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VMWare, vCPU allocation for VM

Posted on 2012-03-17
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If I have a 16 core server, and I allocate only a single VM, and that VM is only allocated 8 vCPU, ......

... will that VM run with only 8 cores, or will VMWare detect that more cores are available and steal cores and give it to the VM, therefor that VM is running with more than 8 cores ?

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Question by:Los Angeles1
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Expert Comment

by:coolsport00
ID: 37733534
The VM will use what it needs at any given time. Unless you use 'CPU affinity', which is a config for a VM to use explicit CPUs, the VM will use whatever is available at any time when the VM needs compute resources. I highly recommend reading over the CPU Scheduler whitepaper:
http://www.vmware.com/files/pdf/techpaper/VMW_vSphere41_cpu_schedule_ESX.pdf

~coolsport00
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Andrew Hancock (VMware vExpert / EE MVE^2) earned 2000 total points
ID: 37733547
if you have allocated 8 vCPU.

8 vCPU = 8 physical cores on the physical host. in simple terms.

It will never run with more than 8vCPUs.

What license do you have?

because the license will limit you to a max of 8 vCPUs!
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Author Comment

by:Los Angeles1
ID: 37733598
Do the 2 posts above disagree.  The first says the scheduler will use whatever it needs, (so I assume that the VM will use extra cores that are not being utilized, even if it exceeds the limit of vCPU assigned to it)  the second says 8 vCPu means only 8 physical cores.

Am I misunderstanding your answers (I know you both generally agree, so I must misunderstand something)
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LVL 123
ID: 37733608
if you assign 8vCPUs, the hypervisor WILL ONLY ever execute your VM on 8 Cores.

(they may not be the same cores, through the process of the execution!)

e.g. the scheduler looks at ALL cores every 20ms, you VM may run on the same 8 Cores, or it could change to another 8 Cores, but by defination of 8vCPUs, it will never be more than this.

AND, if you do not have a license to use 32 vCPUs, license restricts you to 8 Way SMP.
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