Scheduling priority
Every thread is assigned a priority. The thread scheduler selects the next thread to run by comparing the priorities of READY threads (i.e., those capable of using a processor).
Single-core ready queue
On a single-core system, the READY thread with the highest priority is selected to run.
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. Threads A, B, and C are at the highest priority, so they'll share the processor based on their scheduling policies.
Multicore (SMP) ready queue
On a multicore (SMP) system, the scheduler runs the highest-priority READY thread on one of the
available processors (or CPUs, or cores). Additional processors run other threads in the system,
though not necessarily the next highest-priority threads.
Further details are given in the Thread displacement on multicore systems
section of the
Programmer's Guide.
For each scheduling decision on a given processor, the scheduler looks at the highest priority READY threads that can run on each cluster (i.e., group of associated processors) that the processor belongs to, and then selects the highest-priority thread from this set.
- If a high-priority thread preempts the running thread, then it becomes the head of the thread list for its priority.
- When a thread changes priority. For more information, go to the pthread_setschedprio() entry in the QNX OS C Library Reference.
For information about clusters and configuring processor affinity for threads (i.e.,
associating threads with specific clusters), refer to the Processor affinity, clusters, runmasks, and inherit masks
section of the
QNX OS Programmer's Guide.
Scheduling priority levels
Every thread is assigned a priority. It can be explicitly set, but if it's not set explicitly, a thread inherits the priority of its parent thread by default.
Setting the upper end of the range of unprivileged priorities):
| Priority level | Owner |
|---|---|
| 0 | Idle threads |
| 1 through priority − 1 | Unprivileged or privileged processes |
| priority through 253 | Privileged processes with the PROCMGR_AID_PRIORITY ability (see procmgr_ability() in the C Library Reference) |
| 254 | Per-processor clock Interrupt Service Threads (ISTs) and other in-kernel ISTs |
| 255 | Per-processor InterProcessor (IPI) ISTs |
Priority inheritance and mutexeslater in this chapter, and
Priority inheritance and messagesin the Interprocess Communication (IPC) chapter. The initial priority of the kernel's server threads is 10, 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.
Setting the upper end of the range of unprivileged priorities
procnto-smp-instr -P priority
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:
- SCHED_PRIO_LIMIT_ERROR(priority)—indicate an error
- SCHED_PRIO_LIMIT_SATURATE(priority)—use the maximum allowed priority
Setting the priority of a child process
When spawning a child process with a priority that's different than its parent, be aware that posix_spawn() and spawn() calls block until the initialization of the child process is complete. Because the child process’s main thread performs the initialization at the priority set for the child process, it may create a priority inversion if that priority is lower than the parent.