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Deciding when to use virtual symmetric multiprocessing

Should you assign a virtual machine (VM) more than one virtual processor or not? It’s common for admins to configure virtual symmetric multiprocessing, or VMs with multiple CPUs, whether it is needed or not.The decision to use more then one virtual processor in a VM should be based on an actual requirement by the applications installed on the VM and not simply because two processors are better then one. Many physical servers commonly have multiple CPUs regardless if the applications running require them. While being wasteful of server resources, this does not negatively impact a physical server but most VMs will usually run better with one virtual processor and can actually run slower when more than one is assigned to it.

The reason for this is the hypervisor’s CPU scheduler must find simultaneous cores available equal to the number assigned to the VM. So a four VCPU VM will need to have four free cores available on the host for every CPU request that is made by the VM. If there are not four cores available because other VMs are using them then the VM must wait until the cores become available. Single VCPU VMs have a much easier time because they only need there to be a single core available for the scheduler to process CPU requests for it.

Here are some tips on assigning VCPUs to VMs:

  • Limit the number of VSMP VMs on your hosts. The less you have, the better your VMs will perform.
  • Assign a VM multiple VCPUs only if you are running an application that requires it and will make use of them.
  • Don’t assign a VM the same amount of VCPUs as your host system has total cores available.
  • If you are going to use VSMP have at least twice (preferably three or four times) the number of cores available on your host system then that of your VM with the most VCPUs. So if you have a four VCPU VM, have at least eight cores available on your host server and preferably 16.
  • If you are converting a multi-CPU physical Windows server to a single VCPU VM, make sure you change the HAL from multiprocessor to uniprocessor.
  • Don’t use CPU affinity as it restricts the scheduler and makes it harder to process CPU requests. The scheduler is very good at what it does, so let it do its job.

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