From mboxrd@z Thu Jan 1 00:00:00 1970 From: Michal Hocko Subject: Re: [PATCH RFC] mm/memcontrol: reclaim severe usage over high limit in get_user_pages loop Date: Fri, 2 Aug 2019 11:35:07 +0200 Message-ID: <20190802093507.GF6461@dhcp22.suse.cz> References: <156431697805.3170.6377599347542228221.stgit@buzz> <20190729091738.GF9330@dhcp22.suse.cz> <3d6fc779-2081-ba4b-22cf-be701d617bb4@yandex-team.ru> <20190729103307.GG9330@dhcp22.suse.cz> <20190729184850.GH9330@dhcp22.suse.cz> Mime-Version: 1.0 Return-path: Content-Disposition: inline In-Reply-To: Sender: linux-kernel-owner@vger.kernel.org List-ID: Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Yang Shi Cc: Konstantin Khlebnikov , Linux MM , Linux Kernel Mailing List , cgroups@vger.kernel.org, Vladimir Davydov , Johannes Weiner On Thu 01-08-19 14:00:51, Yang Shi wrote: > On Mon, Jul 29, 2019 at 11:48 AM Michal Hocko 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