Limiting per vCPU Usage

Limiting per vCPU Usage

[Peter Lieven]

Hi Qemu folks,


is there a BCP to limit just the maximum usage of a virtual (KVM) cpu?

I know that there are many approaches, but as far as I know they all limit the complete qemu process which is far more

than just the virtual CPUs.

Is it possible to limit just the vCPU threads and leave threads that handle the monitor, vnc, qmp, iothreads, storage backend etc.

unthrottled?


Thanks,

Peter

Re: Limiting per vCPU Usage

[Daniel P. Berrangé]

On Sun, Sep 20, 2020 at 10:24:41PM +0200, Peter Lieven wrote:
> Hi Qemu folks,
> 
> 
> is there a BCP to limit just the maximum usage of a virtual (KVM) cpu?
> 
> I know that there are many approaches, but as far as I know they all limit the complete qemu process which is far more
> 
> than just the virtual CPUs.
> 
> Is it possible to limit just the vCPU threads and leave threads that
> handle the monitor, vnc, qmp, iothreads, storage backend etc.

Libvirt uses cgroups to achieve what you describe. A cgroup for the
QEMU process as a whole, then create child cgroups, one for each
vCPUs, and a further one for non-vCPU threads. CPU limits are then
applied on the child cgroups.

Regards,
Daniel
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Re: Limiting per vCPU Usage

[Peter Lieven]

Am 21.09.20 um 10:29 schrieb Daniel P. Berrangé:
> On Sun, Sep 20, 2020 at 10:24:41PM +0200, Peter Lieven wrote:
>> Hi Qemu folks,
>>
>>
>> is there a BCP to limit just the maximum usage of a virtual (KVM) cpu?
>>
>> I know that there are many approaches, but as far as I know they all limit the complete qemu process which is far more
>>
>> than just the virtual CPUs.
>>
>> Is it possible to limit just the vCPU threads and leave threads that
>> handle the monitor, vnc, qmp, iothreads, storage backend etc.
> Libvirt uses cgroups to achieve what you describe. A cgroup for the
> QEMU process as a whole, then create child cgroups, one for each
> vCPUs, and a further one for non-vCPU threads. CPU limits are then
> applied on the child cgroups.


Hi Daniel,


I came up with the same solution. Is there a nice way to get the thread ids of the vCPUs?

Iterating over all thread IDs and check the names works, but the thread names are not considered

a stable API (text from original commit message).


Thanks,

Peter

Re: Limiting per vCPU Usage

[Daniel P. Berrangé]

On Mon, Sep 21, 2020 at 03:47:56PM +0200, Peter Lieven wrote:
> Am 21.09.20 um 10:29 schrieb Daniel P. Berrangé:
> > On Sun, Sep 20, 2020 at 10:24:41PM +0200, Peter Lieven wrote:
> > > Hi Qemu folks,
> > > 
> > > 
> > > is there a BCP to limit just the maximum usage of a virtual (KVM) cpu?
> > > 
> > > I know that there are many approaches, but as far as I know they all limit the complete qemu process which is far more
> > > 
> > > than just the virtual CPUs.
> > > 
> > > Is it possible to limit just the vCPU threads and leave threads that
> > > handle the monitor, vnc, qmp, iothreads, storage backend etc.
> > Libvirt uses cgroups to achieve what you describe. A cgroup for the
> > QEMU process as a whole, then create child cgroups, one for each
> > vCPUs, and a further one for non-vCPU threads. CPU limits are then
> > applied on the child cgroups.
> 
> 
> Hi Daniel,
> 
> 
> I came up with the same solution. Is there a nice way to get the thread ids of the vCPUs?
> 
> Iterating over all thread IDs and check the names works, but the thread names are not considered
> 
> a stable API (text from original commit message).

The QMP  query-cpus  command tells you thread IDs. There is similar for
I/O threads too.  This is what libvirt uses to determine the mappings.

Regards,
Daniel
-- 
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|: https://libvirt.org         -o-            https://fstop138.berrange.com :|
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