On Wed, Feb 05, 2020 at 09:19:58AM +0100, David Hildenbrand wrote: > 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". So as you pointed out yourself deflate on oom is really under-specified. I would be very happy if you could take a stub at documenting what's expected from guest and how it could be used. Please copy the virtio TC when you do this as this is spec stuff. > > -- > Thanks, > > David / dhildenb
