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Because the scheduler determines when and for how long LWPs run, it has an overriding importance in the performance and perceived performance of a system.
By default, all processes and LWPs are time-sharing processes or LWPs. A process or LWP changes class only as a result of one of the priocntl functions.
In the default configuration, all real-time fixed priority process priorities are above any time-sharing process priority. This implies that as long as any fixed priority process or LWP is runnable, no time-sharing process or LWP or system process ever runs. So if a fixed priority application is not written carefully, it can completely lock out users and essential kernel housekeeping.
Besides controlling process and LWP class and priorities, a fixed priority application must also control several other factors that influence 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. For more information, see ``Managing system performance''. In particular, the sar(1M) command has options for reporting on all the factors discussed in this section.