On Fri, Mar 08, 2019 at 10:06:14AM -0800, Alexander Duyck wrote: > On Thu, Mar 7, 2019 at 6:32 PM Michael S. Tsirkin <mst@xxxxxxxxxx> wrote: > > > > On Thu, Mar 07, 2019 at 02:35:53PM -0800, Alexander Duyck wrote: > > > The only other thing I still want to try and see if I can do is to add > > > a jiffies value to the page private data in the case of the buddy > > > pages. > > > > Actually there's one extra thing I think we should do, and that is make > > sure we do not leave less than X% off the free memory at a time. > > This way chances of triggering an OOM are lower. > > If nothing else we could probably look at doing a watermark of some > sort so we have to have X amount of memory free but not hinted before > we will start providing the hints. It would just be a matter of > tracking how much memory we have hinted on versus the amount of memory > that has been pulled from that pool. It is another reason why we > probably want a bit in the buddy pages somewhere to indicate if a page > has been hinted or not as we can then use that to determine if we have > to account for it in the statistics. > > > > With that we could track the age of the page so it becomes > > > easier to only target pages that are truly going cold rather than > > > trying to grab pages that were added to the freelist recently. > > > > I like that but I have a vague memory of discussing this with Rik van > > Riel and him saying it's actually better to take away recently used > > ones. Can't see why would that be but maybe I remember wrong. Rik - am I > > just confused? > > It is probably to cut down on the need for disk writes in the case of > swap. If that is the case it ends up being a trade off. > > The sooner we hint the less likely it is that we will need to write a > given page to disk. However the sooner we hint, the more likely it is > we will need to trigger a page fault and pull back in a zero page to > populate the last page we were working on. The sweet spot will be that > period of time that is somewhere in between so we don't trigger > unnecessary page faults and we don't need to perform additional swap > reads/writes. Right but the question is - is it better to hint on least recently used, or most recently used pages? It looks like LRU should be better, but I vaguely rememeber there were arguments for why most recently used might be better. Can't figure out why, maybe I am remembering wrong. -- MST
