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vmware provision vmguest CPU question

Posted on 2011-05-04
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Hi All,

We have a requirement to provision a VM guest that uses 4 cores. My understanding is that its not a good idea to mix core assignment to VM guests on the host. Some of the guest has 1 core, some of them have 2 cores.

Could some explain to me why it is not a good idea to mix assign cores ? or what is a better way of doiong this?

Thanks in advance for your help!
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Question by:fireburn11
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LVL 124
ID: 35691157
Do you want to assign the Virtual Guest to actual cores of the phyiscal processors, or assign 4 vCPU to the Virtual Guest?
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Author Comment

by:fireburn11
ID: 35691542
assing 4VCPU to the guest, although if youcould also share how to assign the virtual guest the actual cores of the physical processor , much appreciated. Thanks
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LVL 124
ID: 35691861
Assigning more than one vCPU to a guest, could possible slow it down.

see here

http://communities.vmware.com/message/1439622

it's explaind here in the guide

VMware vSphere™ 4: The CPU Scheduler in VMware ESX™ 4

http://www.vmware.com/files/pdf/perf-vsphere-cpu_scheduler.pdf

Best Practices Using VMware Virtual SMP

http://www.vmware.com/pdf/vsmp_best_practices.pdf

1 core = 1 virtual CPU (vCPU)

A physical processor/core is NOT dedicated to a vCPU but is shared with other virtual machines/vCPUs. The VMware Hypervisor will look to move the workload to another core every 20ms (by default) if required. While your vCPU is running on a core it can consume the whole core but this is time sliced with other vCPUs.

 
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Author Comment

by:fireburn11
ID: 35692275
Hi Hanccocka,

Thanks for the good links.
so in Vsphere, if I provision a vm with 4 cores, and rest of the guests on the host are 1 core or 2 cores, the vm with 4 cores will have to wait until 4 1 core guest release physical cpu or 2 2 core guest release physical core in order to function, right? wouldn't that cause the entire performance of the vms on the host?

Thanks
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LVL 124
ID: 35692318
You allocate VMs, a vCPU (which equals 1 core on a physical processor).

A physical processor/core is NOT dedicated to a vCPU but is shared with other virtual machines/vCPUs. The VMware Hypervisor will look to move the workload to another core every 20ms (by default) if required. While your vCPU is running on a core it can consume the whole core but this is time sliced with other vCPUs.

It's time scheduled very fast, there is no performance degradation.

Make a mental note of assigning 4 vCPU to a virtual machine you could make it slower.
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Author Comment

by:fireburn11
ID: 35692409
ok, so would you say that mixing 2 core / 1 core/ 4 core vm guests on the same esx host will not degrde perofrmance on esx vsphere?

Thanks
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LVL 124

Accepted Solution

by:
Andrew Hancock (VMware vExpert / EE MVE^2) earned 2000 total points
ID: 35692484
No.

But I would advise caution on using Dual vCPU and Quad vCPU guests, unless abosutely necessary, unless you've got applications that are designed for Dual or Quad SMP, and you are not just listening to vendors or following vendors guides.

TEST TEST TEST and TEST again, and if you have a CPU bottlenext, add another, that's the flexibility of vSMP, it doesn't "cost" anything to add another CPU.

Get over excited and adding vCPU everywhere because you can, will be your performance downfall, we've seen it too many times, and when asked, why has this got 2, 4 or 8 vCPUs, the answer "oh, because we could!".
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LVL 124
ID: 35692488
that should be Don't Get over excited!
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