On Tue 03-03-20 16:47:54, Huang, Ying wrote: > Michal Hocko <mhocko@xxxxxxxxxx> writes: > > > On Tue 03-03-20 09:51:56, Huang, Ying wrote: > >> Mel Gorman <mgorman@xxxxxxx> writes: > >> > On Mon, Mar 02, 2020 at 07:23:12PM +0800, Huang, Ying wrote: > >> >> If some applications cannot tolerate the latency incurred by the memory > >> >> allocation and zeroing. Then we cannot discard instead of migrate > >> >> always. While in some situations, less memory pressure can help. So > >> >> it's better to let the administrator and the application choose the > >> >> right behavior in the specific situation? > >> >> > >> > > >> > Is there an application you have in mind that benefits from discarding > >> > MADV_FREE pages instead of migrating them? > >> > > >> > Allowing the administrator or application to tune this would be very > >> > problematic. An application would require an update to the system call > >> > to take advantage of it and then detect if the running kernel supports > >> > it. An administrator would have to detect that MADV_FREE pages are being > >> > prematurely discarded leading to a slowdown and that is hard to detect. > >> > It could be inferred from monitoring compaction stats and checking > >> > if compaction activity is correlated with higher minor faults in the > >> > target application. Proving the correlation would require using the perf > >> > software event PERF_COUNT_SW_PAGE_FAULTS_MIN and matching the addresses > >> > to MADV_FREE regions that were freed prematurely. That is not an obvious > >> > debugging step to take when an application detects latency spikes. > >> > > >> > Now, you could add a counter specifically for MADV_FREE pages freed for > >> > reasons other than memory pressure and hope the administrator knows about > >> > the counter and what it means. That type of knowledge could take a long > >> > time to spread so it's really very important that there is evidence of > >> > an application that suffers due to the current MADV_FREE and migration > >> > behaviour. > >> > >> OK. I understand that this patchset isn't a universal win, so we need > >> some way to justify it. I will try to find some application for that. > >> > >> Another thought, as proposed by David Hildenbrand, it's may be a > >> universal win to discard clean MADV_FREE pages when migrating if there are > >> already memory pressure on the target node. For example, if the free > >> memory on the target node is lower than high watermark? > > > > This is already happening because if the target node is short on memory > > it will start to reclaim and if MADV_FREE pages are at the tail of > > inactive file LRU list then they will be dropped. Please note how that > > follows proper aging and doesn't introduce any special casing. Really > > MADV_FREE is an inactive cache for anonymous memory and we treat it like > > inactive page cache. This is not carved in stone of course but it really > > requires very good justification to change. > > If my understanding were correct, the newly migrated clean MADV_FREE > pages will be put at the head of inactive file LRU list instead of the > tail. So it's possible that some useful file cache pages will be > reclaimed. This is the case also when you migrate other pages, right? We simply cannot preserve the aging. -- Michal Hocko SUSE Labs
