Q. How do I set a CPU reservation for a Hyper-V virtual machine (VM)?

A. Hyper-V allows you to reserve processor resources for a VM in addition to limiting the amount of processor resource usage. To do so, access a VM's settings and select the Processor node.

Initially, you select the number of logical (virtual) processors that the VM can see, from one to four (assuming the physical host has at least four cores). You can then set various Resource control options, as shown below.

Click to expand.

• Virtual machine reserve: This is the amount of the processor's power that's reserved for this VM, and therefore always available. The host in the example screenshot has eight cores and the VM has one logical processor. With Virtual machine reserve set to 50, half of one of the physical cores is always available to this VM. Note that it doesn't mean the VM will always use the same core or that all the processing for the VM will be done by the same core. The hypervisor will just ensure that the VM always has the equivalent of half of a core of processing power available.
  You'll see that the Percent of total system resources box shows 6. One processor out of eight is 12.5 percent of the machine's total processor capacity. Because I set Virtual machine reserve to 50 percent, this VM has 6.25 percent (rounded to 6) of the host's processor power reserved for it
• Virtual machine limit: This is the maximum amount of processor power that the VM can use. I have it set to 100 percent, which means this VM can use the entire resources of the allocated processors—so 12.5 percent of the machine's total processor resources. Note that in times of resource contention, the VM may not get a full 100 percent, but will always get its reserve amount.
• Relative weight: When there's contention for your CPU resources, the weight value is used to determine the importance of a VM getting shares of CPU time. For example, a VM with a weight of 200 would get twice as many CPU cycles as a VM with a weight of 100.

Related Reading:

• Virtualization Myths and Misconceptions
• Q. I'm seeing a big performance difference with my live migration operations when I use Cluster Shared Volumes (CSV), is this normal?
• Q. How can I list all applications being published from a Windows Virtual PC virtual machine (VM)?
• Q. Why are my virtual machines (VMs) migrated between nodes in a cluster using Quick Migration instead of Live Migration when I shut down a cluster node?

Check out hundreds more useful Q&As like this in John Savill's FAQ for Windows. Also, watch instructional videos made by John at ITTV.net.