On Wed, Jan 29, 2020 at 2:31 AM David Hildenbrand <[email protected]> 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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