On Mon 05-08-19 20:28:40, Yang Shi wrote: > On Mon, Aug 5, 2019 at 7:32 AM Michal Hocko <mho...@kernel.org> wrote: > > > > On Fri 02-08-19 11:56:28, Yang Shi wrote: > > > On Fri, Aug 2, 2019 at 2:35 AM Michal Hocko <mho...@kernel.org> wrote: > > > > > > > > On Thu 01-08-19 14:00:51, Yang Shi wrote: > > > > > On Mon, Jul 29, 2019 at 11:48 AM Michal Hocko <mho...@kernel.org> > > > > > wrote: > > > > > > > > > > > > On Mon 29-07-19 10:28:43, Yang Shi wrote: > > > > > > [...] > > > > > > > I don't worry too much about scale since the scale issue is not > > > > > > > unique > > > > > > > to background reclaim, direct reclaim may run into the same > > > > > > > problem. > > > > > > > > > > > > Just to clarify. By scaling problem I mean 1:1 kswapd thread to > > > > > > memcg. > > > > > > You can have thousands of memcgs and I do not think we really do > > > > > > want > > > > > > to create one kswapd for each. Once we have a kswapd thread pool > > > > > > then we > > > > > > get into a tricky land where a determinism/fairness would be non > > > > > > trivial > > > > > > to achieve. Direct reclaim, on the other hand is bound by the > > > > > > workload > > > > > > itself. > > > > > > > > > > Yes, I agree thread pool would introduce more latency than dedicated > > > > > kswapd thread. But, it looks not that bad in our test. When memory > > > > > allocation is fast, even though dedicated kswapd thread can't catch > > > > > up. So, such background reclaim is best effort, not guaranteed. > > > > > > > > > > I don't quite get what you mean about fairness. Do you mean they may > > > > > spend excessive cpu time then cause other processes starvation? I > > > > > think this could be mitigated by properly organizing and setting > > > > > groups. But, I agree this is tricky. > > > > > > > > No, I meant that the cost of reclaiming a unit of charges (e.g. > > > > SWAP_CLUSTER_MAX) is not constant and depends on the state of the memory > > > > on LRUs. Therefore any thread pool mechanism would lead to unfair > > > > reclaim and non-deterministic behavior. > > > > > > Yes, the cost depends on the state of pages, but I still don't quite > > > understand what does "unfair" refer to in this context. Do you mean > > > some cgroups may reclaim much more than others? > > > > > Or the work may take too long so it can't not serve other cgroups in time? > > > > exactly. > > Actually, I'm not very concerned by this. In our design each memcg has > its dedicated work (memcg->wmark_work), so the reclaim work for > different memcgs could be run in parallel since they are *different* > work in fact although they run the same function. And, We could queue > them to a dedicated unbound workqueue which may have maximum 512 or > scale with nr cpus active works. Although the system may have > thousands of online memcgs, I'm supposed it should be rare to have all > of them trigger reclaim at the same time.
I do believe that it might work for your particular usecase but I do not think this is robust enough for the upstream kernel, I am afraid. As I've said I am open to discuss an opt-in per memcg pro-active reclaim (a kernel thread that belongs to the memcg) but it has to be a dedicated worker bound by all the cgroup resource restrictions. -- Michal Hocko SUSE Labs
