On Fri, Jun 9, 2023 at 5:42 AM Charan Teja Kalla <quic_charante@xxxxxxxxxxx> wrote: > > Thanks Suren & Johannes, > > On 6/7/2023 1:18 AM, Suren Baghdasaryan wrote: > > Hi Folks. Sorry for being late to the party. > > Yeah, userspace does not have a crystal ball to predict future user > > behavior, so there will always be pathological cases when usual > > assumptions and resulting madvise() would make things worse. > > > > I think this discussion can be split into several questions/issues: > > 1. Inconsistency in how madvise(MADV_PAGEOUT) would affect PSI > > calculation when the page is refaulted, based on the path it took > > before being evicted by madvise(). In your initial description case > > (a) is inconsistent with (b) and (c) and it's probably worth fixing. > > IMHO (a) should be made consistent with others, not the other way > > around. My reasoning is that page was expelled from the active list, > > so it was part of the active workingset. > > > That means we should be setting Workingset on the page while it is on > the active list and when it is being pageout through madvising. Right? I > see, this makes it consistent. This was my opinion but others might think otherwise, like I found out in some recent conversations. So, it would be great to get some more feedback before making the change. > > On the same note, discussing with Suren offline, Should the refaulted > madvise pages start always at the inactive list? If they are really > active, they get promoted anyway.. > > > 2. Whether refaults caused by incorrect madvise(MADV_PAGEOUT) should > > be counted as workingset refault and affect PSI. > > This one I think is trickier. IMHO it should be counted as workingset > > refault simply because it was refaulted and it was part of the > > workingset. Whether it should affect PSI, which is supposed to be an > > indicator of "pressure" is, I think, debatable. With madvise() in the > > mix, refault might happen without any real memory pressure... So, the > > answer is not obvious to me. > > > > 3. Should refaults caused by incorrect madvise(MADV_PAGEOUT) be > > distinguished from the ones which were evicted by kernel reclaim > > mechanisms. > > I can see use for that from userspace to detect incorrect madvise() > > and adjust its aggressiveness. I think the API might get a bit complex > > because of the need to associate refaults with specific madvise()/VMAs > > to understand which hint was incorrect and adjust the behavior. > > Instead what is the opinion about giving an MADV_PAGEOUT_INACTIVE > interface which does operate on a page only If it is on the inactive > list and !PageWorkingset ? IOW you want a less aggressive mechanism which can be used by the userspace to tell the kernel "I think these pages won't be used but I'm not 100% sure, so drop them only if they are inactive"? I don't know how much that will help when the madvise() ends up being wrong but maybe you can quickly experiment and tell us if the difference is substantial? > > > Hope my feedback is useful and if we can improve Android's userspace > > behavior, I'm happy to help make that happen. > Thanks...
