On Tue 13-09-11 13:03:01, Johannes Weiner wrote: > On Tue, Sep 13, 2011 at 07:27:59PM +0900, KAMEZAWA Hiroyuki wrote: > > On Mon, 12 Sep 2011 12:57:21 +0200 > > Johannes Weiner <jweiner@xxxxxxxxxx> wrote: > > > > > Memory cgroup limit reclaim currently picks one memory cgroup out of > > > the target hierarchy, remembers it as the last scanned child, and > > > reclaims all zones in it with decreasing priority levels. > > > > > > The new hierarchy reclaim code will pick memory cgroups from the same > > > hierarchy concurrently from different zones and priority levels, it > > > becomes necessary that hierarchy roots not only remember the last > > > scanned child, but do so for each zone and priority level. > > > > > > Furthermore, detecting full hierarchy round-trips reliably will become > > > crucial, so instead of counting on one iterator site seeing a certain > > > memory cgroup twice, use a generation counter that is increased every > > > time the child with the highest ID has been visited. > > > > > > Signed-off-by: Johannes Weiner <jweiner@xxxxxxxxxx> > > > > I cannot image how this works. could you illustrate more with easy example ? > > Previously, we did > > mem = mem_cgroup_iter(root) > for each priority level: > for each zone in zonelist: > > and this would reclaim memcg-1-zone-1, memcg-1-zone-2, memcg-1-zone-3 > etc. > > The new code does > > for each priority level > for each zone in zonelist > mem = mem_cgroup_iter(root) > > but with a single last_scanned_child per memcg, this would scan > memcg-1-zone-1, memcg-2-zone-2, memcg-3-zone-3 etc, which does not > make much sense. > > Now imagine two reclaimers. With the old code, the first reclaimer > would pick memcg-1 and scan all its zones, the second reclaimer would > pick memcg-2 and reclaim all its zones. Without this patch, the first > reclaimer would pick memcg-1 and scan zone-1, the second reclaimer > would pick memcg-2 and scan zone-1, then the first reclaimer would > pick memcg-3 and scan zone-2. If the reclaimers are concurrently > scanning at different priority levels, things are even worse because > one reclaimer may put much more force on the memcgs it gets from > mem_cgroup_iter() than the other reclaimer. They must not share the > same iterator. > > The generations are needed because the old algorithm did not rely too > much on detecting full round-trips. After every reclaim cycle, it > checked the limit and broke out of the loop if enough was reclaimed, > no matter how many children were reclaimed from. The new algorithm is > used for global reclaim, where the only exit condition of the > hierarchy reclaim is the full roundtrip, because equal pressure needs > to be applied to all zones. Could you fold this into the patch description, please? -- 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=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>