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* [PATCH] blkio-throttle: Avoid calling blkiocg_lookup_group() for root group
@ 2011-01-17 21:56 Vivek Goyal
  2011-01-17 22:00 ` Jeff Moyer
  2011-01-26 18:57 ` Vivek Goyal
  0 siblings, 2 replies; 7+ messages in thread
From: Vivek Goyal @ 2011-01-17 21:56 UTC (permalink / raw)
  To: linux kernel mailing list, Jens Axboe; +Cc: Moyer Jeff Moyer

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>
---
 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)) {


^ permalink raw reply	[flat|nested] 7+ messages in thread

end of thread, other threads:[~2011-01-31 19:30 UTC | newest]

Thread overview: 7+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2011-01-17 21:56 [PATCH] blkio-throttle: Avoid calling blkiocg_lookup_group() for root group Vivek Goyal
2011-01-17 22:00 ` Jeff Moyer
2011-01-26 18:57 ` Vivek Goyal
2011-01-31 19:20   ` Jeff Moyer
2011-01-31 19:24     ` Jens Axboe
2011-01-31 19:25       ` Jens Axboe
2011-01-31 19:30         ` Jeff Moyer

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