linux-kernel - Re: x86 memcpy performance

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Message-ID: <CAHSGOutdR8dvf+i0NfKCycpui6G3Du6qn0DXftHdtiHfmYCCRw@mail.gmail.com>
Date:	Mon, 5 Dec 2011 17:50:49 +0530
From:	melwyn lobo <linux.melwyn@...il.com>
To:	Borislav Petkov <bp@...en8.de>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Maarten Lankhorst <m.b.lankhorst@...il.com>,
	Borislav Petkov <bp@...64.org>,
	"Valdis.Kletnieks@...edu" <Valdis.Kletnieks@...edu>,
	Ingo Molnar <mingo@...e.hu>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: x86 memcpy performance

The driver has a loop of memcpy the source and destination addresses
based on a runtime computed value and confuses the compiler on the
alignement.
So instead of generating neat 32 bit memcpy, gcc generates "rep movsb"
Example code snippet:
src = (char *)kmap(bo->pages[idx]);
src += offset;
memcpy(des, src, len);
Be replacing ssse3 only for memcpy of length larger than 1K bytes (for
my driver typical length are 2k metadata from SRAM to DDR) I think
overheads of FPU save and restore can be forgiven.
Will SSSE3 work for unlaigned pointers as well ? If it doesn't I am
lucky for past 6 months :)


On Fri, Sep 9, 2011 at 9:05 PM, Borislav Petkov <bp@...en8.de> wrote:
> 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.
>
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