Michal Hocko <mhocko@xxxxxxxxxx> writes: > 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. So you consider the priority of the clean MADV_FREE pages is same as that of page cache pages? Because the penalty difference is so large, I think it may be a good idea to always put clean MADV_FREE pages at the tail of the inactive file LRU list? Best Regards, Huang, Ying
