On Sun, 25 Feb 2024 03:50:48 +0800 Barry Song <21cnbao@xxxxxxxxx> wrote: > On Sun, Feb 25, 2024 at 3:02 AM SeongJae Park <sj@xxxxxxxxxx> wrote: > > > > On Fri, 23 Feb 2024 17:15:50 +1300 Barry Song <21cnbao@xxxxxxxxx> wrote: > > > > > From: Barry Song <v-songbaohua@xxxxxxxx> > > > > > > While doing MADV_PAGEOUT, the current code will clear PTE young > > > so that vmscan won't read young flags to allow the reclamation > > > of madvised folios to go ahead. > > > It seems we can do it by directly ignoring references, thus we > > > can remove tlb flush in madvise and rmap overhead in vmscan. > > > > > > Regarding the side effect, in the original code, if a parallel > > > thread runs side by side to access the madvised memory with the > > > thread doing madvise, folios will get a chance to be re-activated > > > by vmscan. But with the patch, they will still be reclaimed. But > > > this behaviour doing PAGEOUT and doing access at the same time is > > > quite silly like DoS. So probably, we don't need to care. > > > > I think we might need to take care of the case, since users may use just a > > best-effort estimation like DAMON for the target pages. In such cases, the > > page granularity re-check of the access could be helpful. So I concern if this > > could be a visible behavioral change for some valid use cases. > > Hi SeongJae, > > If you read the code of MADV_PAGEOUT, you will find it is not the best-effort. I'm not saying about MADV_PAGEOUT, but the logic of ther user of MADV_PAGEOUT, which being used for finding the pages to reclaim. > It does clearing pte young and immediately after the ptes are cleared, it reads > pte and checks if the ptes are young. If not, reclaim it. So the > purpose of clearing > PTE young is helping the check of young in folio_references to return false. > The gap between clearing ptes and re-checking ptes is quite small at > microseconds > level. > > > > > > > > > A microbench as below has shown 6% decrement on the latency of > > > MADV_PAGEOUT, > > > > I assume some of the users may use MADV_PAGEOUT for proactive reclamation of > > the memory. In the use case, I think latency of MADV_PAGEOUT might be not that > > important. > > > > Hence I think the cons of the behavioral change might outweigh the pros of the > > latench improvement, for such best-effort proactive reclamation use case. Hope > > to hear and learn from others' opinions. > > I don't see the behavioral change for MADV_PAGEOUT as just the ping-pong > is removed. The only chance is in that very small time gap, somebody accesses > the cleared ptes and makes it young again, considering this time gap > is so small, > i don't think it is worth caring. thus, i don't see pros for MADV_PAGEOUT > case, but we improve the efficiency of MADV_PAGEOUT and save the power of > Android phones. Ok, I agree the time gap is small enough and the benefit could be significant on such use case. Thank you for enlightening me with the nice examples and the numbers :) Thanks, SJ [...]
