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

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Message-ID: <4AFD1326.506@zytor.com>
Date:	Fri, 13 Nov 2009 00:04:54 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Ingo Molnar <mingo@...e.hu>
CC:	Pavel Machek <pavel@....cz>, "Ma, Ling" <ling.ma@...el.com>,
	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 11/12/2009 11:33 PM, Ingo Molnar wrote:
> 
> * Pavel Machek <pavel@....cz> wrote:
> 
>>> Ling, if you are interested, could you send a user-space test-app to 
>>> this thread that everyone could just compile and run on various older 
>>> boxes, to gather a performance profile of hand-coded versus string ops 
>>> performance?
>>>
>>> ( And i think we can make a judgement based on cache-hot performance
>>>   alone - if then the strings ops will perform comparatively better in
>>>   cache-cold scenarios, so the cache-hot numbers would be a conservative
>>>   estimate. )
>>
>> Ugh, really? I'd expect cache-cold performance to be not helped at all 
>> (memory bandwidth limit) and you'll get slow down from additional 
>> i-cache misses...
> 
> That's my point - the new code is shorter, which will run comparatively 
> faster in a cache-cold environment.
> 

memcpy_c by itself is by far the shortest variant, of course.

The question is if it makes sense to use the long variants for short (<
1024 bytes) copies.

	-hpa

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

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