Process scheduling

The Kersel's scheduler makes scheduling decisions when:

a process becomes unblocked;
the timeslice for a running process expires;
a running process is preempted.

In QNX, every process has its own priority. The scheduler selects the next process to run by looking at the priority assigned to every process that is READY. The process with the highest priority is selected to run.

The prioriries assigned to processes range from 0 (the lowest) to 31 (the highest). The default priority for a new process is inherited from its parent, this is normally set to 10 for programs started by the shell. You can use getprio() C function to determine the priority of a process and setprio() C function to set the priority of a process. QNX provides three scheduling methods: FIFO, round-robin, and adaptive scheduling. Each process on the system my run any one of these methods. These scheduling methods apply only when two or more processes that shere the same priority are READY. If a higher priority process becomes ready, it immediatelly preempts all lower-priority processes.

The scheduling method of a process is inherited from its parent process, but it can be changed using sched_setscheduler() C function or it cab be determined by the sched_getscheduler() C function.

FIFO scheduling

In FIFO scheduling, a process selected to run continues executing until it:

voluntarily relinquishes (e.g. it blocks)
is preempted by a higher-priority process

Two process that run at the same priority level can use FIFO scheduling to ensure mutual exclusion to a shared resource. Neither process will be preempted by the other while it is executing.

Round-robin scheduling

In round-robin scheduling, a process selected to run continues executing until it:

voluntarily relinquishes control
is preempted by a higher-priority process
consumes its timeslice

A timeslice is the unit of time assigned to every process. Once it consumes its timeslice, a process is preempted and the next READY process at the same priority level is given control. A timeslice is 100 milliseconds. Apart from time slicing, round-robin scheduling is identical to FIFO scheduling.

Adaptive scheduling

In adaptive scheduling, a process behaves as follows:

If the process consumes its timeslice (i.e. it does not block), its priority is reduced by 1 if another process at the same priority is READY. This is known as priority decay.
If the process has decayed and remains unscheduled for one second, its priority is boosted by 1 (a process is never boosted above its original priority).
If the process blocks, it immediately reverts to its original priority.

You can use adaptive scheduling in envirinments where potentially compute-intensive background processes are shering the computer with interactive users. You should find that adaptive scheduling gives retains fast interactive response for other processes. Adaptive scheduling is the default scheduling method for programs created by the shell.