Re: [PATCH] blkio-throttle: Avoid calling blkiocg_lookup_group()for root group

From: Jens Axboe
Date: Mon Jan 31 2011 - 14:24:09 EST

On 2011-01-31 20:20, Jeff Moyer wrote:
> Vivek Goyal <vgoyal@xxxxxxxxxx> 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@xxxxxxxxxx>
>>> Signed-off-by: Vivek Goyal <vgoyal@xxxxxxxxxx>
>> Hi Jens,
>> Do you have any concerns regarding this patch?
> Acked-by: Jeff Moyer <jmoyer@xxxxxxxxxx>
> 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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