ThreadPriority

Thread priority is a value associated with a thread that expresses its relative importance to the scheduler. In a preemptive multitasking system, the scheduler uses priorities as a hint to select which runnable thread to run next. Higher-priority threads are generally preferred over lower-priority ones, but the exact behavior depends on the OS and scheduling policy. Priority does not guarantee execution order and can be affected by synchronization, I/O operations, and fairness constraints. In many systems, priorities are dynamic or can be changed at runtime by the program or the OS.

Platform specifics vary. Windows platforms define thread priority through priority classes and thread priority levels, and the OS maps these to a range of values that influence preemption. POSIX systems typically separate policy and priority: the default SCHED_OTHER policy uses niceness, while real-time policies SCHED_FIFO and SCHED_RR assign sched_priority values; higher values grant more immediate execution. Managed runtimes such as Java expose Thread.setPriority with a range (MIN_PRIORITY to MAX_PRIORITY, typically 1 to 10). The effect of Java thread priority is platform-dependent and often non-guaranteed. Real-time and embedded systems may support fixed-priority preemption, deadline scheduling, or priority inheritance to address timing constraints.

Common issues include priority inversion, where a low-priority thread holds a resource needed by a higher-priority thread. Solutions include priority inheritance, priority ceiling, or using non-blocking techniques. Aging or dynamic priority adjustments can prevent starvation.

Best practices emphasize using thread priorities sparingly and designing concurrency to rely on synchronization and coordination rather than precise priority tuning. For most applications, priorities are a hint rather than a guarantee, and correct synchronization and parallel design are usually more important. See also: scheduling, priority inversion, real-time systems.