Yu Zhao <yuzhao@xxxxxxxxxx> writes: > 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. Hi, Yu, In general, I think it's a good idea to combine the page table scanning and rmap scanning in the page reclaiming. For example, if the working-set is transitioned, we can take advantage of the fast page table scanning to identify the new working-set quickly. While we can fallback to the rmap scanning if the page table scanning doesn't help. Best Regards, Huang, Ying
