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

On Fri, Apr 24, 2020 at 8:34 AM David Hildenbrand <david@redhat.com> wrote:
> >> 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).

Okay. I guess there are a number of other cases I hand't thought of. I
will switch the default to false.


- Alex

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