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    SubjectRe: How can we make page replacement smarter
    Date
    Rik van Riel wrote:
    > Al Boldi wrote:
    > > Good idea, but unless we understand the problems involved, we are bound
    > > to repeat it. So my first question would be: Why is swap-in so slow?
    > >
    > > As I have posted in other threads, swap-in of consecutive pages suffers
    > > a 2x slowdown wrt swap-out, whereas swap-in of random pages suffers over
    > > 6x slowdown.
    > >
    > > Because it is hard to quantify the expected swap-in speed for random
    > > pages, let's first tackle the swap-in of consecutive pages, which should
    > > be at least as fast as swap-out. So again, why is swap-in so slow?
    >
    > I suspect that this is a locality of reference issue.
    >
    > Anonymous memory can get jumbled up by repeated free and
    > malloc cycles of many smaller objects. The amount of
    > anonymous memory is often smaller than or roughly the same
    > size as system memory.

    Sounds exactly like the tmpfs problem.

    > Locality of refenence to anonymous memory tends to be
    > temporal in nature, with the same sets of pages being
    > accessed over and over again.
    >
    > Files are different. File content tends to be grouped
    > in large related chunks, both logically in the file and
    > on disk. Generally there is a lot more file data on a
    > system than what fits in memory.
    >
    > Locality of reference to file data tends to be spatial
    > in nature, with one file access leading up to the system
    > accessing "nearby" data. The data is not necessarily
    > touched again any time soon.
    >
    > > Once we understand this problem, we may be able to suggest a smart
    > > improvement.
    >
    > Like the one on http://linux-mm.org/PageoutFailureModes ?

    Interesting to see that there are known problems, but it doesn't seem to list
    the resume-from-disk swap-in slowdown.

    > I have the LRU lists split and am working on getting SEQ
    > replacement implemented for the anonymous pages.
    >
    > The most recent (untested) patches are attached.

    Applied against 2.6.22; the kernel crashes out on boot.


    Thanks!

    --
    Al

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