On Thu, Feb 02, 2017 at 02:14:10PM +0900, Minchan Kim wrote: > Hi Johannes, > > On Tue, Jan 31, 2017 at 04:38:10PM -0500, Johannes Weiner wrote: > > On Tue, Jan 31, 2017 at 11:45:47AM -0800, Shaohua Li wrote: > > > On Tue, Jan 31, 2017 at 01:59:49PM -0500, Johannes Weiner wrote: > > > > Hi Shaohua, > > > > > > > > On Sun, Jan 29, 2017 at 09:51:17PM -0800, Shaohua Li wrote: > > > > > We are trying to use MADV_FREE in jemalloc. Several issues are found. Without > > > > > solving the issues, jemalloc can't use the MADV_FREE feature. > > > > > - Doesn't support system without swap enabled. Because if swap is off, we can't > > > > > or can't efficiently age anonymous pages. And since MADV_FREE pages are mixed > > > > > with other anonymous pages, we can't reclaim MADV_FREE pages. In current > > > > > implementation, MADV_FREE will fallback to MADV_DONTNEED without swap enabled. > > > > > But in our environment, a lot of machines don't enable swap. This will prevent > > > > > our setup using MADV_FREE. > > > > > - Increases memory pressure. page reclaim bias file pages reclaim against > > > > > anonymous pages. This doesn't make sense for MADV_FREE pages, because those > > > > > pages could be freed easily and refilled with very slight penality. Even page > > > > > reclaim doesn't bias file pages, there is still an issue, because MADV_FREE > > > > > pages and other anonymous pages are mixed together. To reclaim a MADV_FREE > > > > > page, we probably must scan a lot of other anonymous pages, which is > > > > > inefficient. In our test, we usually see oom with MADV_FREE enabled and nothing > > > > > without it. > > > > > > > > Fully agreed, the anon LRU is a bad place for these pages. > > > > > > > > > For the first two issues, introducing a new LRU list for MADV_FREE pages could > > > > > solve the issues. We can directly reclaim MADV_FREE pages without writting them > > > > > out to swap, so the first issue could be fixed. If only MADV_FREE pages are in > > > > > the new list, page reclaim can easily reclaim such pages without interference > > > > > of file or anonymous pages. The memory pressure issue will disappear. > > > > > > > > Do we actually need a new page flag and a special LRU for them? These > > > > pages are basically like clean cache pages at that point. What do you > > > > think about clearing their PG_swapbacked flag on MADV_FREE and moving > > > > them to the inactive file list? The way isolate+putback works should > > > > not even need much modification, something like clear_page_mlock(). > > > > > > > > When the reclaim scanner finds anon && dirty && !swapbacked, it can > > > > again set PG_swapbacked and goto keep_locked to move the page back > > > > into the anon LRU to get reclaimed according to swapping rules. > > > > > > Interesting idea! Not sure though, the MADV_FREE pages are actually anonymous > > > pages, this will introduce confusion. On the other hand, if the MADV_FREE pages > > > are mixed with inactive file pages, page reclaim need to reclaim a lot of file > > > pages first before reclaim the MADV_FREE pages. This doesn't look good. The > > > point of a separate LRU is to avoid scan other anon/file pages. > > > > The LRU code and the rest of VM already use independent page type > > distinctions. That's because shmem pages are !PageAnon - they have a > > page->mapping that points to a real address space, not an anon_vma - > > but they are swapbacked and thus go through the anon LRU. This would > > just do the reverse: put PageAnon pages on the file LRU when they > > don't contain valid data and are thus not swapbacked. > > > > As far as mixing with inactive file pages goes, it'd be possible to > > link the MADV_FREE pages to the tail of the inactive list, rather than > > the head. That said, I'm not sure reclaiming use-once filesystem cache > > before MADV_FREE is such a bad policy. MADV_FREE retains the vmas for > > the sole purpose of reusing them in the (near) future. That is > > actually a stronger reuse signal than we have for use-once file pages. > > If somebody does continuous writes to a logfile or a one-off search > > through one or more files, we should actually reclaim that cache > > before we go after MADV_FREE pages that are temporarily invalidated. > > Yes, we should be careful on this issue. It was main arguable point. > How about moving them to head of inactive file, not tail if we want to > go with inactive file LRU? > > With that, VM try to reclaim file pages first from the tail of list > and if pages reclaimed were workingset, it could be activated by > workingset_refault. Otherwise, we can discard use-once pages without > puring *madv_free* pages so I think it's good compromise. > > What do you think? That's what I tried to say. To address Shaohua's concern in two steps, first, it *would* be possible to move MADV_FREE pages to the tail of the inactive list. But then, taking a step back, I argued that this is probably not be the reclaim policy we actually want. So I agree with you. I think MADV_FREE should move these pages to the *head* of the inactive cache list, so that we reclaim colder use-once cache first. Workingset detection will make any necessary corrections. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
