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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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