On Wed, Apr 14, 2021 at 9:00 PM Andi Kleen <ak@xxxxxxxxxxxxxxx> wrote: > > > We fall back to the rmap when it's obviously not smart to do so. There > > is still a lot of room for improvement in this function though, i.e., > > it should be per VMA and NUMA aware. > > Okay so it's more a question to tune the cross over heuristic. That > sounds much easier than replacing everything. > > Of course long term it might be a problem to maintain too many > different ways to do things, but I suppose short term it's a reasonable > strategy. Hi Rik, Ying, Sorry for being persistent. I want to make sure we are on the same page: Page table scanning doesn't replace the existing rmap walk. It is complementary and only happens when it is likely that most of the pages on a system under pressure have been referenced, i.e., out of *inactive* pages, by definition of the existing implementation. Under such a condition, scanning *active* pages one by one with the rmap is likely to cost more than scanning them all at once via page tables. When we evict *inactive* pages, we still use the rmap and share a common path with the existing code. Page table scanning falls back to the rmap walk if the page tables of a process are apparently sparse, i.e., rss < size of the page tables. I should have clarified this at the very beginning of the discussion. But it has become so natural to me and I assumed we'd all see it this way. Your concern regarding the NUMA optimization is still valid, and it's a high priority. Thanks.
