On Thu, Aug 24, 2017 at 04:13:37PM +0200, Michal Hocko wrote: > On Thu 24-08-17 14:58:42, Roman Gushchin wrote: > > On Thu, Aug 24, 2017 at 02:58:11PM +0200, Michal Hocko wrote: > > > On Thu 24-08-17 13:28:46, Roman Gushchin wrote: > > > > Hi Michal! > > > > > > > There is nothing like a "better victim". We are pretty much in a > > > catastrophic situation when we try to survive by killing a userspace. > > > > Not necessary, it can be a cgroup OOM. > > memcg OOM is no different. The catastrophic is scoped to the specific > hierarchy but tasks in that hierarchy still fail to make a further > progress. > > > > We try to kill the largest because that assumes that we return the > > > most memory from it. Now I do understand that you want to treat the > > > memcg as a single killable entity but I find it really questionable > > > to do a per-memcg metric and then do not treat it like that and kill > > > only a single task. Just imagine a single memcg with zillions of taks > > > each very small and you select it as the largest while a small taks > > > itself doesn't help to help to get us out of the OOM. > > > > I don't think it's different from a non-containerized state: if you > > have a zillion of small tasks in the system, you'll meet the same issues. > > Yes this is possible but usually you are comparing apples to apples so > you will kill the largest offender and then go on. To be honest I really > do hate how we try to kill a children rather than the selected victim > for the same reason. I do hate it too. > > > > > > I guess I have asked already and we haven't reached any consensus. I do > > > > > not like how you treat memcgs and tasks differently. Why cannot we have > > > > > a memcg score a sum of all its tasks? > > > > > > > > It sounds like a more expensive way to get almost the same with less accuracy. > > > > Why it's better? > > > > > > because then you are comparing apples to apples? > > > > Well, I can say that I compare some number of pages against some other number > > of pages. And the relation between a page and memcg is more obvious, than a > > relation between a page and a process. > > But you are comparing different accounting systems. > > > Both ways are not ideal, and sum of the processes is not ideal too. > > Especially, if you take oom_score_adj into account. Will you respect it? > > Yes, and I do not see any reason why we shouldn't. It makes things even more complicated. Right now task's oom_score can be in (~ -total_memory, ~ +2*total_memory) range, and it you're starting summing it, it can be multiplied by number of tasks... Weird. It also will be different in case of system and memcg-wide OOM. > > > I've started actually with such approach, but then found it weird. > > > > > Besides that you have > > > to check each task for over-killing anyway. So I do not see any > > > performance merits here. > > > > It's an implementation detail, and we can hopefully get rid of it at some point. > > Well, we might do some estimations and ignore oom scopes but I that > sounds really complicated and error prone. Unless we have anything like > that then I would start from tasks and build up the necessary to make a > decision at the higher level. Seriously speaking, do you have an example, when summing per-process oom_score will work better? Especially, if we're talking about customizing oom_score calculation, it makes no sence to me. How you will sum process timestamps? > > > > > > How do you want to compare memcg score with tasks score? > > > > > > > > I have to do it for tasks in root cgroups, but it shouldn't be a common case. > > > > > > How come? I can easily imagine a setup where only some memcgs which > > > really do need a kill-all semantic while all others can live with single > > > task killed perfectly fine. > > > > I mean taking a unified cgroup hierarchy into an account, there should not > > be lot of tasks in the root cgroup, if any. > > Is that really the case? I would assume that memory controller would be > enabled only in those subtrees which really use the functionality and > the rest will be sitting in the root memcg. It might be the case if you > are running only containers but I am not really sure this is true in > general. Agree. -- To unsubscribe from this list: send the line "unsubscribe cgroups" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
