Timeshare Scheduling

Timeshare scheduling attempts to distribute processor resources fairly among the LWPs in the timesharing (TS) and interactive (IA) scheduling classes.

The priocntl(2) call sets the class priority of one or more processes or LWPs. The normal range of timesharing class priorities is -60 to +60. The higher the value, the higher the kernel dispatch priority. The default timesharing class priority is 0.

The old concept of a nice value for a process, where a lower nice value means a higher priority, is maintained for all of the TS, IA, and FSS scheduling classes. The old nice-based setpriority(3C) and nice(2) interfaces continue to work by mapping nice values into priority values. Setting a nice value changes the priority and vice-versa. The range of nice values is -20 to +20. A nice value of 0 corresponds to a priority of 0. A nice value of -20 corresponds to a priority of +60.

The dispatch priority of time-shared LWPs is calculated from the instantaneous CPU use rate of the LWP and from its class priority. The class priority indicates the relative priority of the LWPs to the timeshare scheduler.

LWPs with a smaller class priority value get a smaller, but nonzero, share of the total processing. An LWP that has received a larger amount of processing is given lower dispatch priority than an LWP that has received little or no processing.