Re: [PATCH 0/8][for -mm] mem_notify v6

[Tom May]

On Wed, Apr 2, 2008 at 12:31 AM, KOSAKI Motohiro
<kosaki.motohiro@xxxxxxxxxxxxxx> 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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