On Thu, Jan 30, 2020 at 7:03 AM David Hildenbrand <david@xxxxxxxxxx> wrote:
On 29.01.20 20:11, Tyler Sanderson wrote:
>
>
> On Wed, Jan 29, 2020 at 2:31 AM David Hildenbrand <david@xxxxxxxxxx
> <mailto:david@xxxxxxxxxx>> 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.
Just wondering, how does your workaround look like?
The work around is to monitor the memory statistics reported on the stats queue. Keep inflating (inefficiently) -- despite the activity on the deflate queue -- until memory statistics indicate the guest is actually low on available memory.
--
Thanks,
David / dhildenb
