On Wed, Jan 29, 2020 at 2:31 AM David Hildenbrand <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/71994620bb25a8b109388fefa9e99a28e355255a#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.
> >
> > If file IO is ongoing during this balloon inflation then the page cache
> > could be growing which further puts "back pressure" on the balloon
> > trying to inflate. In testing I've seen periods of > 45 seconds where
> > balloon inflation makes no net forward progress.
> >
> > This wasn't a problem before the change to the shrinker API since forced
> > balloon deflation only occurred via the OOM notifier callback which was
> > invoked only after the page cache had depleted.
> >
> > Is this new busy behavior working as intended?
>
> Please note that the shrinker will only be registered in case we have
> VIRTIO_BALLOON_F_DEFLATE_ON_OOM - (which is AFAIK very rare) - to
> implement automatic balloon deflation when the guest is under memory
> pressure.


> Are you actually experiencing issues with that or did you just stumble
> over the code?
>

We have a use case that is encountering this (and that registers
DEFLATE_ON_OOM). We can work around this, but it does seem inefficient.
I understand there were good reasons for moving away from the OOM notifier
callback, but I'm wondering if the balloon driver could specify a "nice"
level to the shrinker API that would cause it to be reclaimed from only as
a last resort?


>
> --
> Thanks,
>
> David / dhildenb
>
>