DESIGN PROCESS OF REAL-TIME OPERATING SYSTEMS (RTOS) - Software Design for Six-Sigma: A Roadmap for Excellence - page 70

TABLE 3.4 Task Scheduling Design Options & Comparison

Purpose Advantages Disadvantages Efficiency Implementation

Periodic Tasks Usually uses a timer to

perform regular
maintenance tasks

Ideal for tasks that must be
performed at regular time
intervals

Code may be executed
more often than
required

Mostly efficient,
although context
switching can cause
latency

Easy with statically
scheduling systems;
complicated if
dynamic

Aperiodic
Ta s k s

Can occur at any time,
usually triggered by
something external
to the system

Good for use when the system
only needs to respond
when an event occurs

May increase WCET
of the system

Mostly efficient,
although there is
latency for context
switching

Relatively easy

Interrupt
Driven

A timer causes an
interrupt signaling
the operating system
to execute a task

Provides an efficient method
of notifying operating
system the that it is time
for the task to execute

Must have an operating
system and
hardware in place to
support interrupts

Usually more effective
than other
alternatives, but
there can be
significant latency if
not implemented
properly

Implementing code to
handle context
switches efficiently
can be moderately
difficult

Preemptive Allows tasks to

interrupt a task that
is executing

Without this, all tasks must
execute until completion,
difficult to support in
multitasking real-time
system

It takes time to switch
out tasks

Depending on the
implementation, the
time to switch tasks
can be minimized

Relatively difficult to
implement, and the
time to perform the
switch must be
known