On Tue, Feb 04, 2020 at 10:52:42AM -0800, Tyler Sanderson wrote: > > > 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. Right. Let's not repeat past mistakes and let's try to specify this new one properly though :) > > > 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 > >
