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Difference between 2 vCPU sockets and 1 vCPU sockets w/ 2 cores

Posted on 2011-09-19
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Last Modified: 2012-05-12
In vsphere 5, does a windows guest think there is any difference if you assign 2 vCPU sockets with 1 core, than if you assigned 1 vCPU socket with 2 cores? With vsphere 5 you can now separate out the cores. Thanks
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Question by:fina27
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7 Comments
 
LVL 123
ID: 36560617
1 vCPU = 1 Core on the physical processor (but it could be a different core due to the scheduler)
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by:fina27
ID: 36560646
But the guest still thinks its 2 cores regardless of either config right? So does it technically not matter?
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LVL 123

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Andrew Hancock (VMware vExpert / EE MVE^2) earned 2000 total points
ID: 36560749
The guest has no knowledge of what a core is. To the guest it's a vCPU. The guest thinks it's 2 vCPU, which may be the same physical Cores, unless you physically allocate them.

Personally I would leave the settings alone, unless you have a real reason to allocate specific cores to a VM, and let the Hypervisor complete the scheduling for the VM.

With vsphere 5 you can now separate out the cores

You've always been able to do it, but now there is a new GUI, that may make it easier, rather than selecting Advanced CPU Affinity Options in vSphere 4.0.
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LVL 123
ID: 36560776
I've not seen the ESX 5.0 whitepaper yet, but this is for 4.1.

check it out

VMware vSphere 4: The CPU Scheduler in VMware ESX 4.1
http://www.vmware.com/files/pdf/techpaper/VMW_vSphere41_cpu_schedule_ESX.pdf
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Author Comment

by:fina27
ID: 36560812
What do you mean by leave the settings alone? So if i was building a new windows server and wanted to give it 3 vCPU's, would I select 3 virtual sockets and 1 core each or 1 virtual socket and 3 cores?

If your answer is it depends, then what is it depending on? Thanks
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LVL 123
ID: 36560897
Just allocate 3 vCPUs, and let the hypervisor schedule the vCPU across the total cores in the phsyical host.

Also remember that adding additional vCPU can also slow a virtual machine down to due the vSMP scheduling.
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LVL 123
ID: 36561014
Using multicore virtual CPUs can be useful when you run operating systems or applications that can take advantage of only a limited number of CPU sockets.

Chapter 4 of the Resource Management Guide explains in more details

http://pubs.vmware.com/vsphere-50/topic/com.vmware.ICbase/PDF/vsphere-esxi-vcenter-server-50-resource-management-guide.pdf
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