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

[Ronen Hod <rhod@redhat.com>]

On 12/20/2011 03:39 PM, Anthony Liguori wrote:
> On 12/20/2011 01:06 AM, Ronen Hod wrote:
>> 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)
>
> QEMU has no way to limit the guest CPU time.

Wouldn't any "yield" (sleep / whatever) limit the guest's CPU time, be 
it in qemu or in KVM.
My intention is to suggest an algorithm that is based on guest 
throttling. Looking at the relevant BZs, I do not see how we can avoid 
it. I certainly have no claims regarding the architecture.
Avi and mst, believe that it is better to continuously control the 
guest's CPU from the outside (libvirt) using cgroups. Although less 
responsive to changes, it should still work.
In the meantime, I also discovered that everybody has a different point 
of view regarding the requirements. Regardless, I believe that the same 
basic mechanics (once decided), can do the work
Some relevant configuration "requirements" are:
1. Max bandwidth
2. Min CPU per guest
3. Max guest stall time
4. Max migration time
These requirements will often conflict, and may imply changes in 
behavior over time.

I would also suggest that the management GUI will let the user select 
the aggression-level (or whatever), and display the implication on all 
the other parameters (total-time, %CPU) based on the current behavior of 
the guest and network.

Regards, Ronen

>
> Regards,
>
> Anthony Liguori