On 05.02.20 08:35, Nadav Amit wrote:

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On Feb 3, 2020, at 2:50 PM, Nadav Amit [off-list ref] wrote:

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On Feb 3, 2020, at 8:34 AM, David Hildenbrand [off-list ref] wrote:

On 03.02.20 17:18, Alexander Duyck wrote:

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On Mon, 2020-02-03 at 08:11 -0500, Michael S. Tsirkin wrote:

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On Thu, Jan 30, 2020 at 11:59:46AM -0800, Tyler Sanderson wrote:

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On Thu, Jan 30, 2020 at 7:31 AM Wang, Wei W [off-list ref] wrote:

  On Thursday, January 30, 2020 11:03 PM, David Hildenbrand wrote:

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On 29.01.20 20:11, Tyler Sanderson wrote:

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On Wed, Jan 29, 2020 at 2:31 AM David Hildenbrand <david@redhat.com
<mailto:david@redhat.com>> wrote:

  On 29.01.20 01:22, Tyler Sanderson via Virtualization wrote:

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A primary advantage of virtio balloon over other memory reclaim
mechanisms is that it can pressure the guest's page cache into

  shrinking.

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However, since the balloon driver changed to using the shrinker

  API

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<https://github.com/torvalds/linux/commit/71994620bb25a8b109388fefa9
e99a28e355255a#diff-fd202acf694d9eba19c8c64da3e480c9> this

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use case has become a bit more tricky. I'm wondering what the

intended

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device implementation is.

When inflating the balloon against page cache (i.e. no free

  memory

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remains) vmscan.c will both shrink page cache, but also invoke

  the

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shrinkers -- including the balloon's shrinker. So the balloon

  driver

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allocates memory which requires reclaim, vmscan gets this memory

by

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shrinking the balloon, and then the driver adds the memory back

  to

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the

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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

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