libvirt users - May 2013 - Re: About the change of a VM's vcpu count in runtime?

Hi,

We know that there is a command "setvcpus" in virsh used to
"change
number of virtual CPUs", now we just have the need to change vcpu count 
dynamically for a running VM, so I tried this command (the vcpu count is 
2 in the VM xml file), below is the result:

virsh # setvcpus 3 --count 4
error: invalid argument: requested vcpus is greater than max allowable 
vcpus for the domain: 4 > 2

virsh # setvcpus 3 --count 1
error: Operation not supported: qemu didn't unplug the vCPUs properly

So I tried another way: log into the running VM, and "echo 0 > 
/sys/devices/system/cpu/cpu1/online", after that I can find only one 
processor in the vm through '/proc/cpuinfo', but in virsh, the vcpuinfo 
still shows:

virsh # vcpuinfo 3
VCPU:           0
CPU:            0
State:          running
CPU time:       13.5s
CPU Affinity:   yyyy

VCPU:           1
CPU:            1
State:          running
CPU time:       6.9s
CPU Affinity:   yyyy

So the question is: is there any method I can't find yet to change the 
vcpu count of a running vm dynamically?

BRs,
Dennis

On 05/28/2013 01:24 AM, Dennis Chen wrote:
> Hi,
> 
> We know that there is a command "setvcpus" in virsh used to
"change
> number of virtual CPUs", now we just have the need to change vcpu
count
> dynamically for a running VM, so I tried this command (the vcpu count is
> 2 in the VM xml file), below is the result:
> 
> virsh # setvcpus 3 --count 4
> error: invalid argument: requested vcpus is greater than max allowable
> vcpus for the domain: 4 > 2
Correct - you can't exceed the maximum number of vcpus that you told
qemu on its initial command line.  Worse, qemu hasn't supported cpu
hotplug or hot-unplug for quite some time (qemu 1.5 added partial
support for hotplug, but still left hot-unplug unimplemented; qemu 1.5
also added support for guest-agent unplug, but we have not yet wired
that up in libvirt).
> 
> virsh # setvcpus 3 --count 1
> error: Operation not supported: qemu didn't unplug the vCPUs properly
Alas, until qemu actually implements vcpu hot-unplug, this will fail for
you (it works for xen:// URI though).
> 
> So I tried another way: log into the running VM, and "echo 0 >
> /sys/devices/system/cpu/cpu1/online", after that I can find only one
> processor in the vm through '/proc/cpuinfo', but in virsh, the
vcpuinfo
> still shows:
That's not hot-unplug from qemu's perspective.  That's just
disabling
the use of the cpu from within the guest (and in fact, that's what the
new qemu 1.5 guest agent commands do, where we are still working on how
best to wire that up into libvirt).
> So the question is: is there any method I can't find yet to change the
> vcpu count of a running vm dynamically?
Wait for newer qemu and libvirt, or else pitch in and help write patches.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

On 05/28/2013 02:24 PM, Eric Blake wrote:
>> So the question is: is there any method I can't find yet to change
the
>> vcpu count of a running vm dynamically?
> 
> Wait for newer qemu and libvirt, or else pitch in and help write patches.
You might not have to wait that long - if you could help test these
patches, it would be appreciated:

https://www.redhat.com/archives/libvir-list/2013-May/msg01857.html

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org