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Our VMware admins tell me that we can add additional virtual CPU cores to our VM's, but they tell me that we can't reduce the number of virtual CPU cores.

One recently stated, "CPU on the other hand is a bigger problem because while you can add CPU cores, you cannot take them away once done." After asking a few why's, he stated, "To the CPU add / subtract problem, that is a limitation of pretty much all OS versions and has nothing to do with the VMware version."

My question is, can we reduce the number of CPU cores for a CentOS 6.8 virtual machine without having to reinstall or reconfigure the OS.

Web searches have turned up very little for me. Most search results are about adding virtual CPU cores or disabling CPU's within the OS.

My motivation... Sometimes, we run into performance issues with our VM's (e.g. this one is running Jenkins) and we need to resolve our performance issues quickly. Asking for more VMware resources (at least temporarily) turns into a slow, painful process because the admins say that once they give it to us, they can't take it back.

Thank you for any help you can provide!

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  • Virtualization is great for testing, I'm not sure why you didn't just do a test run of this. Also, are you sure you're not having miscommunication issues? I'd hate to have a VMware admin who thought reducing CPU cores would break an OS.... Apr 22, 2018 at 7:12
  • @FreeSoftwareServers, agreed, we need to try it out. I haven't tried yet myself for lack of hardware and no access to our VMware systems. Been a lot of back and forth with people involved, unfortunately I'm not mistaken about where the admin I'm currently working with stands. Since posting, I found an addition forum discussion. Looks like at one point there were limitations going between a single CPU core and multiple cores with Windows--correct me if I'm mistaken. Our admins may have run into issues many years ago and put a restriction in place that's never been challenged.
    – Will
    Apr 23, 2018 at 12:45
  • maybe with Windows Server License issues, but for Windows 10 I've changed my cpu core topology many times and Windows never complained. It also is aware it's a VM. Apr 26, 2018 at 17:06

2 Answers 2

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You can change the virtual machine dimensions (RAM, CPU, socket count) without reconfiguring the operating system. CentOS will handle this fine.

You cannot do this without a poweroff/reboot, though. The VM will need to be shut down in order to do this.

The political issue with provisioning and allocating resources is a separate situation. But the admins are not being truthful in their responses.

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Reducing the number of CPU cores merely requires that the VM be stopped. It can't be done while the VM is running. Perhaps that is what they meant? I suppose it's also possible that they don't know this...

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