Re: [RFC][Patch v8 6/7] KVM: Enables the kernel to isolate and report free pages

["Michael S. Tsirkin" <mst@redhat.com>]

On Fri, Feb 08, 2019 at 02:05:09PM -0800, Alexander Duyck wrote:
> On Fri, Feb 8, 2019 at 1:38 PM Michael S. Tsirkin <mst@redhat.com> wrote:
> >
> > On Fri, Feb 08, 2019 at 03:41:55PM -0500, Nitesh Narayan Lal wrote:
> > > >> I am also planning to try Michael's suggestion of using MAX_ORDER - 1.
> > > >> However I am still thinking about a workload which I can use to test its
> > > >> effectiveness.
> > > > You might want to look at doing something like min(MAX_ORDER - 1,
> > > > HUGETLB_PAGE_ORDER). I know for x86 a 2MB page is the upper limit for
> > > > THP which is the most likely to be used page size with the guest.
> > > Sure, thanks for the suggestion.
> >
> > Given current hinting in balloon is MAX_ORDER I'd say
> > share code. If you feel a need to adjust down the road,
> > adjust both of them with actual testing showing gains.
> 
> Actually I'm left kind of wondering why we are even going through
> virtio-balloon for this?

Just look at what does it do.

It improves memory overcommit if guests are cooperative, and it does
this by giving the hypervisor addresses of pages which it can discard.

It's just *exactly* like the balloon with all the same limitations.

> It seems like this would make much more sense
> as core functionality of KVM itself for the specific architectures
> rather than some side thing.

Well same as balloon: whether it's useful to you at all
would very much depend on your workloads.

This kind of cooperative functionality is good for co-located
single-tenant VMs. That's pretty niche.  The core things in KVM
generally don't trust guests.


> In addition this could end up being
> redundant when you start getting into either the s390 or PowerPC
> architectures as they already have means of providing unused page
> hints.

Interesting. Is there host support in kvm?


> I have a set of patches I proposed that add similar functionality via
> a KVM hypercall for x86 instead of doing it as a part of a Virtio
> device[1].  I'm suspecting the overhead of doing things this way is
> much less then having to make multiple madvise system calls from QEMU
> back into the kernel.

Well whether it's a virtio device is orthogonal to whether it's an
madvise call, right? You can build vhost-pagehint and that can
handle requests in a VQ within balloon and do it
within host kernel directly.

virtio rings let you pass multiple pages so it's really hard to
say which will win outright - maybe it's more important
to coalesce exits.

Nitesh, how about trying same tests and reporting performance?


> One other concern that has been pointed out with my patchset that
> would likely need to be addressed here as well is what do we do about
> other hypervisors that decide to implement page hinting. We probably
> should look at making this KVM/QEMU specific code run through the
> paravirtual infrastructure instead of trying into the x86 arch code
> directly.
> 
> [1] https://lkml.org/lkml/2019/2/4/903


So virtio is a paravirtual interface, that's an argument for
using it then.

In any case pls copy the Cc'd crowd on future version of your patches.

-- 
MST