[Qemu-devel] [RFC] Migration convergence - a suggestion

[Ronen Hod]

Well the issue is not new, anyhow, following a conversation with Orit ...

Since we want the migration to finish, I believe that the "migration speed" parameter alone cannot do the job.
I suggest using two distinct parameters:
1. Migration speed - will be used to limit the network resources utilization
2. aggressionLevel - A number between 0.0 and 1.0, where low values imply minimal interruption to the guest, and 1.0 mean that the guest will be completely stalled.

In any case the migration will have to do its work and finish given any actual migration-speed, so even low aggressionLevel values will sometimes imply that the guest will be throttled substantially.

The algorithm:
The aggressionLevel should determine the targetGuest%CPU (how much CPU time we want to allocate to the guest)
With aggressionLevel = 1.0, the guest gets no CPU-resources (stalled).
With aggressionLevel = 0.0, the guest gets minGuest%CPU, such that migrationRate == dirtyPagesRate. This minGuest%CPU is continuously updated based on the running average of the recent samples (more below).

Note that the targetGuest%CPU allocation is continuously updated due to changes guest behavior, network congestion, and alike.

Some more details
- minGuest%CPU (i.e., for dirtyPagesRate == migrationRate) is easy to calculate as a running average of
  (migrationRate / dirtyPagesRate * guest%CPU)
- There are several methods to calculate the running average, my favorite is IIR, where, roughly speaking,
  newVal = 0.99 * oldVal + 0.01 * newSample
- I would use two measures to ensure that there are more migrated pages than "dirty" pages.
  1. The running average (based on recent samples) of the migrated pages is larger than that of the new dirty pages
  2. The total number of migrated pages so far is larger than the total number of new dirty pages.

And yes, many details are still missing.

Ronen.