Overview of the Scheduler

When a process is created, the system assigns a lightweight process (LWP) to the process. If the process is multithreaded, more LWPs might be assigned to the process. An LWP is the object that is scheduled by the UNIX system scheduler, which determines when processes run. The scheduler maintains process priorities that are based on configuration parameters, process behavior, and user requests. The scheduler uses these priorities to determine which process runs next. The six priority classes are real-time, system, interactive (IA), fixed-priority (FX), fair-share (FSS), and time-sharing (TS).

The default scheduling is a time-sharing policy. This policy dynamically adjusts process priorities to balance the response time of interactive processes. The policy also dynamically adjusts priorities to balance the throughput of processes that use a lot of CPU time. The time-sharing class has the lowest priority.

The SunOS 5.11 scheduler also provides a real-time scheduling policy. Real-time scheduling enables the assigning of fixed priorities to specific processes by users. The highest-priority real-time user process always gets the CPU as soon as the process is runnable .

The SunOS 5.11 scheduler also provides a policy for fixed-priority scheduling. Fixed-priority scheduling enables the assignment of fixed priorities to specific processes by users. Fixed-priority scheduling uses the same priority range as the time-sharing scheduling class by default.

A program can be written so that its real-time processes have a guaranteed response time from the system. See Chapter 11, Real-time Programming and Administration for detailed information.

The control of process scheduling provided by real-time scheduling is rarely needed. However, when the requirements for a program include strict timing constraints, real-time processes might be the only way to satisfy those constraints.

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Caution  - Careless use of real-time processes can have a dramatic negative effect on the performance of time-sharing processes.

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Because changes in scheduler administration can affect scheduler behavior, programmers might also need to know something about scheduler administration. The following interfaces affect scheduler administration:

• dispadmin(1M) displays or changes scheduler configuration in a running system.

• ts_dptbl(4) and rt_dptbl(4) are tables that contain the time-sharing and real-time parameters that are used to configure the scheduler.

A process inherits its scheduling parameters, including scheduling class and priority within that class, when the process is created. A process changes class only by user request. The system bases its adjustments of a process' priority on user requests and the policy associated with the scheduler class of the process.

In the default configuration, the initialization process belongs to the time-sharing class. Therefore, all user login shells begin as time-sharing processes.

The scheduler converts class-specific priorities into global priorities. The global priority of a process determines when the process runs. The scheduler always runs the runnable process with the highest global priority. Higher priorities run first. A process assigned to the CPU runs until the process sleeps, uses its time slice, or is preempted by a higher-priority process. Processes with the same priority run in sequence, around a circle.

All real-time processes have higher priorities than any kernel process, and all kernel processes have higher priorities than any time-sharing process.

Administrators specify default time slices in the configuration tables. Users can assign per-process time slices to real-time processes.

You can display the global priority of a process with the -cl options of the ps(1) command. You can display configuration information about class-specific priorities with the priocntl(1) command and the dispadmin(1M) command.

The following sections describe the scheduling policies of the six scheduling classes.