linux-kernel - Re: [PATCH RFC] [X86] performance improvement for memcpy

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Date:	Wed, 11 Nov 2009 15:21:26 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	"Ma, Ling" <ling.ma@...el.com>
CC:	Ingo Molnar <mingo@...e.hu>, Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RFC] [X86] performance improvement for memcpy_64.S by
 fast string.

On 11/10/2009 11:57 PM, Ma, Ling wrote:
> Hi Ingo
> 
> This program is for 64bit version, so please use 'cc -o memcpy  memcpy.c -O2 -m64'
> 

I did some measurements with this program; I added power-of-two
measurements from 1-512 bytes, plus some different alignments, and found
some very interesting results:

Nehalem:
	memcpy_new is a win for 1024+ bytes, but *also* a win for 2-32
	bytes, where the old code apparently performs appallingly bad.

	memcpy_new loses in the 64-512 byte range, so the 1024
	threshold is probably justified.

Core2:
	memcpy_new is a win for <= 512 bytes, but a lose for larger
	copies (possibly a win again for 16K+ copies, but those are
	very rare in the Linux kernel.)  Surprise...

	However, the difference is very small.

However, I had overlooked something much more fundamental about your
patch.  On Nehalem, at least *it will never get executed* (except during
very early startup), because we replace the memcpy code with a jmp to
memcpy_c on any CPU which has X86_FEATURE_REP_GOOD, which includes Nehalem.

So the patch is a no-op on Nehalem, and any other modern CPU.

Am I guessing that the perf numbers you posted originally were all from
your user space test program?

	-hpa
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