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Date: Wed, 18 Jan 2012 16:13:13 +0100 From: Michal Hocko <mhocko@...e.cz> To: Hillf Danton <dhillf@...il.com> Cc: linux-mm@...ck.org, Johannes Weiner <hannes@...xchg.org>, KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>, Hugh Dickins <hughd@...gle.com>, Andrew Morton <akpm@...ux-foundation.org>, LKML <linux-kernel@...r.kernel.org>, Balbir Singh <bsingharora@...il.com> Subject: Re: [PATCH] mm: memcg: remove checking reclaim order in soft limit reclaim On Wed 18-01-12 22:01:57, Hillf Danton wrote: > On Wed, Jan 18, 2012 at 9:40 PM, Michal Hocko <mhocko@...e.cz> wrote: > > On Wed 18-01-12 20:30:41, Hillf Danton wrote: > >> On Tue, Jan 17, 2012 at 10:07 PM, Michal Hocko <mhocko@...e.cz> wrote: > >> > On Tue 17-01-12 21:29:52, Hillf Danton wrote: > >> >> On Tue, Jan 17, 2012 at 9:16 PM, Michal Hocko <mhocko@...e.cz> wrote: > >> >> > Hi, > >> >> > > >> >> > On Tue 17-01-12 20:47:59, Hillf Danton wrote: > >> >> >> If async order-O reclaim expected here, it is settled down when setting up scan > >> >> >> control, with scan priority hacked to be zero. Other than that, deny of reclaim > >> >> >> should be removed. > >> >> > > >> >> > Maybe I have misunderstood you but this is not right. The check is to > >> >> > protect from the _global_ reclaim with order > 0 when we prevent from > >> >> > memcg soft reclaim. > >> >> > > >> >> need to bear mm hog in this way? > >> > > >> > Could you be more specific? Are you trying to fix any particular > >> > problem? > >> > > >> My thought is simple, the outcome of softlimit reclaim depends little on the > >> value of reclaim order, zero or not, and only exceeding is reclaimed, so > >> selective response to swapd's request is incorrect. > > > > OK, got your point, finally. Let's add Balbir (the proposed patch can > > be found at https://lkml.org/lkml/2012/1/17/166) to the CC list because > > this seems to be a design decision. > > > > I always thought that this is because we want non-userspace (high order) > > mem pressure to be handled by the global reclaim only. And it makes some > > sense to me because it is little bit strange to reclaim for order-0 > > while the request is for an higher order. I guess this might lead to an > > extensive and pointless reclaiming because we might end up with many > > free pages which cannot satisfy higher order allocation. > > > > On the other hand, it is true that the documentation says that the soft > > limit is considered when "the system detects memory contention or low > > memory" which doesn't say that the contention comes from memcg accounted > > memory. > > > > Anyway this changes the current behavior so it would better come with > > much better justification which shows that over reclaim doesn't happen > > and that we will not see higher latencies with higher order allocations. > > > > As the function shows, the checked reclaim order is not used, but the > scan control is prepared with order(= 0), which is called async order-0 > reclaim in my tern, then your worries on over reclaim and higher latencies > could be removed, I think 8-) Not really. My concern was that memcg will reclaim for order-0 while the kswapd reclaims for order-N so after we reclaimed something from cgroups we finally start reclaiming for order-N. -- Michal Hocko SUSE Labs SUSE LINUX s.r.o. Lihovarska 1060/12 190 00 Praha 9 Czech Republic -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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