On Fri, Apr 20, 2012 at 12:33:18AM +0200, Johannes Weiner wrote: > On Thu, Apr 19, 2012 at 10:47:27AM -0700, Ying Han wrote: > > 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. > > I'd argue, given the history of cgroups, one of the main purposes is > having a machine of containers where you overcommit their hard limit > and set the soft limit accordingly to provide fairness. > > Yes, we don't want to reclaim hierarchies that are below their soft > limit as long as there are some in excess, of course. This is a flaw > and needs fixing. But it's something completely different than > changing how the soft limit is defined and suddenly allow child > groups, which you may not trust, to override rules defined by parental > groups. > > It bothers me that we should add something that will almost certainly > bite us in the future while we are discussing on the cgroups list what > would stand in the way of getting sane hierarchy semantics across > controllers to provide consistency, nesting, etc. > > To support a single use case, which I feel we still have not discussed > nearly enough to justify this change. > > For example, I get that you want 'meta-groups' that group together > subgroups for common accounting and hard limiting. But I don't see > why such meta-groups have their own processes. Conceptually, I mean, > how does a process fit into A? Is it superior to the tasks in A1 and > A2? Why can't it live in A3? > > So here is a proposal: > > Would it make sense to try to keep those meta groups always free of > their own memory so that they don't /need/ soft limits with weird > semantics? E.g. immediately free the unused memory on rmdir, OR add > mechanisms to migrate the memory to a dedicated group: > > A > A1 (soft-limited) > A2 (soft-limited) > B > unused (soft-limited) > > Move all leftover memory from finished jobs to this 'unused' group. > You could set its soft limit to 0 so that it sticks around only until > you actually need the memory for something else. > > Then you would get the benefits of accounting and limiting A1 and A2 > under a single umbrella without the need for a soft limit in A. We > could keep the consistent semantics for soft limits, because you would > only have to set it on leaf nodes. > > Wouldn't this work for you? Or, if the frequency of job creation and completion permits, just keep the original groups around after completion, set their soft limit to 0, put a watch ("threshold notification") on its usage and reap it when global pressure finally cleaned it out. -- 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=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>