Re: OOM notifications

[Martin Bligh <mbligh@mbligh.org>]

Andrew Morton wrote:
> On Thu, 18 Oct 2007 16:15:31 -0400
> Marcelo Tosatti <marcelo@kvack.org> wrote:
>
> > Hi,
> >
> > AIX contains the SIGDANGER signal to notify applications to free up some
> > unused cached memory:
> >
> > http://www.ussg.iu.edu/hypermail/linux/kernel/0007.0/0901.html
> >
> > There have been a few discussions on implementing such an idea on Linux,
> > but nothing concrete has been achieved.
> >
> > On the kernel side Rik suggested two notification points: "about to
> > swap" (for desktop scenarios) and "about to OOM" (for embedded-like
> > scenarios).
> >
> > With that assumption in mind it would be necessary to either have two
> > special devices for notification, or somehow indicate both events
> > through the same file descriptor.
> >
> > Comments are more than welcome.
>
> Martin was talking about some mad scheme wherin you'd create a bunch of
> pseudo files (say, /proc/foo/0, /proc/foo/1, ..., /proc/foo/9) and each one
> would become "ready" when the MM scanning priority reaches 10%, 20%, ... 
> 100%.
>
> Obviously there would need to be a lot of abstraction to unhook a permanent
> userspace feature from a transient kernel implementation, but the basic
> idea is that a process which wants to know when the VM is getting into the
> orange zone would select() on the file "7" and a process which wants to
> know when the VM is getting into the red zone would select on file "9".
>
> It get more complicated with NUMA memory nodes and cgroup memory
> controllers.

We ended up not doing that, but making a scanner that saw what
percentage of the LRU was touched in the last n seconds, and
printing that to userspace to deal with.

Turns out priority is a horrible metric to use for this - it
stays at default for ages, then falls off a cliff far too
quickly to react to.

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