On Thu, Apr 19, 2012 at 10:04 AM, Michal Hocko <mhocko@xxxxxxx> wrote: > On Wed 18-04-12 11:00:40, Ying Han wrote: >> On Wed, Apr 18, 2012 at 5:24 AM, Johannes Weiner <hannes@xxxxxxxxxxx> wrote: >> > On Tue, Apr 17, 2012 at 09:37:46AM -0700, Ying Han wrote: >> >> The "soft_limit" was introduced in memcg to support over-committing the >> >> memory resource on the host. Each cgroup configures its "hard_limit" where >> >> it will be throttled or OOM killed by going over the limit. However, the >> >> cgroup can go above the "soft_limit" as long as there is no system-wide >> >> memory contention. So, the "soft_limit" is the kernel mechanism for >> >> re-distributing system spare memory among cgroups. >> >> >> >> This patch reworks the softlimit reclaim by hooking it into the new global >> >> reclaim scheme. So the global reclaim path including direct reclaim and >> >> background reclaim will respect the memcg softlimit. >> >> >> >> v3..v2: >> >> 1. rebase the patch on 3.4-rc3 >> >> 2. squash the commits of replacing the old implementation with new >> >> implementation into one commit. This is to make sure to leave the tree >> >> in stable state between each commit. >> >> 3. removed the commit which changes the nr_to_reclaim for global reclaim >> >> case. The need of that patch is not obvious now. >> >> >> >> Note: >> >> 1. the new implementation of softlimit reclaim is rather simple and first >> >> step for further optimizations. there is no memory pressure balancing between >> >> memcgs for each zone, and that is something we would like to add as follow-ups. >> >> >> >> 2. this patch is slightly different from the last one posted from Johannes >> >> http://comments.gmane.org/gmane.linux.kernel.mm/72382 >> >> where his patch is closer to the reverted implementation by doing hierarchical >> >> reclaim for each selected memcg. However, that is not expected behavior from >> >> user perspective. Considering the following example: >> >> >> >> root (32G capacity) >> >> --> A (hard limit 20G, soft limit 15G, usage 16G) >> >> --> A1 (soft limit 5G, usage 4G) >> >> --> A2 (soft limit 10G, usage 12G) >> >> --> B (hard limit 20G, soft limit 10G, usage 16G) >> >> >> >> Under global reclaim, we shouldn't add pressure on A1 although its parent(A) >> >> exceeds softlimit. This is what admin expects by setting softlimit to the >> >> actual working set size and only reclaim pages under softlimit if system has >> >> trouble to reclaim. >> > >> > Actually, this is exactly what the admin expects when creating a >> > hierarchy, because she defines that A1 is a child of A and is >> > responsible for the memory situation in its parent. > > Hmm, I guess that both approaches have cons and pros. > * Hierarchical soft limit reclaim - reclaim the whole subtree of the over > soft limit memcg > + it is consistent with the hard limit reclaim Not sure why we want them to be consistent. Soft_limit is serving different purpose and the one of the main purpose is to preserve the working set of the cgroup. > + easier for top to bottom configuration - especially when you allow > subgroups to create deeper hierarchies. Does anybody do that? As far as I heard, most (if not all) are using flat configuration where everything is running under root. > - harder to set up if soft limit should act as a guarantee - might lead > to an unexpected reclaim. > > * Targeted soft limit reclaim - only reclaim LRUs of over limit memcgs > + easier to set up for the working set guarantee because admin can focus > on the working set of a single group and not the whole hierarchy This is true. > - easier to construct soft unreclaimable hierarchies - whole subtree > contributes but nobody wants to take the responsibility when we reach > the limit. > > Both approaches don't play very well with the default 0 limit because we > either reclaim unless we set up the whole hierarchy properly or we just > burn cycles by trying to reclaim groups wit no or only few pages. Setting the default to 0 is a good optimization which makes everybody to be eligible for reclaim if admin doesn't do anything. In reality, if admin want to preserve working set of cgroups and he/she has to set the softlimit. By doing that, it is easier to only focus on the cgroup itself without looking up its ancestors. > The second approach leads to more expected results though because we do > not touch "leaf" groups unless they are over limit. > I have to think about that some more but it seems that the second approach > is much easier to implement and matches the "guarantee" expectations > more. Agree. > I guess we could converge both approaches if we could reclaim from the > leaf groups upwards to the root but I didn't think about this very much. That is what the current patch does, which only consider softlimit under global pressure :) --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. 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