Re: Balloon pressuring page cache

[Alexander Duyck <alexander.h.duyck@linux.intel.com>]

On Mon, 2020-02-03 at 12:32 -0800, 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:
> 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.
> It lacks understanding of NUMA or other OOM constraints.
> It has a higher potential for bugs due to the subtlety of the callback
> context.
> Given the above, I think the shrinker API certainly makes the most sense
> _if_ the balloon size is static. In that case memory should be reclaimed
> from the balloon early and proportionally to balloon size, which the
> shrinker API achieves.

The problem is the shrinker doesn't have any concept of tiering or
priority. I suspect he reason for using the OOM notification is because in
practice it should be the last thing we are pulling memory out of with
things like page cache and slab caches being first. Once we have pages
that are leaked out of the balloon by the shrinker it will trigger the
balloon wanting to reinflate. Ideally if the shrinker is running we
shouldn't be able to reinflate the balloon, and if we are reinflating the
balloon we shouldn't need to run the shrinker. The fact that we can do
both at the same time is problematic.

> However, if the balloon is inflating and intentionally causing memory
> pressure then this results in the inefficiency pointed out earlier.
> 
> If the balloon is inflating but not causing memory pressure then there
> is no problem with either API.

The entire point of the balloon is to cause memory pressure. Otherwise
essentially all we are really doing is hinting since the guest doesn't
need the memory and isn't going to use it any time soon.

> This suggests another route: rather than cause memory pressure to shrink
> the page cache, the balloon could issue the equivalent of "echo 3 >
> /proc/sys/vm/drop_caches".
> Of course ideally, we want to be more fine grained than "drop
> everything". We really want an API that says "drop everything that
> hasn't been accessed in the last 5 minutes".
> 
> This would eliminate the need for the balloon to cause memory pressure
> at all which avoids the inefficiency in question. Furthermore, this
> pairs nicely with the FREE_PAGE_HINT feature.

Something similar was brought up in the discussion we had about this in my
patch set. The problem is, by trying to use a value like "5 minutes" it
implies that we are going to need to track some extra state somewhere to
determine that value.

An alternative is to essentially just slowly shrink memory for the guest.
We had some discussion about this in another thread, and the following
code example was brought up as a way to go about doing that:
https://github.com/Conan-Kudo/omv-kernel-rc/blob/master/0154-sysctl-vm-Fine-grained-cache-shrinking.patch

The idea is you essentially just slowly bleed the memory from the guest by
specifying some amount of MB of cache to be freed on some regular
interval.

Thanks.

- Alex