vCPU vs Core?

[sbentrup]

Are there any performance differences between a  2 vCPU allocation and a 1 vCPU 2-core allocation?  Is it purely a guest-licensing trick, or are there any real performance impacts (positive or negative?)

I asked an internal discussion group, and the only positive discussion point was that when you set up multiple vCPU - by default it does (or at least used to) allocate a pcore from different pCPU and there was an advanced option to change this.   a vCore may allocate multiple cores from the same pCPU instead.

Anyhow - some clarity around this would be appreciated.

Thanks!

[VMmatty]

I don't believe there is an specific performance advantage over 2 vCPUs vs 1 vCPU with 2 vCores.  The biggest reason why folks would use this for CPU based licensing schemes like you mentioned.

The other reason may be if organizations wanted to take advantage of vNUMA.  By default, vNUMA is enabled if you have 8 vCPUs or more but it can be enabled manually for smaller configurations.

For more info on vNUMA there are a number of great whitepapers that discuss it.

vSphere 5 Resource Management Guide:  http://pubs.vmware.com/vsphere-50/topic/com.vmware.ICbase/PDF/vsphere-esxi-vcenter-server-50-resourc...

Performance Best Practices for vSphere 5:  http://www.vmware.com/pdf/Perf_Best_Practices_vSphere5.0.pdf

Matt

http://www.thelowercasew.com

[Virtualinfra · Accepted solution]

This really a good discussion.

I just figured out few things with this KB and with CPU scheduler with vmware document..

http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=101018...

As per CPU proccessing with VMware.

1vCPU is mapped to 1 physical core ( not to physical cpu (socket)).

lets come to our discussion what happen when you increase the cores for virtual machine for example you have 1vCPU socket and increased the cores to 2. ESXi still see it as 2 individual vCPU and its  mapped to 2 physical CPU cores.Virtual machine see it a 1CPU with 2 cores(i.e is dual core). refer below screen shot with 1 virtual CPU socket and changing the core to 2 and finally it shows as 2vCPU's in summary tab.

So from ESX side the vCPU scheduling doesnt make difference if there is 2 individual vCPU socket or 1 vCPU socket with 2 vCore.. esxi see's it as 2vCPU and does the scheduling.. vmkernel does the scheduling in a much better way to give the good performance to the virtual machine.

Hope this clarifies..

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