About the Scheduler

The scheduler (or dispatcher) is the portion of the kernel that controls the allocation of the CPU to processes. It determines when processes run and for how long, depending on their assigned priorities. Priorities are based on scheduling class and process behavior. Four scheduling classes are supported by default: timesharing, system, real-time and interactive.

The scheduler has an overriding effect on the performance of a system.

The fundamental scheduling entity is the kernel thread. For single-threaded processes, scheduling the kernel thread is synonymous with process scheduling.

The SunOS 5.x scheduler controls the order in which processes run and the amount of CPU time each process may use before another process can run.

The scheduler allocates CPU time to processes according to the scheduling policies defined for each scheduling class. Associated with each scheduling class is a set of priority levels or queues. Ready-to-run processes are moved among these queues. Within a class, you can view these queues as a contiguous set of priority levels. These priority levels are mapped into a set of global scheduling priorities.

The global priority of a process determines when it runs--the scheduler runs the process with the highest global priority that is ready to run. Processes with numerically higher priorities run first, and processes with the same priority run using a round robin scheduling policy.

Once the scheduler assigns a process to a CPU, the process runs until one of the following events occur:

• The process uses up its time slice.

• The process blocks waiting for an event (for example, I/O) or a suspended lock.

• The process is preempted by a higher-priority process.

By default, all real-time processes have higher priorities than any system process, and all system processes have higher priorities than any timesharing process.

A process inherits its scheduler parameters from its parent process, including its scheduler class and its priority within that class. A process changes class only from a user request (with the priocntl command or system call). The system manages the priority of a process based on user requests and the policy associated with the scheduling class of the process.