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Date: Fri, 9 Sep 2011 17:35:53 +0200
From: Borislav Petkov <bp@alien8.de>
To: Linus Torvalds <torvalds@linux-foundation.org>
CC: Maarten Lankhorst <m.b.lankhorst@gmail.com>,
        Borislav Petkov <bp@amd64.org>,
        "Valdis.Kletnieks@vt.edu" <Valdis.Kletnieks@vt.edu>,
        Ingo Molnar <mingo@elte.hu>, melwyn lobo <linux.melwyn@gmail.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>, Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>
Subject: Re: x86 memcpy performance
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On Fri, Sep 09, 2011 at 07:39:18AM -0700, Linus Torvalds wrote:
> Kernel memcpy's are basically almost always smaller than a page size,
> because that tends to be the fundamental allocation size.

Yeah, this is what my trace of a kernel build showed too:

Bytes   Count
=====   =====

...

224     3
225     3
227     3
244     1
254     5
255     13
256     21708
512     21746
848     12907
1920    36536
2048    21708

OTOH, I keep thinking that copy_*_user might be doing bigger sizes, for
example when shuffling network buffers to/from userspace. Converting
those to SSE memcpy might not be as easy as memcpy itself, though.

> Yes, there are exceptions that copy into big vmalloc'ed buffers, but
> they don't tend to matter. Things like module loading etc.

Too small a number of repetitions to matter, yes.

-- 
Regards/Gruss,
Boris.