On Wed, 29 Jan 2020, Tyler Sanderson 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? > Shrinkers aren't priority based so I don't think there's a non-hacky way to avoid reclaiming from them, it's not the correct way to implement a reclaim callback unless the objects can be fairly freed compared to other shrinkers. The oom notifier callbacks are the canonical way for global reclaim to free memory only as a last resort. _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
