On Mon 22-02-21 11:48:37, Tim Chen wrote: > > > On 2/22/21 11:09 AM, Michal Hocko wrote: > > >> > >> I actually have tried adjusting the threshold but found that it doesn't work well for > >> the case with unenven memory access frequency between cgroups. The soft > >> limit for the low memory event cgroup could creep up quite a lot, exceeding > >> the soft limit by hundreds of MB, even > >> if I drop the SOFTLIMIT_EVENTS_TARGET from 1024 to something like 8. > > > > What was the underlying reason? Higher order allocations? > > > > Not high order allocation. > > The reason was because the run away memcg asks for memory much less often, compared > to the other memcgs in the system. So it escapes the sampling update and > was not put onto the tree and exceeds the soft limit > pretty badly. Even if it was put onto the tree and gets page reclaimed below the > limit, it could escape the sampling the next time it exceeds the soft limit. I am sorry but I really do not follow. Maybe I am missing something obvious but the the rate of events (charge/uncharge) shouldn't be really important. There is no way to exceed the limit without charging memory (either a new or via task migration in v1 and immigrate_on_move). If you have SOFTLIMIT_EVENTS_TARGET 8 then you should be 128 * 8 events to re-evaluate. Huge pages can make the runaway much bigger but how it would be possible to runaway outside of that bound. Btw. do we really need SOFTLIMIT_EVENTS_TARGET at all? Why cannot we just stick with a single threshold? mem_cgroup_update_tree can be made a effectivelly a noop when there is no soft limit in place so overhead shouldn't matter for the vast majority of workloads. -- Michal Hocko SUSE Labs
