On Fri, Jul 03, 2020 at 09:27:19AM -0700, Shakeel Butt wrote: > On Fri, Jul 3, 2020 at 8:50 AM Roman Gushchin <guro@xxxxxx> wrote: > > > > On Fri, Jul 03, 2020 at 07:23:14AM -0700, Shakeel Butt wrote: > > > On Thu, Jul 2, 2020 at 11:35 PM Michal Hocko <mhocko@xxxxxxxxxx> wrote: > > > > > > > > On Thu 02-07-20 08:22:22, Shakeel Butt wrote: > > > > [...] > > > > > Interface options: > > > > > ------------------ > > > > > > > > > > 1) memcg interface e.g. 'echo 10M > memory.reclaim' > > > > > > > > > > + simple > > > > > + can be extended to target specific type of memory (anon, file, kmem). > > > > > - most probably restricted to cgroup v2. > > > > > > > > > > 2) fadvise(PAGEOUT) on cgroup_dir_fd > > > > > > > > > > + more general and applicable to other FSes (actually we are using > > > > > something similar for tmpfs). > > > > > + can be extended in future to just age the LRUs instead of reclaim or > > > > > some new use cases. > > > > > > > > Could you explain why memory.high as an interface to trigger pro-active > > > > memory reclaim is not sufficient. Also memory.low limit to protect > > > > latency sensitve workloads? > > > > I initially liked the proposal, but after some thoughts I've realized > > that I don't know a good use case where memory.high is less useful. > > Shakeel, what's the typical use case you thinking of? > > Who and how will use the new interface? > > > > > > > > Yes, we can use memory.high to trigger [proactive] reclaim in a memcg > > > but note that it can also introduce stalls in the application running > > > in that memcg. Let's suppose the memory.current of a memcg is 100MiB > > > and we want to reclaim 20MiB from it, we can set the memory.high to > > > 80MiB but any allocation attempt from the application running in that > > > memcg can get stalled/throttled. I want the functionality of the > > > reclaim without potential stalls. > > > > But reclaiming some pagecache/swapping out anon pages can always > > generate some stalls caused by pagefaults, no? > > > > Thanks for looking into the proposal. Let me answer both of your > questions together. I have added the two use-cases but let me explain > the proactive reclaim a bit more as we actually use that in our > production. > > We have defined tolerable refault rates for the applications based on > their type (latency sensitive or not). Proactive reclaim is triggered > in the application based on their current refault rates and usage. If > the current refault rate exceeds the tolerable refault rate then > stop/slowdown the proactive reclaim. > > For the second question, yes, each individual refault can induce the > stall as well but we have more control on that stall as compared to > stalls due to reclaim. For us almost all the reclaimable memory is > anon and we use compression based swap, so, the cost of each refault > is fixed and a couple of microseconds. > > I think the next question is what about the refaults from disk or > source with highly variable cost. Usually the latency sensitive > applications remove such uncertainty by mlocking the pages backed by > such backends (e.g. mlocking the executable) or at least that is the > case for us. Got it. It feels like you're suggesting something similar to memory.high with something similar to a different gfp flags. In other words, the difference is only which pages can be reclaimed and which not. I don't have a definitive answer here, but I wonder if we can somehow generalize the existing interface? E.g. if the problem is with artificially induced delays, we can have a config option/sysctl/sysfs knob/something else which would disable it. Otherwise we risk ending up with many different kinds of soft memory limits. Thanks!
