CPU over-commitment

[Jerry Jiang]

HI all,



I have question not figured out very well. (may not for CS, but I noticed CS
supports CPU over-commitment)



Imaging the following case,



A host owns 4 physical CPUs

Because apps on VM1 consume 350% CPU during 10am-11am, so I gave VM1 4 vCPUs
to ensure performance

Because apps on VM2 consume 360% CPU during 10am-11am, so I gave VM2 4 vCPUs
to ensure performance



So I would ask if Apps are both happy with vm1 and vm2 during 10am-11am



If not, Is CPU over-commitment meaningful? How is your guys consider the
over-commitment



Thanks

Jerry

[Bharat Kumar]

Hi jiri,

The actual utilization can never be greater than 400% i.e. 100% per vCPu. we are overcommitting by allocating 4 vCpus to both the VMs. if both the VMs are trying to use what is allocated then
there will be contention for resources. During the contention the resource allocation is done differently in different hypervisors.


So in the scenario below both the apps cannot be happy if VM1 and VM2 want to use 350% and 360% at the same time.

Overcommit is useful to increase the overall resource utilization.

Overcommit is useful if the scenario was say VM1 uses 300% and VM2 uses 100% during 10-11 am and
form 11-12am VM2 uses 300% where as VM1 uses 100%. we have overcommitted the resource by allocation more than what we have,
this will keep the cpu utilization of the host near 100% most of the time and we will be able to launch more VMs as not all the VMs try to use the
resource allocated to them at the same time.

Regards,
Bharat.

Post by Jerry Jiang
HI all,
I have question not figured out very well. (may not for CS, but I noticed CS
supports CPU over-commitment)
Imaging the following case,
A host owns 4 physical CPUs
Because apps on VM1 consume 350% CPU during 10am-11am, so I gave VM1 4 vCPUs
to ensure performance
Because apps on VM2 consume 360% CPU during 10am-11am, so I gave VM2 4 vCPUs
to ensure performance
So I would ask if Apps are both happy with vm1 and vm2 during 10am-11am
If not, Is CPU over-commitment meaningful? How is your guys consider the
over-commitment
Thanks
Jerry

[Jerry Jiang]

Thanks. Kumar, what you commented is defiantly what I want.

Well, do you have something to share how to avoid the contention for VM1 and
vm2?

I propose a solution that during 9-10am stopped VM1, running vm2
During 10-11am,stopped vm2 and running vm1

Thanks
Jerry



Jerry

-----邮件原件-----
发件人: Bharat Kumar [mailto:bharat.kumar-Sxgqhf6Nn4DQT0dZR+***@public.gmane.org]
发送时间: 2013年8月16日 星期五 12:58
收件人: <users-***@public.gmane.org>
抄送: Jerry Jiang
主题: Re: CPU over-commitment

Hi jiri,

The actual utilization can never be greater than 400% i.e. 100% per vCPu.
we are overcommitting by allocating 4 vCpus to both the VMs. if both the
VMs are trying to use what is allocated then there will be contention for
resources. During the contention the resource allocation is done
differently in different hypervisors.


So in the scenario below both the apps cannot be happy if VM1 and VM2 want
to use 350% and 360% at the same time.

Overcommit is useful to increase the overall resource utilization.

Overcommit is useful if the scenario was say VM1 uses 300% and VM2 uses 100%
during 10-11 am and
form 11-12am VM2 uses 300% where as VM1 uses 100%. we have overcommitted
the resource by allocation more than what we have,
this will keep the cpu utilization of the host near 100% most of the time
and we will be able to launch more VMs as not all the VMs try to use the
resource allocated to them at the same time.

Regards,
Bharat.

Post by Jerry Jiang
HI all,
I have question not figured out very well. (may not for CS, but I
noticed CS supports CPU over-commitment)
Imaging the following case,
A host owns 4 physical CPUs
Because apps on VM1 consume 350% CPU during 10am-11am, so I gave VM1 4
vCPUs to ensure performance
Because apps on VM2 consume 360% CPU during 10am-11am, so I gave VM2 4
vCPUs to ensure performance
So I would ask if Apps are both happy with vm1 and vm2 during
10am-11am
If not, Is CPU over-commitment meaningful? How is your guys consider
the over-commitment
Thanks
Jerry

[Bharat Kumar]

Hi jerry,

In general overcommit is done when you know the usage pattern and have done some real time testing.

The method suggested by you is also good but not scaleable :). Generally in case of contention the host adjusts the cpu allocated to a vm based on the wight or some reservation.

So in this scenario as we know the usage pattern we should have deployed
them in different host or we can set some reservation based on the importance of the vm.

Regards,
Bharat.

Post by Jerry Jiang
Thanks. Kumar, what you commented is defiantly what I want.
Well, do you have something to share how to avoid the contention for VM1 and
vm2?
I propose a solution that during 9-10am stopped VM1, running vm2
During 10-11am,stopped vm2 and running vm1
Thanks
Jerry
Jerry
-----邮件原件-----
发送时间: 2013年8月16日 星期五 12:58
抄送: Jerry Jiang
主题: Re: CPU over-commitment
Hi jiri,
The actual utilization can never be greater than 400% i.e. 100% per vCPu.
we are overcommitting by allocating 4 vCpus to both the VMs. if both the
VMs are trying to use what is allocated then there will be contention for
resources. During the contention the resource allocation is done
differently in different hypervisors.
So in the scenario below both the apps cannot be happy if VM1 and VM2 want
to use 350% and 360% at the same time.
Overcommit is useful to increase the overall resource utilization.
Overcommit is useful if the scenario was say VM1 uses 300% and VM2 uses 100%
during 10-11 am and
form 11-12am VM2 uses 300% where as VM1 uses 100%. we have overcommitted
the resource by allocation more than what we have,
this will keep the cpu utilization of the host near 100% most of the time
and we will be able to launch more VMs as not all the VMs try to use the
resource allocated to them at the same time.
Regards,
Bharat.

Post by Jerry Jiang
HI all,
I have question not figured out very well. (may not for CS, but I
noticed CS supports CPU over-commitment)
Imaging the following case,
A host owns 4 physical CPUs
Because apps on VM1 consume 350% CPU during 10am-11am, so I gave VM1 4
vCPUs to ensure performance
Because apps on VM2 consume 360% CPU during 10am-11am, so I gave VM2 4
vCPUs to ensure performance
So I would ask if Apps are both happy with vm1 and vm2 during
10am-11am
If not, Is CPU over-commitment meaningful? How is your guys consider
the over-commitment
Thanks
Jerry