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How are CPU's  allocated in ESXi

Posted on 2008-09-29
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Last Modified: 2010-04-21
I have a Dual Xebn Quad Core HP DL360 G5
I am running VMware ESXi
If I install a 32bit Windows Server 2003 guest and assign it 1 CPU  how is this CPU mapped to the cores in the hardware ?
Is it just as it says - a virtual CPU - that really then gets processed by all 8 cores ?
Or is it assigned (or can it be) to one core only?

Furthermore, If I install only 32 bit guests - am I 'wasting' the power of the hardware?
Would I get better performance from a 64bit Windows Server 2003 guest than a 32bit guest ?

Regards
Graeme
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Question by:touch-base
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4 Comments
 
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Expert Comment

by:larstr
ID: 22599694
Graeme,
One vcpu in the guest VM equals one cpu core of the host. When the VM is running, each of the vcpus inside the VM will run on different physical cpu cores. If a VM is allocated a single vcpu, it will be scheduled to run on the physical cpu core with the most available resources.

You can give a single guest a maximum of 4 vcpus, but you should always start with a single vcpu and only give your guest more vcpus if it really needs them.

Giving your guests more than 1 vcpu has some virtualization overhead and might in many cases give not equally good performance as if you give it a single one. Even though ESX as relaxed co-scheduling (compared to strict co-scheduling) you should always start out with a single vcpu.
 
Lars
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Author Comment

by:touch-base
ID: 22602151
OK thanks Lars

In that case - 2 further questions:
with 8 cores - does this mean I can only run up to 8 VM's with single vCPU's ?  

my 32/64 bit query:  If a VM does tie up a single core - should I not be using 64 bit guests instead of 32 bit?  Will a 64bit VM perform better than a 32bit VM ?

Graeme
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Accepted Solution

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larstr earned 250 total points
ID: 22603417
Graeme,
Normally you can run 4-8 VMs per cpu core. The bottle neck is normally the disk subsystem not the cpu. WIth 8 cores you should normally be able to run between 32 and 64 single cpu VMs without much problem if you have a good disk subsystem.

A 64 bit VM will not automatically give you better performance than a 32 bit VM as 64 bit VMs tend to have a bit lower disk io. Memory access should however be faster on 64 bit VMs.


Lars
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Author Closing Comment

by:touch-base
ID: 31501269
Thanks again Lars - Graeme
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