On Wed, Nov 04, 2015 at 09:53:42AM -0800, Shaohua Li wrote: > On Tue, Nov 03, 2015 at 09:52:23AM +0900, Minchan Kim wrote: > > On Fri, Oct 30, 2015 at 10:22:12AM -0700, Shaohua Li wrote: > > > On Fri, Oct 30, 2015 at 04:01:41PM +0900, Minchan Kim wrote: > > > > MADV_FREE is a hint that it's okay to discard pages if there is memory > > > > pressure and we use reclaimers(ie, kswapd and direct reclaim) to free > > > > them > > > > so there is no value keeping them in the active anonymous LRU so this > > > > patch moves them to inactive LRU list's head. > > > > > > > > This means that MADV_FREE-ed pages which were living on the inactive > > > > list > > > > are reclaimed first because they are more likely to be cold rather than > > > > recently active pages. > > > > > > > > An arguable issue for the approach would be whether we should put the > > > > page > > > > to the head or tail of the inactive list. I chose head because the > > > > kernel > > > > cannot make sure it's really cold or warm for every MADV_FREE usecase > > > > but > > > > at least we know it's not *hot*, so landing of inactive head would be a > > > > comprimise for various usecases. > > > > > > > > This fixes suboptimal behavior of MADV_FREE when pages living on the > > > > active list will sit there for a long time even under memory pressure > > > > while the inactive list is reclaimed heavily. This basically breaks the > > > > whole purpose of using MADV_FREE to help the system to free memory which > > > > is might not be used. > > > > > > My main concern is the policy how we should treat the FREE pages. Moving > > > it to > > > inactive lru is definitionly a good start, I'm wondering if it's enough. > > > The > > > MADV_FREE increases memory pressure and cause unnecessary reclaim because > > > of > > > the lazy memory free. While MADV_FREE is intended to be a better > > > replacement of > > > MADV_DONTNEED, MADV_DONTNEED doesn't have the memory pressure issue as it > > > free > > > memory immediately. So I hope the MADV_FREE doesn't have impact on memory > > > pressure too. I'm thinking of adding an extra lru list and wartermark for > > > this > > > to make sure FREE pages can be freed before system wide page reclaim. As > > > you > > > said, this is arguable, but I hope we can discuss about this issue more. > > > > Yes, it's arguble. ;-) > > > > It seems the divergence comes from MADV_FREE is *replacement* of > > MADV_DONTNEED. > > But I don't think so. If we could discard MADV_FREEed page *anytime*, I > > agree > > but it's not true because the page would be dirty state when VM want to > > reclaim. > > There certainly are other usage cases, but even your patch log mainly > describes > the jemalloc usage case, which uses MADV_DONTNEED. > > > I'm also against with your's suggestion which let's discard FREEed page > > before > > system wide page reclaim because system would have lots of clean cold page > > caches or anonymous pages. In such case, reclaiming of them would be better. > > Yeb, it's really workload-dependent so we might need some heuristic which is > > normally what we want to avoid. > > > > Having said that, I agree with you we could do better than the deactivation > > and frankly speaking, I'm thinking of another LRU list(e.g. tentatively > > named > > "ezreclaim LRU list"). What I have in mind is to age (anon|file|ez) > > fairly. IOW, I want to percolate ez-LRU list reclaiming into get_scan_count. > > When the MADV_FREE is called, we could move hinted pages from anon-LRU to > > ez-LRU and then If VM find to not be able to discard a page in ez-LRU, > > it could promote it to acive-anon-LRU which would be very natural aging > > concept because it mean someone touches the page recenlty. > > > > With that, I don't want to bias one side and don't want to add some knob for > > tuning the heuristic but let's rely on common fair aging scheme of VM. > > > > Another bonus with new LRU list is we could support MADV_FREE on swapless > > system. > > > > > > > > Or do you want to push this first and address the policy issue later? > > > > I believe adding new LRU list would be controversial(ie, not trivial) > > for maintainer POV even though code wouldn't be complicated. > > So, I want to see problems in *real practice*, not any theoritical > > test program before diving into that. > > To see such voice of request, we should release the syscall. > > So, I want to push this first. > > The memory pressure issue isn't just in artificial test. In jemalloc, there is > a knob (lg_dirty_mult) to control the rate memory should be purged (using > MADV_DONTNEED). We already had several reports in our production environment > changing the knob can cause extra memory usage (and swap and so on). If > jemalloc uses MADV_FREE, jemalloc will not purge any memory, which is equivent > to disable current MADV_DONTNEED (eg, lg_dirty_mult = -1). I'm sure this will > cause the similar issue, eg (extram memory usage, swap). That said I don't > object to push this first, but the memory pressue issue can happen in real > production, I hope it's not ignored.
I think the question is if application uses MADV_DONTNEED originally, how much better if we replace it to MADV_FREE compared to just delete the MADV_DONTNEED, considering anonymous memory is hard to be reclaimed currently. Thanks, Shaohua -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
