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? -- Thanks, David / dhildenb _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
