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

[Anthony Liguori <anthony@codemonkey.ws>]

On 12/21/2011 03:47 AM, Ronen Hod wrote:
> 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.

Practically speaking, not really.

> 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.

Yes, that's really the only way to do it I think.  You'll need to use the CFS 
bandwidth controller.

> 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.

It really depends on your management GUI.  If you have a task oriented GUI, you 
are unlikely to have a "live migration" feature.  Rather, you'll have a 
mechanism to enable load balancing (which may use live migration) or you'll have 
an "evacuate node" feature to allow system maintenance.

Your downtime policy really depends on the feature.  The best thing for QEMU to 
do is make sure we expose all of the necessary hooks to allow a wide variety of 
policies to be implemented.

Regards,

Anthony Liguori

>
> Regards, Ronen
>
>>
>> Regards,
>>
>> Anthony Liguori
>
>