Re: [PATCH v3 9/9] tests/qtest: massively speed up migration-test

[Juan Quintela]

Peter Xu <peterx@redhat.com> wrote:
> On Wed, May 31, 2023 at 02:24:00PM +0100, Daniel P. Berrangé wrote:
>> The migration test cases that actually exercise live migration want to
>> ensure there is a minimum of two iterations of pre-copy, in order to
>> exercise the dirty tracking code.
>> 
>> Historically we've queried the migration status, looking for the
>> 'dirty-sync-count' value to increment to track iterations. This was
>> not entirely reliable because often all the data would get transferred
>> quickly enough that the migration would finish before we wanted it
>> to. So we massively dropped the bandwidth and max downtime to
>> guarantee non-convergance. This had the unfortunate side effect
>> that every migration took at least 30 seconds to run (100 MB of
>> dirty pages / 3 MB/sec).
>> 
>> This optimization takes a different approach to ensuring that a
>> mimimum of two iterations. Rather than waiting for dirty-sync-count
>> to increment, directly look for an indication that the source VM
>> has dirtied RAM that has already been transferred.
>> 
>> On the source VM a magic marker is written just after the 3 MB
>> offset. The destination VM is now montiored to detect when the
>> magic marker is transferred. This gives a guarantee that the
>> first 3 MB of memory have been transferred. Now the source VM
>> memory is monitored at exactly the 3MB offset until we observe
>> a flip in its value. This gives us a guaranteed that the guest
>> workload has dirtied a byte that has already been transferred.
>> 
>> Since we're looking at a place that is only 3 MB from the start
>> of memory, with the 3 MB/sec bandwidth, this test should complete
>> in 1 second, instead of 30 seconds.
>> 
>> Once we've proved there is some dirty memory, migration can be
>> set back to full speed for the remainder of the 1st iteration,
>> and the entire of the second iteration at which point migration
>> should be complete.
>> 
>> On a test machine this further reduces the migration test time
>> from 8 minutes to 1 minute 40.
>
> The outcome is definitely nice, but it does looks slightly hacky to me and
> make the test slightly more complicated.
>
> If it's all about making sure we finish the 1st iteration, can we simply
> add a src qemu parameter "switchover-hold"?  If it's set, src never
> switchover to dst but keeps the iterations.
>
> Then migrate_ensure_non_converge() will be as simple as setting
> switchover-hold to true.
>
> I am even thinking whether there can even be real-life use case for that,
> e.g., where a user might want to have a pre-heat of a migration of some VM,
> and trigger it immediately when the admin really wants (the pre-heats moved
> most of the pages and keep doing so).
>
> It'll be also similar to what Avihai proposed here on switchover-ack, just
> an ack mechanism on the src side:
>
> https://lore.kernel.org/r/20230530144821.1557-3-avihaih@nvidia.com

That was basically my idea and that is why I am holding the last two
patches and see if I can came with something in the next couple of days.

Later, Juan.