4vcpu on 4CPU host

[ErikErik]

In the past I always got the advice to avoid 4VCPU's on a 4 CPU host because of the overhead this is causing and possible lock on the 4CPu's.

I want to gather some ideas about this because I think this is controversial ?

Kind Regards

Erik

[MR-T]

Becasue a virtual machine with multuiple vCPU's needs to co-schedule it's requests to seperate physical CPU's simultaneously, you end up with really high ready-times.

Basically a VM can't execute any instructions unless all 4 physical CPU's are free. So even if the ESX host has 2 CPU's idle and available, your virtual machine will not be able to make use of them.

[ErikErik]

The reason why I asked this is based on this paper:

they mention 4VCPU. That's why I wanted to gather some opinions

[TomHowarth]

I would not even consider a 4vCPU machine unless I had at least 16pCPUs or cores to run against. the overhead due to CPU Ready times on a machine with less CPU/cores would negate any performance benefits you could receive by using 4vCPU.

As Mr T has mentioned you need 4 pCPU/Cores available at the same time to process a single processor cycle in your VM and as CPU0 is utilised by the Service console unless you have a larger number of core or pCPU available you will be waiting for the Service console to release CPU0.

therefore a minimum of 8 logical processors would be needed to guarantee any amount of performance. Just because you can do something does not necessarily mean you should.

Tom Howarth

VMware Communities User Moderator

[ErikErik]

I agree. One last question : Is there a risk in counting on the cores and not on the physical CPU's?

[sureshsatya]

this is a gerat idea.....

[TomHowarth]

Sorry, what exactly do you mean by that?

Tom Howarth

VMware Communities User Moderator

[ErikErik]

I mean : e.g. an 8 pCPU compared to a 4pCPU with dual Cores

[TomHowarth]

no, there was a risk in counting HT cores as they were not complete logical processors, but that is not true with Cores as they are a full CPU.

Tom Howarth

VMware Communities User Moderator