On Thu, Jan 30, 2020 at 11:59:46AM -0800, Tyler Sanderson wrote: > > > On Thu, Jan 30, 2020 at 7:31 AM Wang, Wei W <wei.w.wang@xxxxxxxxx> wrote: > > 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? > > I could be wrong about the mechanism, but the device sees lots of activity on > the deflate queue. The balloon is being shrunk. And this only starts once all > free memory is depleted and we're inflating into page cache. So given this looks like a regression, maybe we should revert the patch in question 71994620bb25 ("virtio_balloon: replace oom notifier with shrinker") Besides, with VIRTIO_BALLOON_F_FREE_PAGE_HINT shrinker also ignores VIRTIO_BALLOON_F_MUST_TELL_HOST which isn't nice at all. So it looks like all this rework introduced more issues than it addressed ... I also CC Alex Duyck for an opinion on this. Alex, what do you use to put pressure on page cache? > > > > > > > > > > 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. > > As noted above the deflate queue is active, so it's not just memory allocation > failures. > > > > > Best, > Wei >
