Timesharing Class Policies

In the default configuration, the initialization process (init) belongs to the timesharing class. Because processes inherit their scheduler parameters, all user login shells--and consequently the processes run from those shells--begin as timesharing processes.

The goal of the timesharing policy is to provide good response time for interactive processes and good throughput for processes that use a lot of CPU time. The scheduler tries to divide the CPU's time fairly between processes, subject to the priorities associated with the processes. Those with higher priorities get more attention than those with lower priorities. However, to prevent any one job (process) from hogging the CPU, the scheduler can move jobs from high priorities to low priorities and vice versa.

The scheduler switches CPU allocation frequently enough to provide good response time, but not so frequently that it spends too much time doing the switching. Time slices are typically on the order of a few hundredths of a second.

The timesharing policy changes priorities dynamically and assigns time slices of different lengths. Once a process has started, its timesharing priority varies according to how much CPU time it's getting, how much time it's spending in queues, and other factors. The scheduler raises the priority of a process that "sleeps." (A process sleeps, for example, when it starts an I/O operation such as a terminal read or a disk read.) Entering sleep states frequently is characteristic of interactive tasks such as editing and running simple shell commands. On the other hand, the timesharing policy lowers the priority of a process that uses the CPU for long periods without sleeping.

The default timesharing policy gives larger time slices to processes with lower priorities. A process with a low priority is likely to be stuck in the CPU. Other processes get the CPU first, but when a lower-priority process finally gets the CPU, it gets a bigger chunk of time. If a higher-priority process becomes ready to run during a time slice, however, it preempts the running process.

The scheduler manages timesharing processes using parameters in the timesharing parameter table ts_dptbl. This table contains information specific to the timesharing class. It is automatically loaded into core memory from the TS_DPTBL loadable module located in the /kernel/sched directory.