On Tue, Jan 31, 2012 at 09:34:23PM +0100, Peter Zijlstra wrote: > On Wed, 2011-07-20 at 20:29 -0400, Mikulas Patocka wrote: > > Hi Jens > > > > Please would you consider taking this into the block tree? It seems to > > speed up device deletion enormously. > > > > Mikulas > > > > --- > > > > backing-dev: use synchronize_rcu_expedited instead of synchronize_rcu > > > > synchronize_rcu sleeps several timer ticks. synchronize_rcu_expedited is > > much faster. > > > > With 100Hz timer frequency, when we remove 10000 block devices with > > "dmsetup remove_all" command, it takes 27 minutes. With this patch, > > removing 10000 block devices takes only 15 seconds. > > > > Signed-off-by: Mikulas Patocka <mpatocka@xxxxxxxxxx> > > > > --- > > mm/backing-dev.c | 2 +- > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > Index: linux-3.0-rc7-fast/mm/backing-dev.c > > =================================================================== > > --- linux-3.0-rc7-fast.orig/mm/backing-dev.c 2011-07-19 18:01:00.000000000 +0200 > > +++ linux-3.0-rc7-fast/mm/backing-dev.c 2011-07-19 18:01:07.000000000 +0200 > > @@ -505,7 +505,7 @@ static void bdi_remove_from_list(struct > > list_del_rcu(&bdi->bdi_list); > > spin_unlock_bh(&bdi_lock); > > > > - synchronize_rcu(); > > + synchronize_rcu_expedited(); > > } > > > > Urgh, I just noticed this crap in my tree.. You realize that what you're > effectively hammering a global sync primitive this way? Depending on > what RCU flavour you have any SMP variant will at least do a machine > wide IPI broadcast for every sync_rcu_exp(), some do significantly more. > > The much better solution would've been to batch your block-dev removals > and use a single sync_rcu as barrier. > > This is not cool. Indeed, synchronize_rcu_expedited() is quite heavyweight, so as Peter suggests, if you can use batching you will get even better performance with much less load on the rest of the system. Thanx, Paul -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
