[ovirt-users] Ovirt VM Performance abd CPU times

[Xavier Naveira]

Ok, so we found a "solution" that I thought I'll share in case this thread
pops up in a future search.

The problem was that our Redhat 5.x guests still run with a kernel that
lacks the "tickeless" feature, meaning that they are poking the host a 1000
times per second even if they haven't anything to do.

Disabling hyperthreading and most importantly, adding the parameter
"divider=10" to the kernel boot line in grub does indeed lower the cpu
utilization in the hosts almost to 0% when idling.

In our case we use the command

sudo /sbin/grubby --update-kernel=ALL --args="divider=10"

To upgrade the /boot/grub/grub.conf file and then we restart the vm.

Thank you everyone for the help.

Xavier

On Mon, Nov 10, 2014 at 8:32 AM, Xavier Naveira <xnaveira at gmail.com> wrote:

> We tried a minimal installation from CD of RedHat 5.10 and it is the same.
>
> This should be fairly easy to reproduce:
>
> - Install a RedHat 6.5 hypervisor
> - Install a RedHat 5.10 guest in it
> - Enjoy your overused CPU
>
> Is there someone with a similar setup out there?
>
> Xavier
>
> On Fri, Nov 7, 2014 at 12:30 PM, Gianluca Cecchi <
> gianluca.cecchi at gmail.com> wrote:
>
>> On Fri, Nov 7, 2014 at 10:48 AM, Xavier Naveira <xnaveira at gmail.com>
>> wrote:
>>
>>> Hi,
>>>
>>> We have installed and added a new hypervisor into the ovirt cluster but
>>> this time with disabled HT.
>>>
>>> I migrated a RedHat 5.10 machine to it and immediately the qemu-kvm
>>> process running the vm (freshly installed, just basic packages) began to
>>> consume 20-40% CPU as showed running top on the hypervisor.
>>>
>>> Now that I have a hypervisor to run tests in, what would you suggest the
>>> next step is?
>>>
>>> Thank you.
>>>
>>> Xavier
>>>
>>>
>>>
>> If I remember correctly you had to test plain Qemu/KVM on CentOS 6.5 and
>> see if the difference is made by oVirt itself or by the OS changed from 5.x
>> to 6.y...
>> And also compare command line (eventually both in 5.x and 6.x) between
>> plain Qemu/KVM and oVirt spawned VMs
>> Gianluca
>>
>
>
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