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Giving Windows VMs more processors

The Problem

I am working on porting some VMs from ESXi over to SmartOS and had encountered a problem. One of the VMs in ESXi is a Windows 7 machine with 4 cores. When I created the VM in SmartOS, I specified "vcpus: 4", but then upon booting the VM it did not appear to have 4 cores.

It turns out that SmartOS/qemu sets up the VM to have 4 individual CPU sockets when you specify vcpus: 4, not 4 cores, and although Windows allows many CPU cores, it has seemingly arbitrary limits on CPU sockets like:

  • Windows 7 Home Premium: 1
  • Windows 7 Professional: 2
  • Windows 8: 2
  • Windows Small Business Server 2003: 2
  • Windows Server 2012 Foundation: 1
  • Windows Server 2012 Essentials: 2
  • Windows Server 2012 Standard: 64
  • Windows Server 2012 Datacenter: 64

My SmartOS server is running a Xeon E3-1230 v3 processor which is one processor with 4 cores, and with hyperthreading normally shows up as 8 virtual processors if one was to run Windows on this processor, bare metal.

I want my Windows VM to be able to run 4 virtual processors, but the only way you can run Windows with 4 CPU sockets is to run with the full Windows Server packages, which in my use case is overkill and my applications aren't even supported on Windows Server.

The Solution

  1. Shut down the vm
  2. Update the value of vcpus to 1. (this will stop smartos from passing -smp option to QEMU)
  3. Add -smp threads=4 to your qemu_extra_opts
  4. (Optional) Adjust cpu shares / caps accordingly
  5. Boot the VM