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    SubjectRe: [RFC][Patch v9 2/6] KVM: Enables the kernel to isolate guest free pages
    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@redhat.com> 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

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