Sommerville-Software-Engineering-10ed

626 Chapter 21 ■ Real-time software engineering
Neutron flux sensors
Sensor
identifier and
flux value
A-D
convertor
Raw data
buffer
Flux
processing
Processed
flux level
Flux value
buffer
Storage
Display
Figure 21.14 Neutron
flux data acquisition
by an independent process. This architecture is efficient for systems that use multiple
processors or multicore processors. Each process in the pipeline can be associated
with a separate processor or core, so that the processing steps can be carried
out in parallel.
Figure 21.12 is a brief description of the data pipeline pattern, and Figure 21.13
shows the process architecture for this pattern. Notice that the processes involved
produce and consume information. The processes exchange information using
synchronized buffers, as I explained in Section 21.1. Producer and consumer processes
can thereby operate at different speeds without data losses.
An example of a system that may use a process pipeline is a high-speed data
acquisition system. Data acquisition systems collect data from sensors for subsequent
processing and analysis. These systems are used in situations where the sensors
are collecting large volumes of data from the system’s environment and it isn’t
possible or necessary to process that data in real time. Rather, it is collected and
stored for later analysis. Data acquisition systems are often used in scientific experiments
and process control systems where physical processes, such as chemical reactions,
are very rapid. In these systems, the sensors may be generating data very
quickly, and the data acquisition system has to ensure that a sensor reading is collected
before the sensor value changes.
Figure 21.14 is a simplified model of a data acquisition system that might be part
of the control software in a nuclear reactor. This system collects data from sensors
monitoring the neutron flux (the density of neutrons) in the reactor. The sensor data
is placed in a buffer from which it is extracted and processed. The average flux level
is displayed on an operator’s display and stored for future processing.
21.3 Timing analysis
As I discussed in the introduction to this chapter, the correctness of a real-time system
depends not just on the correctness of its outputs but also on the time at which
these outputs were produced. Therefore, timing analysis is an important activity in
the embedded, real-time software development process. In such an analysis, you calculate
how often each process in the system must be executed to ensure that all inputs