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

["Daniel P. Berrangé" <berrange@redhat.com>]

On Thu, Jun 01, 2023 at 11:46:01AM -0400, Peter Xu 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.

For *most* of the tests, we want to ensure there are a minimum
of 2 iterations. For the XBZRLE test we want to ensure there are
a minimum of 3 iterations, so the cache gets  workout.

> 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

In general I strongly wanted to avoid adding special logic to the
migration code that makes it directly synchronize with the  test
suite, because once we do that I don't think the test suite is a
providing coverage of the real world usage scenario.

IOW, if we add a switchover-ack feature, we should certainly have
*a* test that uses it, but we should not modify other tests because
we want to exercise the logic as it would run with apps that don't
rely on switchover-ack.

Also, this slow migration test is incredibly painful for people right
now, so I'd like to see us get a speed up committed to git quickly.
I don't want to block it on a feature proposal that might yet take
months to merge.

With regards,
Daniel
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