Sommerville-Software-Engineering-10ed

630 Chapter 21 ■ Real-time software engineering
50 Hz (0.5 ms)
Door sensor
process
50 Hz (1 ms)
Movement
detector process
250 Hz (0.5 ms)
Voltage monitor
process
50 Hz (0.5 ms)
Window sensor
process
50 Hz (0.5 ms)
Control panel
process
250 Hz
(1 ms)
System
controller
B
50 Hz (1 ms)
Testing process
Console display
process
R (20 ms)
Power management
process
Figure 21.17
Alarm process timing
R (5 ms)
Audible alarm
process
R (5 ms)
Lighting control
process
R (10 ms)
External alert
process
they are doing is checking whether or not a sensor has changed its status (e.g., from
off to on). It is reasonable to assume that the execution time to check and assess the
state of one sensor is less than 1 millisecond.
To ensure that you meet the deadlines defined by the timing requirements, you
then have to decide how frequently the related processes have to run and how many
sensors should be examined during each execution of the process. There are obvious
trade-offs here between frequency and execution time:
1. The deadline for detecting a change of state is 0.25 second, which means that
each sensor has to be checked 4 times per second. If you examine one sensor
during each process execution, then if there are N sensors of a particular type,
you must schedule the process 4N times per second to ensure that all sensors are
checked within the deadline.
2. If you examine four sensors, say, during each process execution, then the execution
time is increased to about 4 ms, but you need only run the process N times/
second to meet the timing requirement.
In this case, because the system requirements define actions when two or more
sensors are positive, the best strategy is to examine sensors in groups, with groups
based on the physical proximity of the sensors. If an intruder has entered the building,
then it will probably be adjacent sensors that are positive.
When you have completed the timing analysis, you may then annotate the process
model with information about frequency of execution and their expected execution
time (see Figure 21.17). Here, periodic processes are annotated with their frequency,
processes that are started in response to a stimulus are annotated with R, and the testing
process is a background process, annotated with B. This background process