On Tue, Jan 24, 2012 at 12:43 AM, Michal Hocko <mhocko@xxxxxxx> wrote: > On Mon 23-01-12 14:05:33, Ying Han wrote: >> On Wed, Jan 18, 2012 at 3:53 PM, KAMEZAWA Hiroyuki >> <kamezawa.hiroyu@xxxxxxxxxxxxxx> wrote: >> > On Wed, 18 Jan 2012 11:47:03 +0100 >> > Michal Hocko <mhocko@xxxxxxx> wrote: >> > >> >> On Wed 18-01-12 09:12:26, KAMEZAWA Hiroyuki wrote: >> >> > On Tue, 17 Jan 2012 17:46:05 +0100 >> >> > Michal Hocko <mhocko@xxxxxxx> wrote: >> >> > >> >> > > On Fri 13-01-12 17:40:19, KAMEZAWA Hiroyuki wrote: >> >> [...] >> >> > > > This patch removes PCG_MOVE_LOCK and add hashed rwlock array >> >> > > > instead of it. This works well enough. Even when we need to >> >> > > > take the lock, >> >> > > >> >> > > Hmmm, rwlocks are not popular these days very much. >> >> > > Anyway, can we rather make it (source) memcg (bit)spinlock instead. We >> >> > > would reduce false sharing this way and would penalize only pages from >> >> > > the moving group. >> >> > > >> >> > per-memcg spinlock ? >> >> >> >> Yes >> >> >> >> > The reason I used rwlock() is to avoid disabling IRQ. This routine >> >> > will be called by IRQ context (for dirty ratio support). So, IRQ >> >> > disable will be required if we use spinlock. >> >> >> >> OK, I have missed the comment about disabling IRQs. It's true that we do >> >> not have to be afraid about deadlocks if the lock is held only for >> >> reading from the irq context but does the spinlock makes a performance >> >> bottleneck? We are talking about the slowpath. >> >> I could see the reason for the read lock when doing hashed locks because >> >> they are global but if we make the lock per memcg then we shouldn't >> >> interfere with other updates which are not blocked by the move. >> >> >> > >> > Hm, ok. In the next version, I'll use per-memcg spinlock (with hash if necessary) >> >> Just want to make sure I understand it, even we make the lock >> per-memcg, there is still a false sharing of pc within one memcg. > > Yes that is true. I have missed that we might fault in several pages at > once but this would happen only during task move, right? And that is not > a hot path anyway. Or? I was thinking of page-statistics update which is hot path. If the moving task and non-moving task share the same per-memcg lock, any page-statistic update from the non-moving task will be blocked? Sorry If i missed something here :) > >> Do we need to demonstrate the effect ? >> >> Also, I don't get the point of why spinlock instead of rwlock in this case? > > spinlock provides a fairness while with rwlocks might lead to > starvation. that is true. --Ying > > > -- > Michal Hocko > SUSE Labs > SUSE LINUX s.r.o. > Lihovarska 1060/12 > 190 00 Praha 9 > Czech Republic -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href