linux-kernel - Re: x86 memcpy performance

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Message-ID: <20110812195220.GA29051@elte.hu>
Date:	Fri, 12 Aug 2011 21:52:20 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	melwyn lobo <linux.melwyn@...il.com>
Cc:	linux-kernel@...r.kernel.org, "H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: x86 memcpy performance

* melwyn lobo <linux.melwyn@...il.com> wrote:

> Hi All,
> Our Video recorder application uses memcpy for every frame. About 2KB
> data every frame on Intel® Atom™ 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 %.
> C0 residency also reduced from 75% to 67%. This means power benefits too.
> My questions:
> 1. Is kernel memcpy profiled for optimal performance.
> 2. Does the default kernel configuration for i386 include the best
> memcpy implementation (AMD 3DNOW, __builtin_memcpy .... etc)
> 
> Any suggestions, prior experience on this is welcome.

Sounds very interesting - it would be nice to see 'perf record' + 
'perf report' profiles done on that workload, before and after your 
patches.

The thing is, we obviously want to achieve those gains of 12.2% fps 
and while we probably do not want to switch the kernel's memcpy to 
SSE right now (the save/restore costs are significant), we could 
certainly try to optimize the specific codepath that your video 
playback path is hitting.

If it's some bulk memcpy in a key video driver then we could offer a 
bulk-optimized x86 memcpy variant which could be called from that 
driver - and that could use SSE3 as well.

So yes, if the speedup is real then i'm sure we can achieve that 
speedup - but exact profiles and measurements would have to be shown.

Thanks,

	Ingo
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