On Mon, Jun 03, 2019 at 09:16:07AM +0200, Michal Hocko wrote: > On Fri 31-05-19 23:34:07, Minchan Kim wrote: > > On Fri, May 31, 2019 at 04:03:32PM +0200, Michal Hocko wrote: > > > 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? > > > > Because page table in one of process shared the page would have access bit > > so finally we couldn't reclaim the page. The more process it is shared, > > the more fail to reclaim. > > So what? In other words why should it be restricted solely based on the > map count. I can see a reason to restrict based on the access > permissions because we do not want to simplify all sorts of side channel > attacks but memory reclaim is capable of reclaiming shared pages and so > far I haven't heard any sound argument why madvise should skip those. > Again if there are any reasons, then document them in the changelog. I will go with removing the part so that defer to decision to the VM reclaim based on the review. > > [...] > > > > 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. > > > > IIRC, the problem of MADV_FREE was that we couldn't discard freeable > > pages because VM never scan anonymous LRU with swapless system. > > However, it's not the our case because we should reclaim them, not > > discarding. > > Right. But there is still the page cache reclaim. Is it expected that > an explicitly cold memory doesn't get reclaimed because we have a > sufficient amount of page cache (a very common case) and we never age > anonymous memory because of that? If there are lots of used-once pages in file-LRU, I think there is no need to reclaim anonymous pages because it needs bigger overhead due to IO. It has been true for a long time in current VM policy. Reclaim preference model based on hints is as following based on cost: MADV_DONTNEED >> MADV_PAGEOUT > used-once pages > MADV_FREE >= MADV_COLD It is desirable for the new hints to be placed in the reclaiming preference order such that a) they don't overlap functionally with existing hints and b) we have a balanced ordering of disruptive and non-disruptive hints.
