On Wed, Jan 29, 2020 at 2:31 AM David Hildenbrand <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?
--
Thanks,
David / dhildenb
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