JAVA-BASED REAL-TIME PROGRAMMING

3. Multi-Threaded Programming
}
System.out.println("World!");
}
public void run() {
System.out.print("and goodbye ");
}
A few test runs on a 200MHz Pentium-MMX running Windows-NT4 gave:
> javac HelloWorld.java
> java HelloWorld
Hello and goodbye World!
> java HelloWorld
Hello World!
and goodbye
> java HelloWorld
Hello World!
and goodbye
> java HelloWorld
Hello and goodbye World!
Hence, the output from the program varies from time to time. In most cases
we need a well defined output, independent of the timing. For instance, "Hello"
and "World!" could correspond to parts of a ticket printout and the printing of
"and goodbye" could correspond to the result from a data-base access. Then
waiting for the data-base result by calling sleep(1) clearly is the wrong way of
waiting since it makes the correctness of the output dependent on the timing
of computing platform. The problem is, however, that developing and testing
the application on one type of machine does not imply correct output on
another (customer) machine. In particular, if the machine is a robot or a
vehicle (instead of a desktop computer), we certainly need additional rules for
how to program.
Note that even if we would know the properties of the execution environment
(OS, CPU-type, clock frequency, speed and amount of memory,
other applications, level of patches or service packs, etc.), and testing verifies
the correctness of the system, the program as such is not correct
from a concurrency point of view.
3.1.5 Scheduling – Priorities
The number of concurrently executing activities (threads or processes) is in
practice always greater than the number CPUs available, which implies that
all work cannot be worked on immediately. The support for concurrent execution
(multi threading etc.) solves this problem, if we have no demands on
timely response. In other words, for purely concurrent software, we can let
the run-time system schedule our activities in any feasible way that lets all
activities run. However, if the application requires certain response times or
performance, in particular when the CPU load is high, we may need to tell
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