Peter Xu <pet...@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-avih...@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.
