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Why Did My Thread Priority Change?

There are many reasons that a thread’s priority can change. This section attempts to explain the root cause of these thread priority changes.

A real-time thread, as mentioned previously, is penalized (and may even be knocked down to normal thread priority) if it exceeds its time quantum without blocking repeatedly. For this reason, it is very important to make a reasonable guess about your thread’s workload if it needs to run in the real-time band.

Threads that are heavily compute-bound are given lower priority to help minimize response time for interactive tasks so that high–priority compute–bound threads cannot monopolize the system and prevent lower–priority I/O-bound threads from running. Even at a lower priority, the compute–bound threads still run frequently, since the higher–priority I/O-bound threads do only a short amount of processing, block on I/O again, then allow the compute-bound threads to execute.

All of these mechanisms are operating continually in the Mach scheduler. This means that threads are frequently moving up or down in priority based upon their behavior and the behavior of other threads in the system.

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