On Thu, Jul 11, 2019 at 10:25:25AM +0900, Minchan Kim wrote: > When a process expects no accesses to a certain memory range, it could > give a hint to kernel that the pages can be reclaimed when memory pressure > happens but data should be preserved for future use. This could reduce > workingset eviction so it ends up increasing performance. > > This patch introduces the new MADV_COLD hint to madvise(2) syscall. > MADV_COLD can be used by a process to mark a memory range as not expected > to be used in the near future. The hint can help kernel in deciding which > pages to evict early during memory pressure. > > It works for every LRU pages like MADV_[DONTNEED|FREE]. IOW, It moves > > active file page -> inactive file LRU > active anon page -> inacdtive anon LRU > > Unlike MADV_FREE, it doesn't move active anonymous pages to inactive > file LRU's head because MADV_COLD is a little bit different symantic. > MADV_FREE means it's okay to discard when the memory pressure because > the content of the page is *garbage* so freeing such pages is almost zero > overhead since we don't need to swap out and access afterward causes just > minor fault. Thus, it would make sense to put those freeable pages in > inactive file LRU to compete other used-once pages. It makes sense for > implmentaion point of view, too because it's not swapbacked memory any > longer until it would be re-dirtied. Even, it could give a bonus to make > them be reclaimed on swapless system. However, MADV_COLD doesn't mean > garbage so reclaiming them requires swap-out/in in the end so it's bigger > cost. Since we have designed VM LRU aging based on cost-model, anonymous > cold pages would be better to position inactive anon's LRU list, not file > LRU. Furthermore, it would help to avoid unnecessary scanning if system > doesn't have a swap device. Let's start simpler way without adding > complexity at this moment. However, keep in mind, too that it's a caveat > that workloads with a lot of pages cache are likely to ignore MADV_COLD > on anonymous memory because we rarely age anonymous LRU lists. > > * man-page material > > MADV_COLD (since Linux x.x) > > Pages in the specified regions will be treated as less-recently-accessed > compared to pages in the system with similar access frequencies. > In contrast to MADV_FREE, the contents of the region are preserved > regardless of subsequent writes to pages. > > MADV_COLD cannot be applied to locked pages, Huge TLB pages, or VM_PFNMAP > pages. > > * v2 > * add up the warn with lots of page cache workload - mhocko > * add man page stuff - dave > > * v1 > * remove page_mapcount filter - hannes, mhocko > * remove idle page handling - joelaf > > * RFCv2 > * add more description - mhocko > > * RFCv1 > * renaming from MADV_COOL to MADV_COLD - hannes > > * internal review > * use clear_page_youn in deactivate_page - joelaf > * Revise the description - surenb > * Renaming from MADV_WARM to MADV_COOL - surenb > > Acked-by: Michal Hocko <mhocko@xxxxxxxx> > Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx> Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>
