Re: [RFC] simple_lmk: Introduce Simple Low Memory Killer for Android

[Sultan Alsawaf]

On Tue, Mar 12, 2019 at 09:05:32AM +0100, Michal Hocko wrote:
> The only way to control the OOM behavior pro-actively is to throttle
> allocation speed. We have memcg high limit for that purpose. Along with
> PSI, I can imagine a reasonably working user space early oom
> notifications and reasonable acting upon that.

The issue with pro-active memory management that prompted me to create this was
poor memory utilization. All of the alternative means of reclaiming pages in the
page allocator's slow path turn out to be very useful for maximizing memory
utilization, which is something that we would have to forgo by relying on a
purely pro-active solution. I have not had a chance to look at PSI yet, but
unless a PSI-enabled solution allows allocations to reach the same point as when
the OOM killer is invoked (which is contradictory to what it sets out to do),
then it cannot take advantage of all of the alternative memory-reclaim means
employed in the slowpath, and will result in killing a process before it is
_really_ necessary.

> If you design is relies on the speed of killing then it is fundamentally
> flawed AFAICT. You cannot assume anything about how quickly a task dies.
> It might be blocked in an uninterruptible sleep or performin an
> operation which takes some time. Sure, oom_reaper might help here but
> still.

In theory we could instantly zap any process that is not trapped in the kernel
at the time that the OOM killer is invoked without any consequences though, no?

Thanks,
Sultan