scheduling

Scheduling virtual CPUs

Recently someone asked me what VMware scheduling is, so I thought I would cover that in this blog post. Scheduling, or virtual CPU scheduling, happens behind the scenes and is not a very visible component of virtualization, but is absolutely critical for virtualization to work properly. You should have at least a basic understanding of how it works so you understand how it impacts virtual machine (VM) performance and what to look for when troubleshooting malfunctions.

Scheduling 101

The scheduler is a component of the VMkernel that schedules requests for the virtual CPUs assigned to virtual machines to the physical CPUs of the host server. Whenever a virtual machine (VM) uses its virtual CPU, the VMkernel has to find a free physical CPU (or core) for the VM to use. On a typical host server, the number of virtual CPUs usually outnumbers the number of physical CPUs, so the VMs are all competing to use the limited number of physical CPUs that the host has. The scheduler’s job is to find CPU time for all the VMs that are requesting it and to do it in a balanced way, so performance for any one VM does not suffer. This is not always an easy task, especially when VMs are assigned multiple virtual CPUs (virtual symmetric multiprocessing, or vSMP) as this further complicates the scheduling.

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