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SubjectRe: [PATCH] blkio-throttle: Avoid calling blkiocg_lookup_group() for root group
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@redhat.com>
> Signed-off-by: Vivek Goyal <vgoyal@redhat.com>

Hi Jens,

Do you have any concerns regarding this patch?

Thanks
Vivek

> ---
> 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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