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    SubjectRe: [PATCH 0/8][for -mm] mem_notify v6
    On Wed, Apr 2, 2008 at 12:31 AM, KOSAKI Motohiro
    <kosaki.motohiro@jp.fujitsu.com> wrote:
    > Hi Tom,
    >
    > Thank you very useful comment.
    > that is very interesting.
    >
    >
    > > I tried it with a real-world program that, among other things, mmaps
    > > anonymous pages and touches them at a reasonable speed until it gets
    > > notified via /dev/mem_notify, releases most of them with
    > > madvise(MADV_DONTNEED), then loops to start the cycle again.
    > >
    > > What tends to happen is that I do indeed get notifications via
    > > /dev/mem_notify when the kernel would like to be swapping, at which
    > > point I free memory. But the notifications come at a time when the
    > > kernel needs memory, and it gets the memory by discarding some Cached
    > > or Mapped memory (I can see these decreasing in /proc/meminfo with
    > > each notification). With each mmap/notify/madvise cycle the Cached
    > > and Mapped memory gets smaller, until eventually while I'm touching
    > > pages the kernel can't find enough memory and will either invoke the
    > > OOM killer or return ENOMEM from syscalls. This is precisely the
    > > situation I'm trying to avoid by using /dev/mem_notify.
    >
    > Could you send your test program?

    Unfortunately, no, it's a Java Virtual Machine (which is a perfect
    user of /dev/mem_notify since it can garbage collect on notification,
    among other times).

    But it should be possible to make a small program with the same
    behavior; I'll do that.

    > I can't reproduce that now, sorry.
    >
    >
    >
    > > The criterion of "notify when the kernel would like to swap" feels
    > > correct, but in addition I seem to need something like "notify when
    > > cached+mapped+free memory is getting low".
    >
    > Hmmm,
    > I think this idea is only useful when userland process call
    > madvise(MADV_DONTNEED) periodically.

    Do you have a recommendation for freeing memory? I could maybe use
    munmap/mmap, but that's not atomic and may be "worse" (more overhead,
    etc.) than madvise(MADV_DONTNEED).

    > but I hope improve my patch and solve your problem.
    > if you don' mind, please help my testing ;)

    It's my pleasure to help in any way I can.

    .tom


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