Re: Balloon pressuring page cache

[Tyler Sanderson <tysand@google.com>]

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On Mon, Feb 3, 2020 at 1:22 PM Alexander Duyck <
alexander.h.duyck@linux.intel.com> wrote:

> 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.

Deciding whether to trade IO performance (page cache) for memory-usage
efficiency (balloon) seems use-case dependent.
Deciding when to re-inflate is a similar policy choice.

If the balloon's shrinker priority is hard-coded to "last-resort" then
there would be no way to implement a policy where page cache growth could
shrink the balloon.
The current balloon implementation allows the host to implement this policy
and tune the tradeoff between balloon and page cache.


> 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.
>
I agree that this is inefficient.


>
> > 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.
>
Causing memory pressure is just a mechanism to achieve increased reclaim.
If there was a better mechanism (like the fine-grained-cache-shrinking one
discussed below) then I think the balloon device would be perfectly
justified in using that instead (and maybe "balloon" becomes a misnomer. Oh
well).


>
> > 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.
>
Makes sense. Whatever API is settled on, I'd just propose that we allow the
host to invoke it via the balloon device since the host has a host-global
view of memory and can make decisions that an individual guest cannot.

Alex, what is the status of your fine-grained-cache-shrinking patch? It
seems like a really good idea.


> Thanks.
>
> - Alex
>
>

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