Re: Balloon pressuring page cache

[Nadav Amit <namit@vmware.com>]

> On Feb 3, 2020, at 2:50 PM, Nadav Amit <namit@vmware.com> wrote:
> 
>> On Feb 3, 2020, at 8:34 AM, David Hildenbrand <david@redhat.com> 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@intel.com> 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@redhat.com
>>>>>>> <mailto:david@redhat.com>> 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.

So there is no such issue in VMware balloon driver, unless someone
intentionally enables the shrinker through a module parameter.