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Core/CPU recomendations for VMs

Posted on 2014-09-30
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Last Modified: 2014-10-01
Hi,

When others receive build specs for new systems (Windows Servers on vSphere) how do you spec out the CPU for a VM?  Say you receive a spec of Two (2) Six-core Xeon (12 cores total) do you build a VM with 12 vCPUs?  What about potential CPU wait issues?
Or, do you just build a 4 vCPU and make sure it's running on a host with Two (2) Six-core Xeon (12 cores total)?
When we ask vendors about visualizing we often get different or non-committal answers,  so wondering what others do?

Thanks
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Question by:kswan_expert
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by:ambatihp
ID: 40353874
If you are using vsphere ent. ed. with a cluster the load balancing portion is handled by the vsphere itself and there is no hard-limit of 12 (unless you are limited by Guest Licensing).

The I/O wait and CPU resources are largely a trail/error, in my experience its not the CPU that kills the VM's performance its actually the DISK I/O that limits the actual guest VM's  The faster the disk speed is the more you can cram in.

Now we pretty much started doing two disk 1U's with one SAS 15K for OS and SSD disk for caching and the Guest VM's over either on on direct attached or iscsi SAN.
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Andrew Hancock (VMware vExpert / EE MVE^2) earned 500 total points
ID: 40353987
We always start with a single vCPU, and add as required. (we ignore most vendors requirements, because they do not have a clue, and come up with stupid requirements!)

There is no difference in adding Cores or Sockets, performance is the same. Unless you have a real need for Licensing Restrictions, use sockets (e.g. vCPU).

Be careful that you do not over commit sockets, it's very easy to.

vSMP (virtual SMP) can affect virtual machine performance, when adding too many vCPUs to virtual machines that cannot use the vCPUs effectly, e.g. Servers than can use vSMP correctly :- SQL Server, Exchange Server.

This is true, many VMware Administrators, think adding lots of processors, will increase performance - wrong! (and because they can, they just go silly!). Sometimes there is confusion between cores and processors. But what we are adding is additional processors in the virtual machine.

So 4 vCPU, to the VM is a 4 Way SMP (Quad Processor Server), if you have Enterprise Plus license you can add 8, (and only if you have the correct OS License will the OS recognise them all).

If applications, can take advantage e.g. Exchange, SQL, adding additional processors, can/may increase performance.

So usual rule of thumb is try 1 vCPU, then try 2 vCPU, knock back to 1 vCPU if performance is affected. and only use vSMP if the VM can take advantage.

Example, VM with 4 vCPUs allocated!

My simple laymans explaination of the "scheduler!"

As you have assigned 4 vCPUs, to this VM, the VMware scheulder, has to wait until 4 cores are free and available, to do this, it has to pause the first cores, until the 4th is available, during this timeframe, the paused cores are not available for processes, this is my simplistic view, but bottom line is adding more vCPUs to a VM, may not give you the performance benefits you think, unless the VM, it's applications are optimised for additional vCPUs.

See here
http://www.vmware.com/resources/techresources/10131

see here
http://www.gabesvirtualworld.com/how-too-many-vcpus-can-negatively-affect-your-performance/

http://www.zdnet.com/virtual-cpus-the-overprovisioning-penalty-of-vcpu-to-pcpu-ratios-4010025185/

also there is a document here about the CPU scheduler

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

https://blogs.vmware.com/vsphere/2013/10/does-corespersocket-affect-performance.html
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by:kswan_expert
ID: 40355542
Cheers for great answer!
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No problems.
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