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I have several Windows 7, CentOS, and Ubuntu virtual machines running within a VMWare ESXi server. To get better performance while building on them via Jenkins, I would like to increase their number of processors from 1 to 2-4.

The VMWare UI warns me that "Changing the number of virtual CPUs after the guest OS might make your virtual machine unstable".

Is increasing the number of virtual CPUs on these OSes a safe option?

This is the opposite of question Can I safely reduce the number of processors on a VM?

To follow up with my results, I increased the number of processors to 4 on each VM and have seen no problems.

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On a somewhat related topic you might also have licensing issues for software that is licensed by processor or core. This is usually more of an issue on the server side, but I just wanted to bring it to light. –  squillman Jan 8 '13 at 14:38
    
Yes. I've even changed the number of vCPUs on a VM right in the middle of a build. Not only were there no ill effects, the make began parallelizing immediately. –  Michael Hampton Jan 10 '13 at 17:21

2 Answers 2

up vote 4 down vote accepted

Increasing the number of CPU from 1 to anything more is safe on modern versions of the OSes/Distros you mentioned. Modern kernels dynamically adapt to the number of CPUs at startup and have no problems with you adding more. Old versions of Linux and Windows (Around 2000 and before) do not adapt well to change. The same is essentially true for reducing cores; though you also run the risk of bottlenecking the VM.

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More exact - modern Windows run a Multi core / socket Hardware layer anyway because this is what they get hit with MOST of the time These days. Only very old Versions "casually" assume a single core / processor. –  TomTom Jan 10 '13 at 14:38

The answer is the same as the question that you just linked it.

In OSes that have a multi-CPU kernel/HAL enabled, this isn't a problem. In a Windows XP/2003 or earlier VM with a single CPU HAL installed, there will be a performance hit. I've never seen actual instability from this, though.

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