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

[Wei Wang <wei.w.wang@intel.com>]

On 07/23/2018 10:36 PM, Dr. David Alan Gilbert wrote:
> * Michael S. Tsirkin (mst@redhat.com) 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