Re: Balloon pressuring page cache

[Tyler Sanderson <tysand@google.com>]

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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:

   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.
   2. It lacks understanding of NUMA or other OOM constraints.
   3. 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.

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.

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.


On Mon, Feb 3, 2020 at 9:04 AM Michael S. Tsirkin <mst@redhat.com> wrote:

> On Mon, Feb 03, 2020 at 05:34:20PM +0100, David Hildenbrand wrote:
> > On 03.02.20 17:18, Alexander Duyck wrote:
> > > On Mon, 2020-02-03 at 08:11 -0500, Michael S. Tsirkin wrote:
> > >> On Thu, Jan 30, 2020 at 11:59:46AM -0800, Tyler Sanderson wrote:
> > >>>
> > >>> On Thu, Jan 30, 2020 at 7:31 AM Wang, Wei W <wei.w.wang@intel.com>
> wrote:
> > >>>
> > >>>     On Thursday, January 30, 2020 11:03 PM, David Hildenbrand wrote:
> > >>>     > On 29.01.20 20:11, Tyler Sanderson wrote:
> > >>>     > >
> > >>>     > >
> > >>>     > > On Wed, Jan 29, 2020 at 2:31 AM David Hildenbrand <
> david@redhat.com
> > >>>     > > <mailto:david@redhat.com>> wrote:
> > >>>     > >
> > >>>     > >     On 29.01.20 01:22, Tyler Sanderson via Virtualization
> wrote:
> > >>>     > >     > A primary advantage of virtio balloon over other
> memory reclaim
> > >>>     > >     > mechanisms is that it can pressure the guest's page
> cache into
> > >>>     > >     shrinking.
> > >>>     > >     >
> > >>>     > >     > However, since the balloon driver changed to using the
> shrinker
> > >>>     API
> > >>>     > >     >
> > >>>     > >
> > >>>     > <
> https://github.com/torvalds/linux/commit/71994620bb25a8b109388fefa9
> > >>>     > e99a28e355255a#diff-fd202acf694d9eba19c8c64da3e480c9> this
> > >>>     > >     > use case has become a bit more tricky. I'm wondering
> what the
> > >>>     > intended
> > >>>     > >     > device implementation is.
> > >>>     > >     >
> > >>>     > >     > When inflating the balloon against page cache (i.e. no
> free
> > >>>     memory
> > >>>     > >     > remains) vmscan.c will both shrink page cache, but
> also invoke
> > >>>     the
> > >>>     > >     > shrinkers -- including the balloon's shrinker. So the
> balloon
> > >>>     driver
> > >>>     > >     > allocates memory which requires reclaim, vmscan gets
> this memory
> > >>>     > by
> > >>>     > >     > shrinking the balloon, and then the driver adds the
> memory back
> > >>>     to
> > >>>     > the
> > >>>     > >     > balloon. Basically a busy no-op.
> > >>>
> > >>>     Per my understanding, the balloon allocation won’t invoke
> shrinker as
> > >>>     __GFP_DIRECT_RECLAIM isn't set, no?
> > >>>
> > >>> I could be wrong about the mechanism, but the device sees lots of
> activity on
> > >>> the deflate queue. The balloon is being shrunk. And this only starts
> once all
> > >>> free memory is depleted and we're inflating into page cache.
> > >>
> > >> So given this looks like a regression, maybe we should revert the
> > >> patch in question 71994620bb25 ("virtio_balloon: replace oom notifier
> with shrinker")
> > >> Besides, with VIRTIO_BALLOON_F_FREE_PAGE_HINT
> > >> shrinker also ignores VIRTIO_BALLOON_F_MUST_TELL_HOST which isn't nice
> > >> at all.
> > >>
> > >> So it looks like all this rework introduced more issues than it
> > >> addressed ...
> > >>
> > >> I also CC Alex Duyck for an opinion on this.
> > >> Alex, what do you use to put pressure on page cache?
> > >
> > > I would say reverting probably makes sense. I'm not sure there is much
> > > value to having a shrinker running deflation when you are actively
> trying
> > > to increase the balloon. It would make more sense to wait until you are
> > > actually about to start hitting oom.
> >
> > I think the shrinker makes sense for free page hinting feature
> > (everything on free_page_list).
> >
> > So instead of only reverting, I think we should split it up and always
> > register the shrinker for VIRTIO_BALLOON_F_FREE_PAGE_HINT and the OOM
> > notifier (as before) for VIRTIO_BALLOON_F_MUST_TELL_HOST.
>
> OK ... I guess that means we need to fix shrinker to take
> VIRTIO_BALLOON_F_MUST_TELL_HOST into account correctly.
> Hosts ignore it at the moment but it's a fragile thing
> to do what it does and ignore used buffers.
>
> > (Of course, adapting what is being done in the shrinker and in the OOM
> > notifier)
> >
> > --
> > Thanks,
> >
> > David / dhildenb
>
>

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