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

From: Anthony Liguori
Subject: Re: [Qemu-devel] [RFC] Migration convergence - a suggestion
Date: Wed, 21 Dec 2011 08:11:29 -0600
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv: Gecko/20110922 Lightning/1.0b2 Thunderbird/3.1.15

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

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

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.


Anthony Liguori

Regards, Ronen


Anthony Liguori

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