On Thu 01-08-19 14:00:51, Yang Shi wrote: > On Mon, Jul 29, 2019 at 11:48 AM Michal Hocko <mhocko@xxxxxxxxxx> 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. I can imagine a middle ground where the background reclaim would have to be an opt-in feature and a dedicated kernel thread would be assigned to the particular memcg (hierarchy). -- Michal Hocko SUSE Labs
