Re: Balloon pressuring page cache

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

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.

As far as putting pressure on the page cache I don't have anything
actively doing it in my current test environment. I was keeping things
simple and just resorting to using "echo 3 > /proc/sys/vm/drop_caches"
when I needed to flush out page cache entries. I was planning to work on
the page cache pressure piece as a next step.

Did you see the thread recently in response to my v16.1 patch set? We
actually had a similar discussion about how best to exert pressure on the
page cache and one thing I realized with that is that there are already
several similar efforts ongoing so my hope is to discuss this at LSF/MM,
decide on a way to consolidate the existing work to get to something that
will work for most, and get that submitted over the next year. A link to
that discussion can be found here:
https://lore.kernel.org/lkml/20200122173040.6142.39116.stgit@localhost.localdomain/

Thanks.

- Alex