OS/2 performs prioritized, preemptive, multitasking. Prioritized means that OS/2 does not divide CPU time equally among all threads. All programs do not get equal access to the CPU. A prioritizing, time-slicing strategy is used to allocate access to the CPU among competing threads. Each thread has a priority and OS/2 runs the highest priority thread that is ready to run. Programs with higher priorities (a real-time robotic application, for example), are given access to the CPU before programs with lower priorities. If a thread with a higher priority than the currently running thread becomes ready to run, the current thread is stopped immediately, or preempted, and the higher priority thread is given the CPU. The lower priority thread does not get to complete its time slice. Threads of equal priority are given CPU time in a round-robin manner.

Preemptive means that the multitasking activity needs no cooperation from the executing programs. OS/2 maintains control over executing programs, and stops, or preempts, them when their time slice with the CPU is over or when a higher priority program is ready to run.

CPU scheduling is based on four priority classes-Time Critical, Fixed-High, Regular, and Idle-Time. Each class has 32 levels of execution ordering. Scheduling parameters are user-selectable at the time the system is started or can be varied dynamically based on system load.

Depending on a thread's priority class and level, OS/2 periodically gives each thread in each process a small slice of CPU time. Threads with higher priorities always run before threads having lower priorities. A thread runs until its time is up or until a thread with a higher priority is ready to run. At that time, OS/2 preempts the thread and starts another thread. Threads can also voluntarily relinquish the CPU (for example, by calling DosSleep).

The amount of time in each time slice is defined by the TIMESLICE command in the CONFIG.SYS file. The TIMESLICE command can be used by the user to customize the size of the time slices that a thread gets. The default is for OS/2 to dynamically vary the size of the time slice based on the activity of the thread and the overall system load.

When a thread is created (using DosCreateThread), it inherits the priority of the thread that started it. DosSetPriority enables threads to change their priority classes and levels in response to changes in their execution environments. DosSetPriority enables a thread to change its own priority, or the priority of any thread within its process. DosSetPriority also enables changing priorities for the entire process and for descendant processes. Within each class, the priority level of a thread can vary because of a DosSetPriorty request or, if dynamic priority variation is being used, because of action taken by OS/2.