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

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Message-ID: <x497he3mbr8.fsf@segfault.boston.devel.redhat.com>
Date:	Mon, 17 Jan 2011 17:00:27 -0500
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	linux kernel mailing list <linux-kernel@...r.kernel.org>,
	Jens Axboe <axboe@...nel.dk>
Subject: Re: [PATCH] blkio-throttle: Avoid calling blkiocg_lookup_group() for root group

Vivek Goyal <vgoyal@...hat.com> writes:

> 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>

Acked-by: Jeff Moyer <jmoyer@...hat.com>

In case it wasn't obvious, this patch was tested and confirmed to fix
the performance degradation.

Cheers,
Jeff

> ---
>  block/blk-throttle.c |   10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
>
> Index: linux-2.6/block/blk-throttle.c
> ===================================================================
> --- linux-2.6.orig/block/blk-throttle.c	2011-01-17 16:23:37.041280712 -0500
> +++ linux-2.6/block/blk-throttle.c	2011-01-17 16:36:09.362940975 -0500
> @@ -168,7 +168,15 @@ static struct throtl_grp * throtl_find_a
>  	 * tree of blkg (instead of traversing through hash list all
>  	 * the time.
>  	 */
> -	tg = tg_of_blkg(blkiocg_lookup_group(blkcg, key));
> +
> +	/*
> +	 * This is the common case when there are no blkio cgroups.
> + 	 * Avoid lookup in this case
> + 	 */
> +	if (blkcg == &blkio_root_cgroup)
> +		tg = &td->root_tg;
> +	else
> +		tg = tg_of_blkg(blkiocg_lookup_group(blkcg, key));
>  
>  	/* Fill in device details for root group */
>  	if (tg && !tg->blkg.dev && bdi->dev && dev_name(bdi->dev)) {
--
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