linux-kernel - CPU utilization between physical CPU and virtual CPU in KVM

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Message-ID: <CA+U0gVjD9mBxR5=r1qv69MHN9b7Dd1PPRnEzchZpYuOU_W1eNw@mail.gmail.com>
Date:	Mon, 8 Oct 2012 15:01:57 +0800
From:	Dennis Chen <kernel.org.gnu@...il.com>
To:	linux-kernel@...r.kernel.org
Cc:	qemu-devel@...gnu.org, libvir-list@...hat.com,
	dennis.chen@...oft.com.cn
Subject: CPU utilization between physical CPU and virtual CPU in KVM

Hi All,

I am confused by the following observed scenario:

In my 4-CPU (KVM supported, 2 core with 2 thread for each) host
machine box, I create only one VM with 3-vCPU through virsh/libvirt
tools and also I pin this VM process to the physical processor 3. I
guess the CPU utilization for the processor 3 will not exceed 100%,
then I create 3 process (dead loop-- while(1);) and bind each of them
to vCPU[0-2] respectively, through the "top -c" command in VM
environment, I can see the CPU utilization for each of the vCPU is
about 100%, but interesting, I found that the CPU utilization of
processor 3 in the host machine is about 300% with "toc -c" command.
why does a single process bound to a CPU can get ~300% cpu bandwidth
in this case, does the kernel scheduler dispatch the idle cycle
capacity of the CPUs to the virtual CPU of the VM, other word, the
scheduler knows the vCPU info in the VM process?

For the same case, if I create another 4 new dead-loop processes and
bind them to the physical CPU[0-3] equally, then I find the vCPU0/1 in
VM will not be 100%, eg. 32%,  (I think the scheduler in the guest OS
doesn't know it's running in a virtual environment, so the utilization
of the vCPU will not change to adapt to the physical processor
utilization, but it did, why?

-org-gnu
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