 EST

melwyn lobo <linux.melwyn@xxxxxxxxx> writes:

> Hi All,
> Our Video recorder application uses memcpy for every frame. About 2KB
> data every frame on Intel Atomâ Z5xx processor.
> With default 2.6.35 kernel we got 19.6 fps. But it seems kernel
> implemented memcpy is suboptimal, because when we replaced
> with an optmized one (using ssse3, exact patches are currently being
> finalized) ew obtained 22fps a gain of 12.2 %.

SSE3 in the kernel memcpy would be incredible expensive,
it would need a full FPU saving for every call and preemption

I haven't seen your patches, but until you get all that
right (and add a lot more overhead to most copies) you
have a good change currently to corrupting user FPU state.

> C0 residency also reduced from 75% to 67%. This means power benefits too.
> My questions:
> 1. Is kernel memcpy profiled for optimal performance.

It depends on the CPU

There have been some improvements for Atom on newer kernels
I believe.

But then kernel memcpy is usually optimized for relatively
small copies (<= 4K) because very few kernel loads do more.


ak@xxxxxxxxxxxxxxx -- Speaking for myself only
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