Assign tasks with a large number of I/O operations a higher priority than tasks with little I/O, because the tasks with much I/O will be in a wait condition for a greater amount of time. The lower priority tasks will be executed when the higher priority tasks are in a wait condition. As the I/O operations are completed, the higher priority tasks get control, so that more I/O can be started.
You can change the priorities of subtasks by using the CHAP macro. The CHAP macro changes the dispatching priority of the active task or one of its subtasks by adding a positive or negative value. The dispatching priority of an active task can be made less than the dispatching priority of another task. If this occurs and the other task is dispatchable, it would be given control after execution of the CHAP macro.
You can also use the CHAP macro to increase the limit priority of any of an active task's subtasks. An active task cannot change its own limit priority. The dispatching priority of a subtask can be raised above its own limit priority, but not above the limit of the originating task. When the dispatching priority of a subtask is raised above its own limit priority, the subtask's limit priority is automatically raised to equal its new dispatching priority.