On Thu 03-01-19 10:40:54, Yang Shi wrote: > > > On 1/3/19 10:13 AM, Michal Hocko wrote: > > On Thu 03-01-19 09:33:14, Yang Shi wrote: > > > > > > On 1/3/19 2:12 AM, Michal Hocko wrote: > > > > On Thu 03-01-19 04:05:30, Yang Shi wrote: > > > > > Currently, force empty reclaims memory synchronously when writing to > > > > > memory.force_empty. It may take some time to return and the afterwards > > > > > operations are blocked by it. Although it can be interrupted by signal, > > > > > it still seems suboptimal. > > > > Why it is suboptimal? We are doing that operation on behalf of the > > > > process requesting it. What should anybody else pay for it? In other > > > > words why should we hide the overhead? > > > Please see the below explanation. > > > > > > > > Now css offline is handled by worker, and the typical usecase of force > > > > > empty is before memcg offline. So, handling force empty in css offline > > > > > sounds reasonable. > > > > Hmm, so I guess you are talking about > > > > echo 1 > $MEMCG/force_empty > > > > rmdir $MEMCG > > > > > > > > and you are complaining that the operation takes too long. Right? Why do > > > > you care actually? > > > We have some usecases which create and remove memcgs very frequently, and > > > the tasks in the memcg may just access the files which are unlikely accessed > > > by anyone else. So, we prefer force_empty the memcg before rmdir'ing it to > > > reclaim the page cache so that they don't get accumulated to incur > > > unnecessary memory pressure. Since the memory pressure may incur direct > > > reclaim to harm some latency sensitive applications. > > Yes, this makes sense to me. > > > > > And, the create/remove might be run in a script sequentially (there might be > > > a lot scripts or applications are run in parallel to do this), i.e. > > > mkdir cg1 > > > do something > > > echo 0 > cg1/memory.force_empty > > > rmdir cg1 > > > > > > mkdir cg2 > > > ... > > > > > > The creation of the afterwards memcg might be blocked by the force_empty for > > > long time if there are a lot page caches, so the overall throughput of the > > > system may get hurt. > > Is there any reason for your scripts to be strictly sequential here? In > > other words why cannot you offload those expensive operations to a > > detached context in _userspace_? > > I would say it has not to be strictly sequential. The above script is just > an example to illustrate the pattern. But, sometimes it may hit such pattern > due to the complicated cluster scheduling and container scheduling in the > production environment, for example the creation process might be scheduled > to the same CPU which is doing force_empty. I have to say I don't know too > much about the internals of the container scheduling. In that case I do not see a strong reason to implement the offloding into the kernel. It is an additional code and semantic to maintain. I think it is more important to discuss whether we want to introduce force_empty in cgroup v2. -- Michal Hocko SUSE Labs
