2020년 7월 17일 (금) 오후 10:59, Johannes Weiner <hannes@xxxxxxxxxxx>님이 작성: > > On Wed, Jun 17, 2020 at 02:26:19PM +0900, js1304@xxxxxxxxx wrote: > > From: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx> > > > > In current implementation, newly created or swap-in anonymous page > > is started on active list. Growing active list results in rebalancing > > active/inactive list so old pages on active list are demoted to inactive > > list. Hence, the page on active list isn't protected at all. > > > > Following is an example of this situation. > > > > Assume that 50 hot pages on active list. Numbers denote the number of > > pages on active/inactive list (active | inactive). > > > > 1. 50 hot pages on active list > > 50(h) | 0 > > > > 2. workload: 50 newly created (used-once) pages > > 50(uo) | 50(h) > > > > 3. workload: another 50 newly created (used-once) pages > > 50(uo) | 50(uo), swap-out 50(h) > > > > This patch tries to fix this issue. > > Like as file LRU, newly created or swap-in anonymous pages will be > > inserted to the inactive list. They are promoted to active list if > > enough reference happens. This simple modification changes the above > > example as following. > > > > 1. 50 hot pages on active list > > 50(h) | 0 > > > > 2. workload: 50 newly created (used-once) pages > > 50(h) | 50(uo) > > > > 3. workload: another 50 newly created (used-once) pages > > 50(h) | 50(uo), swap-out 50(uo) > > > > As you can see, hot pages on active list would be protected. > > > > Note that, this implementation has a drawback that the page cannot > > be promoted and will be swapped-out if re-access interval is greater than > > the size of inactive list but less than the size of total(active+inactive). > > To solve this potential issue, following patch will apply workingset > > detection that is applied to file LRU some day before. > > > > v6: Before this patch, all anon pages (inactive + active) are considered > > as workingset. However, with this patch, only active pages are considered > > as workingset. So, file refault formula which uses the number of all > > anon pages is changed to use only the number of active anon pages. > > I can see that also from the code, but it doesn't explain why. > > And I'm not sure this is correct. I can see two problems with it. > > After your patch series, there is still one difference between anon > and file: cache trim mode. If the "use-once" anon dominate most of > memory and you have a small set of heavily thrashing files, it would > not get recognized. File refaults *have* to compare their distance to > the *entire* anon set, or we could get trapped in cache trimming mode > even as file pages with access frequencies <= RAM are thrashing. > > On the anon side, there is no cache trimming mode. But even if we're > not in cache trimming mode and active file is already being reclaimed, > we have to recognize thrashing on the anon side when reuse frequencies > are within available RAM. Otherwise we treat an inactive file that is > not being reused as having the same value as an anon page that is > being reused. And then we may reclaim file and anon at the same rate > even as anon is thrashing and file is not. That's not right. > > We need to activate everything with a reuse frequency <= RAM. Reuse > frequency is refault distance plus size of the inactive list the page > was on. This means anon distances should be compared to active anon + > inactive file + active file, and file distances should be compared to > active file + inactive_anon + active anon. You're right. Maybe, I'm confused about something at that time. I will change it as you suggested. Thanks.
