linux-kernel - Re: [PATCH] blkio-throttle: Avoid calling blkiocg_lookup

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Date:	Mon, 31 Jan 2011 20:24:00 +0100
From:	Jens Axboe <axboe@...nel.dk>
To:	Jeff Moyer <jmoyer@...hat.com>
CC:	Vivek Goyal <vgoyal@...hat.com>,
	linux kernel mailing list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] blkio-throttle: Avoid calling blkiocg_lookup_group()
 for root group

On 2011-01-31 20:20, Jeff Moyer wrote:
> Vivek Goyal <vgoyal@...hat.com> writes:
> 
>> On Mon, Jan 17, 2011 at 04:56:06PM -0500, Vivek Goyal wrote:
>>> o Jeff Moyer was doing some testing on a RAM backed disk and
>>>   blkiocg_lookup_group() showed up high overhead after memcpy(). Similarly
>>>   somebody else reported that blkiocg_lookup_group() is eating 6% extra
>>>   cpu. Though looking at the code I can't think why the overhead of
>>>   this function is so high. One thing is that it is called with very high
>>>   frequency (once for every IO).
>>>
>>> o For lot of folks blkio controller will be compiled in but they might
>>>   not have actually created cgroups. Hence optimize the case of root
>>>   cgroup where we can avoid calling blkiocg_lookup_group() if IO is happening
>>>   in root group (common case).
>>>
>>> Reported-by: Jeff Moyer <jmoyer@...hat.com>
>>> Signed-off-by: Vivek Goyal <vgoyal@...hat.com>
>>
>> Hi Jens,
>>
>> Do you have any concerns regarding this patch?
> 
> Acked-by: Jeff Moyer <jmoyer@...hat.com>
> 
> Jens, this is a pretty easy performance regression fixup.  I think it
> should be pulled in sooner rather than later.  We've also witnessed this
> slowdown on big performance testing rigs, so it's not just a ramdisk
> issue.

Yep, will fold it into 2.6.38, thanks!


-- 
Jens Axboe

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