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Chapter 5
5-1 ASSIGNING TASK PRIORITIES
Sometimes task priorities are both obvious and intuitive. For example, if the most important
aspect of the embedded system is to perform some type of control and it is known that the
control algorithm must be responsive then it is best to assign the control task(s) a high
priority while display and operator interface tasks are assigned low priority. However, most
of the time, assigning task priorities is not so cut and dry because of the complex nature of
real-time systems. In most systems, not all tasks are considered critical, and non-critical
tasks should obviously be given low priorities.
An interesting technique called rate monotonic scheduling (RMS) assigns task priorities
based on how often tasks execute. Simply put, tasks with the highest rate of execution are
given the highest priority. However, RMS makes a number of assumptions, including:
■
All tasks are periodic (they occur at regular intervals).
■
Tasks do not synchronize with one another, share resources, or exchange data.
■
The CPU must always execute the highest priority task that is ready to run. In other
words, preemptive scheduling must be used.
Given a set of n tasks that are assigned RMS priorities, the basic RMS theorem states that all
task hard real-time deadlines are always met if the following inequality holds true:
Where Ei corresponds to the maximum execution time of task i , and Ti corresponds to the
execution period of task i . In other words, Ei/Ti corresponds to the fraction of CPU time
required to execute task i .
Table 5-1 shows the value for size n(2 1/n – 1) based on the number of tasks. The upper
bound for an infinite number of tasks is given by ln(2) , or 0.693 , which means that
meeting all hard real-time deadlines based on RMS, CPU use of all time-critical tasks should
be less than 70 percent!
84
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