Scheduling priority

Every thread is assigned a priority. The thread scheduler selects the next thread to run by looking at the priority assigned to every thread that's READY (i.e., capable of using the CPU).

The following diagram shows the ready queue for a single-core system with five threads (B–F) that are READY. Thread A is currently running. All other threads (G–Z) are BLOCKED. Thread A, B, and C are at the highest priority, so they'll share the processor based on the running thread's scheduling policy.

Figure 1. The ready queue in a single-core system.

The OS supports a total of 256 scheduling priority levels. An unprivileged thread can set its priority to a level from 1 to 63 (the highest unprivileged priority), independent of the scheduling policy. Only threads with the PROCMGR_AID_PRIORITY ability enabled (see procmgr_ability() in the C Library Reference) are allowed to set priorities above 63. The special idle thread (in the process manager) has priority 0 and is always ready to run. A thread inherits the priority of its parent thread by default.

You can change the allowed priority range for unprivileged processes with the -P option for procnto:

procnto-smp-instr -P priority

In QNX Neutrino 6.6 or later, you can append an s or S to this option if you want out-of-range priority requests by default to use the maximum allowed priority (reach a “maximum saturation point”) instead of resulting in an error. When you're setting a priority, you can wrap it in one these (non-POSIX) macros to specify how to handle out-of-range priority requests:

Here's a summary of the ranges:

Priority level Owner
0 Idle thread
1 through priority − 1 Unprivileged or privileged
priority through 255 Privileged

Note that in order to prevent priority inversion, the kernel may temporarily boost a thread's priority. For more information, see Priority inheritance and mutexes later in this chapter, and Priority inheritance and messages in the Interprocess Communication (IPC) chapter. The initial priority of the kernel's threads is 255, but the first thing they all do is block in a MsgReceive(), so after that they operate at the priority of threads that send messages to them.

The threads on the ready queue are ordered by priority. The ready queue is actually implemented as 256 separate queues, one for each priority. The first thread in the highest-priority queue is selected to run.

Most of the time, threads are queued in FIFO order in the queue of their priority, but there are some exceptions: