Chapter 7: Portability - TOC - 7.4 TASKING

7.4.5 Task Scheduling Algorithm

guideline

Do not assume that tasks execute uninterrupted until they reach a synchronization point.

Use pragma Priority to distinguish general levels of importance only (see Guideline 6.1.6).

rationale

The Ada tasking model requires that tasks be synchronized only through the explicit means provided in the language (i.e., rendezvous, task dependence, pragma Atomic). The scheduling algorithm is not defined by the language and may vary from time sliced to preemptive priority. Some implementations provide several choices that a user may select for the application.

notes

The number of priorities may vary between implementations. In addition, the manner in which tasks of the same priority are handled may vary between implementations even if the implementations use the same general scheduling algorithm.

exceptions

In real-time systems, it is often necessary to tightly control the tasking algorithm to obtain the required performance. For example, avionics systems are frequently driven by cyclic events with limited asynchronous interruptions. A nonpreemptive tasking model is traditionally used to obtain the greatest performance in these applications. Cyclic executives can be programmed in Ada, as can a progression of scheduling schemes from cyclic through multiple-frame-rate to full asynchrony (MacLaren 1980), although an external clock is usually required.