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VMWare, 5 core VM, and resulting processes

Posted on 2014-01-08
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I need to ask the difference between a native 5 core machine and a 5 core VM

As I understand it a 5 core VM essentially has only 5 true processes, and everything else runs as a thread off of that process

In other words, an app that uses 5 processes on 5 cores native is not the same as the same app on a 5 core VM.  In the 5 core VM you have 5 processes, and evrything else runs as threads under that process.  Is that correct

If I have a 5 core HT (allocating a total of 10 cores to make the extra threads visable), I would then have 5 core processes  with the apps processes being resolved to threds under the 5 core VM ?

How is my logic ?

Thanks
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Question by:Los Angeles1
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by:Netman66
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From everything I've read, a vCPU maps directly to a CPU core.

This may help with understanding the scheduling:

http://www.vmware.com/files/pdf/techpaper/VMware-vSphere-CPU-Sched-Perf.pdf
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Andrew Hancock (VMware vExpert / EE MVE) earned 500 total points
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When creating VMs, and allocating processors to the VMs which are virtual e.g. vCPUs

1 vCPU = 1 physical core of the physical processor, but it may not be the same physical core, throughout the operation of the VM. e.g. the hypervisor will move the process from one core to another, based on loading.

You are correct physical computers are different to virtual computers.

A physical computer will have access to all the processors, cores at one time, in a virtual computer, we allocate 1 virtual processor to 1 physical core.

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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