RE: [Newbie question] Why usage of CPU inside VM and outside VM is different

[Gregory Esnaud <Gregory.ESNAUD@exfo.com>]

Hi Paolo,

Thanks for your quick answer.

Are you using only 2 CPUs at 100%, or are you using 9 CPUs at a total of 200%? 

Inside the VM, a top command show that we are using 2 CPU @ 100%: https://framadrop.org/r/gNf3erJVf6#t2HLIpzPHGtxHzHeLrnGGjsRSZZpmv9HPVZt/LIIP8s=
From the hypervisor (ie, *outside* the vm), a top command show that the VM consuming 9 cpu: https://framadrop.org/r/-FmrBhMGX5#q/6aZ8Fq2Mnuzz3+6F/pa83U+jzPox1hd984tw7DkuA=

So, if I'm understanding you correctly, an parameter of kernel of our VM is idel=poll that we should change?


Thanks again,

-----Message d'origine-----
De : Paolo Bonzini <pbonzini@redhat.com> 
Envoyé : mardi 7 janvier 2020 17:59
À : Gregory Esnaud <Gregory.ESNAUD@exfo.com>; kvm@vger.kernel.org
Objet : Re: [Newbie question] Why usage of CPU inside VM and outside VM is different

On 07/01/20 16:57, Gregory Esnaud wrote:
> 
> From an hypervisor (via top command) point of view our VM is consuming 
> 9 CPU (900%). This was reported by our platform provider.
> From a VM point of view we are consuming only 2 CPU (200%), with a top 
> also.
> 
> Our provider claim us to explain why we are consuming so much CPU.
> But we cannot troubleshoot the infra as it's not our responsibility.
> From our point of view, everything is ok.

Are you using only 2 CPUs at 100%, or are you using 9 CPUs at a total of 200%?  If the former, it is possible that you are using something like idle=poll, so the machine _is_ idle but still consuming 100% host CPU.
This is something that you should know, however.

If the latter, the host is probably doing a little bit of busy waiting to improve your performance.  The provider can disable it using the
kvm.halt_poll_ns=0 module parameter and there is nothing that you can do about it.  But it is unlikely that it causes the utilization to jump so much.

Paolo