Processor running time

The processor running time is the length of continuous time (determined by the dispatch interval) allowed for the dispatch of a logical core.

When the processor running time is dynamically determined by the system, the calculation of the default running time is performed separately for general purpose and ICF processors. All logical processors that are shared, either general purpose or ICF, will be assigned a default running time but the values used for each type of processor may be different. If a user-defined run time is specified, the value applies to all shared general purpose and shared ICF processors. For shared logical partitions using HiperDispatch, any vertical high polarity logical processor's run time is not affected by this user-defined setting. Rather, vertical high polarity logical processors have a run time of 100 milliseconds.

The default value is dynamically calculated and changes when the number of active, scheduled logical core changes.

The default running time is determined using the formula:

The default value is used whenever the processor running time is dynamically determined by the system. The run-time value can change whenever an LP is activated or deactivated and when a logical core stops or starts (for instance, when a logical cores is configured online or offline). The default processor running time is limited to the range of 12.5 to 25 milliseconds.

The logical cores might not use all of its run time because it goes into a wait state. With event-driven dispatching, when a logical cores goes into a wait state, the physical core is reassigned to another logical cores ready to do work. When a logical cores does not go into a wait state during its run time, it loses the physical cores when it reaches the end of its run time. Therefore, an LP with CP-bound work cannot permanently take control of the physical cores.

You can choose to set the runtime value yourself. However, when event-driven dispatching is enabled, it is generally recommended that the processor running time be dynamically determined. If event-driven dispatching is disabled, you should consider setting runtime values of 2 to 8 milliseconds. The recommended procedure is to start by using the default processor running time. That value should be acceptable when all sharing LPs have similar proportions of interactive work; for example, two LPs each running 40% - 70% of interactive work.

Adjustments to the runtime value might be necessary when one sharing LP contains a large proportion of CP-bound work and the other sharing LPs contain only short, interactive transactions. Degraded response time in one LP can indicate that the runtime value should be reduced to decrease the length of continuous time given to CP-bound work. The run-time value should be decreased by approximately 10% several times over several days while monitoring performance carefully. The processing weights should also be adjusted to favor the interactive LP. See Processing weights.

Use the Customize/Delete Activation Profiles task to open a reset profile to define processor running time. Select Dynamically determined by the system or Determined by the user on the Options page for the LP. See Figure 1.