On Fri 31-05-19 22:39:04, Minchan Kim wrote: > On Fri, May 31, 2019 at 10:47:52AM +0200, Michal Hocko wrote: > > On Fri 31-05-19 15:43:08, 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. > > > > > > Internally, it works via deactivating pages from active list to inactive's > > > head if the page is private because inactive list could be full of > > > used-once pages which are first candidate for the reclaiming and that's a > > > reason why MADV_FREE move pages to head of inactive LRU list. Therefore, > > > if the memory pressure happens, they will be reclaimed earlier than other > > > active pages unless there is no access until the time. > > > > [I am intentionally not looking at the implementation because below > > points should be clear from the changelog - sorry about nagging ;)] > > > > What kind of pages can be deactivated? Anonymous/File backed. > > Private/shared? If shared, are there any restrictions? > > Both file and private pages could be deactived from each active LRU > to each inactive LRU if the page has one map_count. In other words, > > if (page_mapcount(page) <= 1) > deactivate_page(page); Why do we restrict to pages that are single mapped? > > Are there any restrictions on mappings? E.g. what would be an effect of > > this operation on hugetlbfs mapping? > > VM_LOCKED|VM_HUGETLB|VM_PFNMAP vma will be skipped like MADV_FREE|DONTNEED OK documenting that this is restricted to the same vmas as MADV_FREE|DONTNEED is really useful to mention. > > > > > Also you are talking about inactive LRU but what kind of LRU is that? Is > > it the anonymous LRU? If yes, don't we have the same problem as with the > > active file page -> inactive file LRU > active anon page -> inacdtive anon LRU > > > early MADV_FREE implementation when enough page cache causes that > > deactivated anonymous memory doesn't get reclaimed anytime soon. Or > > worse never when there is no swap available? > > I think MADV_COLD is a little bit different symantic with MAVD_FREE. > MADV_FREE means it's okay to discard when the memory pressure because > the content of the page is *garbage*. Furthemore, freeing such pages is > almost zero overhead since we don't need to swap out and access > afterward causes minor fault. Thus, it would make sense to put those > freeable pages in inactive file LRU to compete other used-once pages. > > However, MADV_COLD doesn't means it's a garbage and freeing requires > swap out/swap in afterward. So, it would be better to move inactive > anon's LRU list, not file LRU. Furthermore, it would avoid unnecessary > scanning of those cold anonymous if system doesn't have a swap device. Please document this, if this is really a desirable semantic because then you have the same set of problems as we've had with the early MADV_FREE implementation mentioned above. -- Michal Hocko SUSE Labs
