OS/2 can be configured to dynamically alter the priority of a process. The PRIORITY statement in CONFIG.SYS can be set to either ABSOLUTE or DYNAMIC. If PRIORITY specifies the ABSOLUTE option, all processes receive CPU time based on the priority established by calls to DosSetPriority. If the PRIORITY command in the CONFIG.SYS file specifies the DYNAMIC option, OS/2 adjusts process priorities based on system load and process activity, and on whether the process is in the foreground. DYNAMIC is the default setting; if the PRIORITY command is not specified, the system uses DYNAMIC priority. DYNAMIC is designed to gives the best overall system performance under most conditions.
When dynamic priority variation is enabled, OS/2 grants higher priority to the foreground process than to background processes. System load and process activity are also taken into consideration. The priority of the process consists of a computed priority value that is based upon the display status of the process (foreground or background), and its recent I/O and CPU time usage history. When dynamic priority variation is enabled, I/O priority boosts are generated for keyboard input, window, foreground, processor starvation, device I/O, and DOS Session interrupts. This ensures that the foreground process-the process most likely to be in use-receives enough CPU time to provide quick response to user input.
There are times when dynamic priority variation can interfere with a multi-threaded application's execution. For example, if you are doing a lot of keyboard input on a thread, its priority will get boosted and other threads may not get enough CPU time. A communication thread is an example of a time sensitive background thread which would be one case. The solution is to either set PRIORITY = ABSOLUTE or to call DosSetPriority on a regular basis to keep the threads priority at the desired level.