JAVA-BASED REAL-TIME PROGRAMMING

2.4. Concurrency
in functions like run. But before we learn how to design and program such
functions, there are some things we should observe.
2.4.3 Programming of parallel sequences
The functionality we are aiming at resembles that of an operating system;
several programs that run at the same time should be scheduled to share
the CPU in order to let each of the programs behave timely. However, the
concurrency of complete programs is another topic handled later in this book.
For now, we assume that the concurrency stems from the parallelism of a
single application or program. It is then a big advantage that separately
programmed concurrent activities can share and operate on the same data
structures or objects. Otherwise, we would need additional files, pipes, sockets,
etc. to exchange data, and that would be slower and much more work to
program. Furthermore, assume that our program containing the concurrently
executing activities is the only program we are running. (We cannot make
assumptions about other programs anyway, and when we load our software
into an embedded computer this assumption is normally fulfilled.)
When an ordinary sequential program executes, from entering the main
program until the final end, there is at each time one specific point of execution.
We refer to that point in terms of a program counter (PC). We have
so far realized that we require the program to be concurrently ongoing, typically
executing in several objects simultaneously. That is, the program has
to switch between its concurrent activities in such a way that, with the time
scale of the application at hand, it appears (to the user and to the controlled
system) to run in parallel (on different CPUs). To achieve this, the run-time
system has to provide:
1. A function for creating a new concurrently executing activity, i.e., an
additional PC and the states comprising the context associated with the
point of execution.
2. A scheduler that provides sharing of the CPU in common by interleaving
the executions. This should be without requiring the source code to be
cluttered with statements not modeling the task to be performed.
3. When concurrent activities share the same data or is dependent on each
others computational results, there must be some means of synchronization
and communication.
The need for a scheduler is related to the obligations of the operating system
for running complete programs, but as mentioned, here we are concerned with
the parallel behavior of one program. Assuming that our system is equipped
with such a scheduler, interlacing the different points of execution, we may
define the following requirement for a concurrent program to be correct:
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