linux-kernel - Re: question about splice performance

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Message-ID: <4839D05C.4040309@garzik.org>
Date:	Sun, 25 May 2008 16:47:24 -0400
From:	Jeff Garzik <jeff@...zik.org>
To:	Francis Moreau <francis.moro@...il.com>
CC:	linux-kernel@...r.kernel.org, jens.axboe@...cle.com
Subject: Re: question about splice performance

Francis Moreau wrote:
> Hi,
> 
> I did a really basic benchmark and was suprise about the results.
> 
> $ echo 3 > /proc/sys/vm/drop_caches
> $ time  splice-cp  vmlinux /tmp/vmlinux
> 
> real    0m10.158s
> user    0m0.002s
> sys     0m0.401s
> 
> $ echo 3 > /proc/sys/vm/drop_caches
> $ time  cp  vmlinux /tmp/vmlinux
> 
> real    0m10.590s
> user    0m0.017s
> sys     0m0.459s
> 
> 
> I was expecting splice-cp case to be faster.
> 
> Could someone enlight me ?

If you drop caches you are not measuring splice speed.

Use ramfs for your tests (guarantees data is in cache) instead.

	Jeff



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