Virtual processors

A virtual processor is a representation of a physical processor core to the operating system of a logical partition that uses shared processors.

When you install and run an operating system on a server that is not partitioned, the operating system calculates the number of operations that it can perform concurrently by counting the number of processors on the server. For example, if you install an operating system on a server that has eight processors, and each processor can perform two operations at a time, the operating system can perform 16 operations at a time. In the same way, when you install and run an operating system on a logical partition that uses dedicated processors, the operating system calculates the number of operations that it can perform concurrently by counting the number of dedicated processors that are assigned to the logical partition. In both cases, the operating system can easily calculate how many operations it can perform at a time by counting the whole number of processors that are available to it.

However, when you install and run an operating system on a logical partition that uses shared processors, the operating system cannot calculate a whole number of operations from the fractional number of processing units that are assigned to the logical partition. The server firmware must therefore represent the processing power available to the operating system as a whole number of processors. This allows the operating system to calculate the number of concurrent operations that it can perform. A virtual processor is a representation of a physical processor to the operating system of a logical partition that uses shared processors.

The server firmware distributes processing units evenly among the virtual processors assigned to a logical partition. For example, if a logical partition has 1.80 processing units and two virtual processors, each virtual processor has 0.90 processing units supporting its workload.

There are limits to the number of processing units that you can have for each virtual processor. The minimum number of processing units that you can have for each virtual processor is 0.10 (or ten virtual processors for every processing unit). When the firmware is at level FW760, or later, the minimum number of processing units is further lowered to 0.05 (or 20 virtual processors for every processing unit). The maximum number of processing units that you can have for each virtual processor is always 1.00. This means that a logical partition cannot use more processing units than the number of virtual processors that it is assigned, even if the logical partition is uncapped.

A logical partition generally performs best if the number of virtual processors is close to the number of processing units available to the logical partition. This lets the operating system manage the workload on the logical partition effectively. In certain situations, you might be able to increase system performance slightly by increasing the number of virtual processors. If you increase the number of virtual processors, you increase the number of operations that can run concurrently. However, if you increase the number of virtual processors without increasing the number of processing units, the speed at which each operation runs will decrease. The operating system also cannot shift processing power between processes if the processing power is split between many virtual processors.

On HMC-managed systems, virtual processors are assigned to logical partitions using partition profiles.