Re: [PATCH v21 QEMU 5/5] virtio-balloon: Provide an interface for free page reporting

[David Hildenbrand <david@redhat.com>]

>> Also, I do wonder if we want to default-enable it. It can still have a
>> negative performance impact and some people might not want that.
> 
> The negative performance impact should be minimal. At this point the
> hinting process ends up being very light since it only processes a
> small chunk of the memory before it shuts down for a couple seconds.
> In my original data the only time any significant performance
> regression was seen was with page shuffling enabled, and that was
> before I put limits on how many pages we could process per pass and
> how frequently we would make a pass through memory. My thought is that
> we are much better having it enabled by default rather than having it
> disabled by default. In the worst case scenario we have a little bit
> of extra thread noise from it popping up running for a few
> milliseconds and then sleeping for about two seconds, but the benefit
> side is that the VMs will do us a favor and restrict themselves to a
> much smaller idle footprint until such time as the memory is actually
> needed in the guest.

While I agree that the impact is small, there *is* a noticeable
performance impact. One of the main users is memory overcommit.

Also, imagine the following scenarios:
- RT workload in the guest. Just because you have a virtio-balloon
  device defined would mean something is suddenly active and
  discarding/trying to discard pages.
- vfio: free page reporting is completely useless right now and
  *only* overhead.
- prealloc does not expect that pages get suddenly discarded
- s390x, which has CMM and might not benefit at all (except when using
  huge pages as backing storage)

No, I don't think it's acceptable to enable this as default. I remember
that it is quite common to just define a balloon device but never use it.

E.g., "A virtual memory balloon device is added to all Xen and KVM/QEMU
guest virtual machines. It will be seen as <memballoon> element" [1].

I think we should let upper layers decide (just as we do for free page
hinting, for example).


[1]
https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/6/html/virtualization_administration_guide/section-libvirt-dom-xml-memory-baloon-device

-- 
Thanks,

David / dhildenb