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

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Date:	Thu, 12 Nov 2009 22:04:06 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	"Ma, Ling" <ling.ma@...el.com>
CC:	Pavel Machek <pavel@....cz>, "mingo@...e.hu" <mingo@...e.hu>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RFC] [X86] performance improvement for memcpy_64.S by
 fast string.

On 11/12/2009 09:33 PM, Ma, Ling wrote:
>> Well, so you are running cache hot and it is only a win on huge
>> copies... how common are those?
>>
> Hi Pavel Machek
> Yes, we intend to introduce movsq for huge hot size(over 1024bytes)
> and avoid regression for less 1024bytes. I guess you suggest using
> prefetch instruction for cold data (if I was wrong please correct me).
> memcpy don't know whether data has been in cache or not,
> so only when copy size is over (first level 1 cache)/2 and lower
> (last level cache)/2 , prefetch will get benefit. Currently first
> level cache size of most cpus is around 32KB, so it is useful for prefetch 
> when copy size is over 16KB, but as H. Peter Anvin mentioned in last email,
> over 16KB copy in kernel is rare.
> 

What it sounds to me is that for Nehalem, we want to use memcpy_c for >=
1024 bytes and the old code for < 1024 bytes; for Core2 it might be the
exact opposite.

Either way, whatever we do should use the appropriate static replacement
mechanism.

	-hpa

-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.

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