Re: [PATCH v2 2/2] virtio_balloon: replace oom notifier with shrinker

[Wei Wang]

On 08/02/2018 07:47 PM, Michal Hocko wrote:
> On Thu 02-08-18 18:32:44, Wei Wang wrote:
> > On 08/01/2018 07:34 PM, Michal Hocko wrote:
> > > On Wed 01-08-18 19:12:25, Wei Wang wrote:
> > > > On 07/30/2018 05:00 PM, Michal Hocko wrote:
> > > > > On Fri 27-07-18 17:24:55, Wei Wang wrote:
> > > > > > The OOM notifier is getting deprecated to use for the reasons mentioned
> > > > > > here by Michal Hocko: https://lkml.org/lkml/2018/7/12/314
> > > > > >
> > > > > > This patch replaces the virtio-balloon oom notifier with a shrinker
> > > > > > to release balloon pages on memory pressure.
> > > > > It would be great to document the replacement. This is not a small
> > > > > change...
> > > > OK. I plan to document the following to the commit log:
> > > >
> > > >     The OOM notifier is getting deprecated to use for the reasons:
> > > >       - As a callout from the oom context, it is too subtle and easy to
> > > >         generate bugs and corner cases which are hard to track;
> > > >       - It is called too late (after the reclaiming has been performed).
> > > >         Drivers with large amuont of reclaimable memory is expected to be
> > > >         released them at an early age of memory pressure;
> > > >       - The notifier callback isn't aware of the oom contrains;
> > > >       Link: https://lkml.org/lkml/2018/7/12/314
> > > >
> > > >       This patch replaces the virtio-balloon oom notifier with a shrinker
> > > >       to release balloon pages on memory pressure. Users can set the amount of
> > > >       memory pages to release each time a shrinker_scan is called via the
> > > >       module parameter balloon_pages_to_shrink, and the default amount is 256
> > > >       pages. Historically, the feature VIRTIO_BALLOON_F_DEFLATE_ON_OOM has
> > > >       been used to release balloon pages on OOM. We continue to use this
> > > >       feature bit for the shrinker, so the shrinker is only registered when
> > > >       this feature bit has been negotiated with host.
> > > Do you have any numbers for how does this work in practice?
> > It works in this way: for example, we can set the parameter,
> > balloon_pages_to_shrink, to shrink 1GB memory once shrink scan is called.
> > Now, we have a 8GB guest, and we balloon out 7GB. When shrink scan is
> > called, the balloon driver will get back 1GB memory and give them back to
> > mm, then the ballooned memory becomes 6GB.
> >
> > When the shrinker scan is called the second time, another 1GB will be given
> > back to mm. So the ballooned pages are given back to mm gradually.
> >
> > > Let's say
> > > you have a medium page cache workload which triggers kswapd to do a
> > > light reclaim? Hardcoded shrinking sounds quite dubious to me but I have
> > > no idea how people expect this to work. Shouldn't this be more
> > > adaptive? How precious are those pages anyway?
> > Those pages are given to host to use usually because the guest has enough
> > free memory, and host doesn't want to waste those pieces of memory as they
> > are not used by this guest. When the guest needs them, it is reasonable that
> > the guest has higher priority to take them back.
> > But I'm not sure if there would be a more adaptive approach than "gradually
> > giving back as the guest wants more".
> I am not sure I follow. Let me be more specific. Say you have a trivial
> stream IO triggering reclaim to recycle clean page cache. This will
> invoke slab shrinkers as well. Do you really want to drop your batch of
> pages on each invocation? Doesn't that remove them very quickly? Just
> try to dd if=large_file of=/dev/null and see how your pages are
> disappearing. Shrinkers usually scale the number of objects they are
> going to reclaim based on the memory pressure (aka targer to be
> reclaimed).

Thanks, I think it looks better to make it more adaptive. I'll send out 
a new version for review.

Best,
Wei