Re: [PATCH v36 0/5] Virtio-balloon: support free page reporting

[Wei Wang]

On 07/23/2018 10:36 PM, Dr. David Alan Gilbert wrote:
> * Michael S. Tsirkin (mst@xxxxxxxxxx) wrote:
> > On Fri, Jul 20, 2018 at 04:33:00PM +0800, Wei Wang wrote:
> > > This patch series is separated from the previous "Virtio-balloon
> > > Enhancement" series. The new feature, VIRTIO_BALLOON_F_FREE_PAGE_HINT,
> > > implemented by this series enables the virtio-balloon driver to report
> > > hints of guest free pages to the host. It can be used to accelerate live
> > > migration of VMs. Here is an introduction of this usage:
> > >
> > > Live migration needs to transfer the VM's memory from the source machine
> > > to the destination round by round. For the 1st round, all the VM's memory
> > > is transferred. From the 2nd round, only the pieces of memory that were
> > > written by the guest (after the 1st round) are transferred. One method
> > > that is popularly used by the hypervisor to track which part of memory is
> > > written is to write-protect all the guest memory.
> > >
> > > This feature enables the optimization by skipping the transfer of guest
> > > free pages during VM live migration. It is not concerned that the memory
> > > pages are used after they are given to the hypervisor as a hint of the
> > > free pages, because they will be tracked by the hypervisor and transferred
> > > in the subsequent round if they are used and written.
> > >
> > > * Tests
> > > - Test Environment
> > >      Host: Intel(R) Xeon(R) CPU E5-2699 v4 @ 2.20GHz
> > >      Guest: 8G RAM, 4 vCPU
> > >      Migration setup: migrate_set_speed 100G, migrate_set_downtime 2 second
> > >
> > > - Test Results
> > >      - Idle Guest Live Migration Time (results are averaged over 10 runs):
> > >          - Optimization v.s. Legacy = 409ms vs 1757ms --> ~77% reduction
> > > 	(setting page poisoning zero and enabling ksm don't affect the
> > >           comparison result)
> > >      - Guest with Linux Compilation Workload (make bzImage -j4):
> > >          - Live Migration Time (average)
> > >            Optimization v.s. Legacy = 1407ms v.s. 2528ms --> ~44% reduction
> > >          - Linux Compilation Time
> > >            Optimization v.s. Legacy = 5min4s v.s. 5min12s
> > >            --> no obvious difference
> > I'd like to see dgilbert's take on whether this kind of gain
> > justifies adding a PV interfaces, and what kind of guest workload
> > is appropriate.
> >
> > Cc'd.
> Well, 44% is great ... although the measurement is a bit weird.
>
> a) A 2 second downtime is very large; 300-500ms is more normal

No problem, I will set downtime to 400ms for the tests.

> b) I'm not sure what the 'average' is  - is that just between a bunch of
> repeated migrations?

Yes, just repeatedly ("source<---->destination" migration) do the tests 
and get an averaged result.


> c) What load was running in the guest during the live migration?

The first one above just uses a guest without running any specific 
workload (named idle guests).
The second one uses a guest with the Linux compilation workload running.

> An interesting measurement to add would be to do the same test but
> with a VM with a lot more RAM but the same load;  you'd hope the gain
> would be even better.
> It would be interesting, especially because the users who are interested
> are people creating VMs allocated with lots of extra memory (for the
> worst case) but most of the time migrating when it's fairly idle.

OK. I will add tests of a guest with larger memory.

Best,
Wei