linux-kernel - Re: [PATCH] x86-64: fix memset() to support sizes of 4Gb and above

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Date:	Thu, 19 Jan 2012 07:48:43 +0000
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"Linus Torvalds" <torvalds@...ux-foundation.org>
Cc:	"Ingo Molnar" <mingo@...e.hu>, <tglx@...utronix.de>,
	"Andrew Morton" <akpm@...ux-foundation.org>,
	<linux-kernel@...r.kernel.org>, <hpa@...or.com>
Subject: Re: [PATCH] x86-64: fix memset() to support sizes of 4Gb and
 above

>>> On 18.01.12 at 19:16, Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> On Wed, Jan 18, 2012 at 2:40 AM, Jan Beulich <JBeulich@...e.com> wrote:
>>
>>> For example the kernel's memcpy routine in slightly faster than
>>> glibc's:
>>
>> This is an illusion - since the kernel's memcpy_64.S also defines a
>> "memcpy" (not just "__memcpy"), the static linker resolves the
>> reference from mem-memcpy.c against this one. Apparent
>> performance differences rather point at effects like (guessing)
>> branch prediction (using the second vs the first entry of
>> routines[]). After fixing this, on my Westmere box glibc's is quite
>> a bit slower than the unrolled kernel variant (4% fewer
>> instructions, but about 15% more cycles).
> 
> Please don't bother doing memcpy performance analysis using hot-cache
> cases (or entirely cold-cache for that matter) and/or big memory
> copies.

I realize that - I just was asked to do this analysis, to (hopefully)
turn down arguments against the $subject patch.

> The *normal* memory copy size tends to be in the 10-30 byte range, and
> the cache issues (both code *and* data) are unclear. Running
> microbenchmarks is almost always counter-productive, since it actually
> shows numbers for something that has absolutely *nothing* to do with
> the actual patterns.

This is why I added a way to do meaningful measurement on small
size operations (albeit still cache-hot) with perf.

Jan

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