Number of virtual processors per Virtual machine

[brini]

Hi,

I would like to understand why - on a ESX3.5 U1 with 2 quadricore CPU - a virtual machine cannot have more than 4 virtual CPU.

During a VM creation, the assistant only let me choose 1, 2 or 4 vcpu.

Why this limitation ? Why can't I choose 8 virtual processors?

Could someone give me an explantion to this ?

BRegards,

Sabrina

[AWo]

ESX 3.5 only supports 4 vCPU's. That is the maximum number of vCPU's you can assign to a guest.

Check the specs: http://pubs.vmware.com/vi35/config_max/config_max.1.2.html

Message was edited by: AWo

[sshamilt]

Why would you need to have a VM with 8 vCPUs anyway?

[AWo]

If you really have an application which stresses 8 CPU's and you would have 8 vCPU's that could lead to a severe performance degradation because VMware uses symmetric multiprocessoring. If your 8 vCPU guest wants to use them it would have to wait until all 8 vCPU's are available and if it has aquired the CPU's all other guests can't run because no vCPU is free at that moment. Therefore it is a good idea to fgrant less vCPU's to a guest that you have pCPU's in the host. A rule of thumb is 50%.

[brini]

Thx all for your help.

Actually the situation is pretty simple: I've got 3 identical ESX in a same cluster and today only 2 are hardly used (all VM could run in a single ESX).A developper needs to run tests on a 8 CPU servers for few days only: I could have dedicated to him the 3rd ESX for that short periode of time without disturbing the other VM. Is this limitation due to a technical VMWare ESX issue?

Rgds.

[admin · Accepted solution]

Hi Sabrina,

this isn´t a issue. It is just a limitation from VMware. In VI4 you can use 8 cores á VM.

I think there is not a chanch for your collegue to use a 8 core vm.

Thomas

[ChrisDearden]

Yes. ( it may increase in future releases but I wouldn't hold your breath)

[brini]

It is just a limitation from VMware.

Ok.

In VI4 you can use 8 cores á VM.

Is independant of the physical CPU architecture: I mean would it be the same if there are 2 quadripro or only 1 octopro ?

Is VI4 available now ?

[admin]

I think VI4 will be available in Q1 09.

Is independant of the physical CPU architecture: I mean would it be the same if there are 2 quadripro or only 1 octopro ?

What do you mean?

[brini]

What do you mean?

I initially thought that the ESX could not allow a VM to use cpu cores split on several processors.

In our case I thought that this cpu limitation was due to the 2 quadricore and that it would have not existed with a 1 octocore processor. But apparently the Pb would have been the same.

I now understood that limitation is not due to the cpu architecture.