On Thursday, January 30, 2020 11:03 PM, David Hildenbrand 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/71994620bb25a8b109388fefa9 > e99a28e355255a#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. Per my understanding, the balloon allocation won’t invoke shrinker as __GFP_DIRECT_RECLAIM isn't set, no? > > > > > > 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. I think this is intentional (but could be improved). As inflation does not stop when the allocation fails (it simply sleeps for a while and resumes.. repeat till there are memory to inflate) That's why you see no inflation progress for long time under memory pressure. Best, Wei
