Most RTOSs do their scheduling of tasks using a scheme called “priority-based preemptive scheduling.” Each task in a software application must be assigned a priority, with higher priority values representing the need for quicker responsiveness. Very quick responsiveness is made possible by the “preemptive” nature of the task scheduling. “Preemptive” means that the scheduler is allowed to stop any task at any point in its execution, if it determines that another task needs to run immediately.
The basic rule that governs priority-based preemptive scheduling is that at every moment in time, “The Highest Priority Task that is Ready to Run, will be the Task that Must be Running.” In other words, if both a low-priority task and a higher-priority task are ready to run, the scheduler will allow the higher-priority task to run first. The low-priority task will only get to run after the higher-priority task has finished with its current work.
What if a low-priority task has already begun to run, and then a higher-priority task becomes ready? This might occur because of an external world trigger such as a switch closing. A priority-based preemptive scheduler will behave as follows: It will allow the low-priority task to complete the current assembly-language instruction that it is executing. [But it won’t allow it to complete an entire line of high-level language code; nor will it allow it to continue running until the next clock tick.] It will then immediately stop the execution of the low-priority task, and allow the higher-priority task to run. After the higher-priority task has finished its current work, the low-priority task will be allowed to continue running. This is shown in Figure 3, where the higher-priority task is called “Mid-Priority Task.”
Of course, while the mid-priority task is running, an even higher-priority task might become ready. This is represented in Figure 3 by “Trigger_2″ causing the “High-Priority Task” to become ready. In that case, the running task (“Mid-Priority Task”) would be preempted to allow the high-priority task to run. When the high-priority task has finished its current work, the mid-priority task would be allowed to continue. And after both the high-priority task and the mid-priority task complete their work, the low-priority task would be allowed to continue running. This situation might be called “nested preemption.”Each time the priority-based preemptive scheduler is alerted by an external world trigger (such as a switch closing) or a software trigger (such as a message arrival), it must go through the following 5 steps:
- Determine whether the currently running task should continue to run. If not …
- Determine which task should run next.
- Save the environment of the task that was stopped (so it can continue later).
- Set up the running environment of the task that will run next.
- Allow this task to run.
These 5 steps together are called “task switching.”
