Programming Interfaces Guide

Scheduling and System Performance

The scheduler determines when and for how long processes run. Therefore, the scheduler's behavior strongly affects a system's performance.

By default, all user processes are time-sharing processes. A process changes class only by a priocntl(2) call.

All real-time process priorities have a higher priority than any time-sharing process. Time-sharing processes or system processes cannot run while any real-time process is runnable. A real-time application that occasionally fails to relinquish control of the CPU can completely lock out other users and essential kernel housekeeping.

Besides controlling process class and priorities, a real-time application must also control other factors that affect its performance. The most important factors in performance are CPU power, amount of primary memory, and I/O throughput. These factors interact in complex ways. The sar(1) command has options for reporting on all performance factors.

Process State Transition

Applications that have strict real-time constraints might need to prevent processes from being swapped or paged out to secondary memory. A simplified overview of UNIX process states and the transitions between states is shown in the following figure.

Figure 4–2 Process State Transition Diagram

 A running process can be preempted to memory, where
it is runnable, or sleep in memory. A process in memory can be swapped.

An active process is normally in one of the five states in the diagram. The arrows show how the process changes states.

Both paging and swapping cause delay when a process is ready to run again. For processes that have strict timing requirements, this delay can be unacceptable.

To avoid swapping delays, real-time processes are never swapped, though parts of such processes can be paged. A program can prevent paging and swapping by locking its text and data into primary memory. For more information, see the memcntl(2) man page. How much memory can be locked is limited by how much memory is configured. Also, locking too much can cause intolerable delays to processes that do not have their text and data locked into memory.

Trade-offs between the performance of real-time processes and the performance of other processes depend on local needs. On some systems, process locking might be required to guarantee the necessary real-time response.

Note –

See Dispatch Latency for information about latencies in real-time applications.