On Thu, 21 Jul 2011 10:28:10 +0200 Michal Hocko <mhocko@xxxxxxx> wrote: > If we fail to charge an allocation for a cgroup we usually have to fall > back into direct reclaim (mem_cgroup_hierarchical_reclaim). > The charging code, however, currently doesn't care about per-cpu charge > caches which might have up to (nr_cpus - 1) * CHARGE_BATCH pre charged > pages (the current cache is already drained, otherwise we wouldn't get > to mem_cgroup_do_charge). > That can be quite a lot on boxes with big amounts of CPUs so we can end > up reclaiming even though there are charges that could be used. This > will typically happen in a multi-threaded applications pined to many CPUs > which allocates memory heavily. > Do you have example and score, numbers on your test ? > Currently we are draining caches during reclaim > (mem_cgroup_hierarchical_reclaim) but this can be already late as we > could have already reclaimed from other groups in the hierarchy. > > The solution for this would be to synchronously drain charges early when > we fail to charge and retry the charge once more. > I think it still makes sense to keep async draining in the reclaim path > as it is used from other code paths as well (e.g. limit resize). It will > not do any work if we drained previously anyway. > > Signed-off-by: Michal Hocko <mhocko@xxxxxxx> I don't like this solution, at all. Assume 2 cpu SMP, (a special case), and 2 applications running under a memcg. - one is running in SCHED_FIFO. - another is running into mem_cgroup_do_charge() and call drain_all_stock_sync(). Then, the application stops until SCHED_FIFO application release the cpu. In general, I don't think waiting for schedule_work() against multiple cpus is not quicker than short memory reclaim. Adding flush_work() here means that a context switch is requred before calling direct reclaim. That's bad. (At leaset, please check __GFP_NOWAIT.) Please find another way, I think calling synchronous drain here is overkill. There are not important file caches in the most case and reclaim is quick. (And async draining runs.) How about automatically adjusting CHARGE_BATCH and make it small when the system is near to limit ? or flushing ->stock periodically ? Thanks, -Kame -- 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>