On 05.02.20 08:35, Nadav Amit wrote: >> On Feb 3, 2020, at 2:50 PM, Nadav Amit <namit@xxxxxxxxxx> wrote: >> >>> On Feb 3, 2020, at 8:34 AM, David Hildenbrand <david@xxxxxxxxxx> wrote: >>> >>> On 03.02.20 17:18, Alexander Duyck wrote: >>>> On Mon, 2020-02-03 at 08:11 -0500, Michael S. Tsirkin wrote: >>>>> 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? >>>> >>>> I would say reverting probably makes sense. I'm not sure there is much >>>> value to having a shrinker running deflation when you are actively trying >>>> to increase the balloon. It would make more sense to wait until you are >>>> actually about to start hitting oom. >>> >>> I think the shrinker makes sense for free page hinting feature >>> (everything on free_page_list). >>> >>> So instead of only reverting, I think we should split it up and always >>> register the shrinker for VIRTIO_BALLOON_F_FREE_PAGE_HINT and the OOM >>> notifier (as before) for VIRTIO_BALLOON_F_MUST_TELL_HOST. >>> >>> (Of course, adapting what is being done in the shrinker and in the OOM >>> notifier) >> >> David, >> >> Please keep me posted. I decided to adapt the same solution as the virtio >> balloon for the VMware balloon. If the verdict is that this is damaging and >> the OOM notifier should be used instead, I will submit patches to move to >> OOM notifier as well. > > Adding some information for the record (if someone googles this thread): > > In the VMware balloon driver, the shrinker is disabled by default since we > encountered a performance degradation in testing. I tried to avoid rapid > inflation/shrinker-deflation cycles by adding a timeout, but apparently it > did not help in avoiding the performance regression. Thanks for that info. To me that sounds like the shrinker is the wrong approach to "auto-deflation". It's not just "some slab cache". -- Thanks, David / dhildenb
