Re: Balloon pressuring page cache

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Jan 30, 2020 at 04:02:34PM +0100, 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/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?
> >  
> 
> Cc-ing linux-mm, Michal and Michael.


Interesting.  VIRTIO_BALLOON_F_DEFLATE_ON_OOM is really
underspecified in a bunch of ways.

I'll wait to see what does Michal say from Linux POV.





> Just wondering, how does your workaround look like?
> 
> -- 
> Thanks,
> 
> David / dhildenb