I haven't tested the performance differences with or without HT. We were running pure kvm-libvirt hosts on these machines and we're migrating them to oVirt and that's what triggered the problem with the Redhat 5 vms.

I'll probably give it a try and disable HT on the next hypervisor that we'll be adding to the cluster next week and see if that solves the problem or it just mitigates it.

X

On Thu, Oct 30, 2014 at 10:09 AM, Arman Khalatyan <arm2arm@gmail.com> wrote:
On our clusters with pure computation we use always HT off, HT was slowing down scientific calculations.
Just I'm interested if any benefit of enabling HT on the Ovirt host?
At least on my system host-Centos6.5, guest-Centos7VM-LAMPstack with separate CentOS7-MariaDBVM did not show any performance boost if I switch on/off HT on 2650v2.
a.



On Wed, Oct 29, 2014 at 5:24 PM, Xavier Naveira <xnaveira@gmail.com> wrote:
On 10/29/2014 05:15 PM, Daniel Helgenberger wrote:


On 29.10.2014 16:44, Xavier Naveira wrote:
On 10/29/2014 04:29 PM, Xavier Naveira wrote:
On 10/29/2014 04:06 PM, Daniel Helgenberger wrote:


On 29.10.2014 15:57, Xavier Naveira wrote:
On 10/29/2014 03:07 PM, Xavier Naveira wrote:
On 10/29/2014 01:26 PM, Daniel Helgenberger wrote:
On 29.10.2014 11:48, Xavier Naveira wrote:
On 10/29/2014 11:47 AM, Xavier Naveira wrote:
On 10/29/2014 11:40 AM, Daniel Helgenberger wrote:


On 29.10.2014 10:21, Xavier Naveira wrote:
Hi,

We are migrating our ifrastructure from kvm+libvirt hypervisors to
ovirt.

Everything is working fine but we're noticing that all the
qemu-kvm
processes in the hypervisors take a lot of CPU.
Without further details of the workload this is hard tell. One
Reason I
can think of might be KSM [1]. Is it enabled on your cluster(s)?
What is
your mem over-commitment setting?

Note, IIRC the KSM policy is currently hard coded; it will start at
80%
host mem usage.

[1] http://www.ovirt.org/Sla/host-mom-policy

The typical example is an idle machine, running top from the
machine
itself it reports cpu use percentages below 10% and loads (with 2
processors) of 0.0x. The process running that machine in the
hypervisor
rports cpu uses in the order of the 80-100%.

Should the values look like this? Why are the idle machines eating
up so
much CPU time?

Thank you.
Xavier

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Hi, thank you for the answer.

I've been trying to work out some pattern and realized that the VMs
using that much cpu all are Redhat 5.x, the Readhat 6.x doesn't
exhibit
this kind of high cpu use. (we run only redhat/centos 5.x/6.x on the
cluster)
What OS are the hosts running? In case of EL6, make sure you have
tuned-0.2.19-13.el6.noarch installed [1].

That's exactly the version we've in the hypervisors.


To further investigate please post Engine, VDSM, libvirt and kernel
versions from the hosts.

vdsm-xmlrpc-4.14.11.2-0.el6.noarch
vdsm-cli-4.14.11.2-0.el6.noarch
vdsm-python-4.14.11.2-0.el6.x86_64
vdsm-4.14.11.2-0.el6.x86_64
vdsm-python-zombiereaper-4.14.11.2-0.el6.noarch

libvirt-client-0.10.2-29.el6_5.12.x86_64
libvirt-0.10.2-29.el6_5.12.x86_64
libvirt-lock-sanlock-0.10.2-29.el6_5.12.x86_64
libvirt-python-0.10.2-29.el6_5.12.x86_64

2.6.32-431.23.3.el6.x86_64 #1 SMP Wed Jul 16 06:12:23 EDT 2014 x86_64
x86_64 x86_64 GNU/Linux



[1] https://access.redhat.com/solutions/358033

I'll take a look to the KSM config.

Cheers,

Xavier




Actually, this seems to be it. But I'm already at a newer kernel:
https://bugzilla.redhat.com/show_bug.cgi?id=705082
Well, I do not have such hardware so I never run into the issue. You
could disable HT as I suspect your physical cores are less then 64?

Your workload might differ but my VMs usually do not benefit from
'threaded' cores and I want HT disabled anyway. Also, you can check
cluster settings and disable 'count threads as cores' if enabled. But I
think this might not make any difference.


Yeah, these are machines with 4 sockets, 6 core per socket and HT
enabled, so total 48 "CPU".
Good to know; yet the largest host I have has 32 (2 sockets, 8 cores, HT
enabled) CPUs and is not showing this issue (at least I just looked and
everything seems fine).

So, are you implying that the problem is the number of "CPUs"? We were
hoping to add some more hypervisors to the cluster next week that have
even more cores...

I can probably try to disable HT when we add the next hypervisor next
week but it feels that it'd be just a workaround?
Maybe, but not a bad one as you should not have any disadvantages.

I opened a bug at redhat just in
case:https://bugzilla.redhat.com/show_bug.cgi?id=1158547
I have to ask as I cannot see the BZ because I have no subscription any
more. Against witch component did you open it?


I did as in the original bug: kernel, and then I took KVM as subsystem.

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