Re: Balloon pressuring page cache

[Tyler Sanderson <tysand@google.com>]

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On Tue, Feb 4, 2020 at 11:17 AM David Hildenbrand <david@redhat.com> wrote:

> On 04.02.20 19:52, Tyler Sanderson wrote:
> >
> >
> > On Tue, Feb 4, 2020 at 12:29 AM David Hildenbrand <david@redhat.com
> > <mailto:david@redhat.com>> 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.
>
> You want the old behavior back, so why should we introduce a new one? Or
> am I missing something? (you did want us to revert to old handling, no?)
>
Reverting actually doesn't help me because this has been the behavior since
Linux 4.19 which is already widely in use. So my device implementation
needs to handle the shrinker behavior anyways. I started this conversation
to ask what the intended device implementation was.

I think there are reasonable device implementations that would prefer the
shrinker behavior (it turns out that mine doesn't).
For example, an implementation that slowly inflates the balloon for the
purpose of memory overcommit. It might leave the balloon inflated and
expect any memory pressure (including page cache usage) to deflate the
balloon as a way to dynamically right-size the balloon.

Two reasons I didn't go with the above implementation:
1. I need to support guests before Linux 4.19 which don't have the shrinker
behavior.
2. Memory in the balloon does not appear as "available" in /proc/meminfo
even though it is freeable. This is confusing to users, but isn't a deal
breaker.

If we added a DEFLATE_ON_PRESSURE feature bit that indicated shrinker API
support then that would resolve reason #1 (ideally we would backport the
bit to 4.19).

In any case, the shrinker behavior when pressuring page cache is more of an
inefficiency than a bug. It's not clear to me that it necessitates
reverting. If there were/are reasons to be on the shrinker interface then I
think those carry similar weight as the problem itself.


>
> I consider virtio-balloon to this very day a big hack. And I don't see
> it getting better with new config knobs. Having that said, the
> technologies that are candidates to replace it (free page reporting,
> taming the guest page cache, etc.) are still not ready - so we'll have
> to stick with it for now :( .
>
> >
> > 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.
>
> Using better suited technologies that are not ready yet (well, some form
> of free page reporting is available under IBM z already but in a
> proprietary form) ;) Anyhow, I remember that DEFLATE_ON_OOM only makes
> it less likely to crash your guest, but not that you are safe to squeeze
> the last bit out of your guest VM.
>
Can you elaborate on the danger of DEFLATE_ON_OOM? I haven't seen any
problems in testing but I'd really like to know about the dangers.
Is there a difference in safety between the OOM notifier callback and the
shrinker API?


>
> --
> Thanks,
>
> David / dhildenb
>
>

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