On Tue, Feb 4, 2020 at 12:29 AM David Hildenbrand <david@xxxxxxxxxx> wrote:
On 03.02.20 21:32, Tyler Sanderson wrote:
> There were apparently good reasons for moving away from OOM notifier
> callback:
> https://lkml.org/lkml/2018/7/12/314
> https://lkml.org/lkml/2018/8/2/322
>
> In particular the OOM notifier is worse than the shrinker because:
The issue is that DEFLATE_ON_OOM is under-specified.
>
> 1. It is last-resort, which means the system has already gone through
> heroics to prevent OOM. Those heroic reclaim efforts are expensive
> and impact application performance.
That's *exactly* what "deflate on OOM" suggests.
It seems there are some use cases where "deflate on OOM" is desired and others where "deflate on pressure" is desired.
This suggests adding a new feature bit "DEFLATE_ON_PRESSURE" that registers the shrinker, and reverting DEFLATE_ON_OOM to use the OOM notifier callback.
This lets users configure the balloon for their use case.
Assume you are using virtio-balloon for some weird way of memory
hotunplug (which is what some people do) and you want to minimize the
footprint of your guest. Then you really only want to give the guest
more memory (or rather, let it take back memory automatically in this
case) in case it really needs more memory. It should try to reclaim first.
Under-specified.
> 2. It lacks understanding of NUMA or other OOM constraints.
Ballooning in general lacks the understanding of NUMA.
> 3. It has a higher potential for bugs due to the subtlety of the
> callback context.
While that is a valid point, it doesn't explain why existing
functionality is changed.
Personally, I think DEFLATE_ON_OOM should never have been introduced (at
least not in this form).
I'm actually not sure how you would safely do memory overcommit without DEFLATE_ON_OOM. So I think it unlocks a huge use case.
--
Thanks,
David / dhildenb
