Specification of Reactive Hardware/Software Systems - Electronic ...

150 Modelling of Concurrent Reactive Behaviour
execution-time of an operation (algorithm) is determined by processor speed as well as
by value(s) of data to be transformed.
If two objects can perform their operations independently and concurrently we do not
want them to be waiting for each other until they both finish their operations. We want
them to perform a next operation immediately after a former operation is finished. As
long as there is no need for communication there is no need for synchronisation.
Operations as well as communications take time in a realistic model. In general, the
actual times cannot be assessed until implementation decisions have been taken. Most
analysis methods do not take time into account during specification. They focus on the
order of events in the system, such as timeless actions and timeless communications.
If behaviour is studied with this abstraction of time, independence of two processes is
interpreted as the independence of the order of their actions. This means that it does
not matter whether a process performs some action after or before an action of another
(concurrent) process. This reasoning leads to an interpretation of concurrency according
to the so-called interleaving semantics (see Paragraph 6.2.2.8).
6.2.2.3 Synchronous Concurrency
The alternative for asynchronous concurrency is synchronous concurrency. This form of
concurrency matches well to the description of systems with a synchronous implementation.
A system with only synchronous concurrency consists of modules that perform
operations concurrently and synchronously start a new operation. All modules must
be ready with their operations before they can synchronously start a new operation.
The time intervals between the points of synchronisation can be arbitrary or fixed (periodical).
If a synchronous system is directed by a common clock the duration of the
operations of all modules must all fit in a fixed time interval. Systems implemented in
digital hardware usually have a synchronous common clock.
6.2.2.4 Computation Time and the Grain of Concurrency
Our decision for an asynchronous approach towards concurrency is confirmed by Wegner
[Weg90]. He notices that a synchronous approach is not practical for modelling
concurrency ’when the computation time within a module or the communication time among
modules is greater than the synchronous computation interval.’ This is often the case in complex
reactive (distributed) systems. However, because of the importance of synchronous
implementations we must offer the possibility to describe synchronous systems as well.
In general complex systems are a mix of asynchronous and synchronous subsystems.
Although processes have asynchronous concurrency we can describe synchronous systems
if processes can be modelled such that they always synchronise their actions.
Communication can be used for synchronisation.