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

6.2 Concurrency and Synchronisation 151
6.2.2.5 Efficiency
There is a trend towards a growing number of features in systems. The improved
performance of new generations of hardware is used up immediately due to advanced
functionality of the software. Concurrency is still required in many systems to satisfy
timing requirements, in spite of the improved performance.
In a synchronous system all modules must wait at each synchronisation point for the
module with the most time consuming operation. This becomes more inefficient as
the variations in computation time are bigger among the modules. In synchronous
hardware systems we observe an extreme small grain of computation. All concurrent
modules are optimised for having a delay path which is as short as possible.
The grain of operations determines how synchronous concurrency works out for modelling
software or mixed hardware/software. A model can be designed to have synchronisation
points on the level of statements or collections of statements such as procedures.
A particular statement can take very long to execute, for instance if it is recursive. A
synchronous model is then not adequate because it forces all other statements that can
be executed in other modules to be waiting.
The efficiency of a system will become poor when all modules must synchronise (wait)
all the time without having a real need for synchronisation. Often these modules are
rather independent. They require synchronisation only when synchronous communication
is required. In such a case synchronisation can be limited to the partners in a
particular communication. Other modules must be allowed to continue their operations
independently. Therefore the modules must be able to operate in an asynchronous
concurrent way.
6.2.2.6 Examples of Concurrency in Object-Oriented Analysis Methods
Objects are promoted as a concept for modelling real world entities in most books
about object-oriented analysis. Because entities in a real world usually coexist and
behave concurrently most readers will intuitively think that an object is an entity with
asynchronous concurrent behaviour. Many methods are not very explicit about the
meaning of their objects concerning synchronisation and concurrency. Some methods
are not intended for concurrent systems while others only offer synchronous concurrency
instead of asynchronous concurrency, or they offer both forms.
A relative new object-oriented method like Fusion [C 94] clearly states its application
area. Fusion is intended to model sequential systems. However, ’although Fusion is a
method for sequential systems, it can be applied to the development of concurrent systems because
many applications exhibit very limited form of concurrency...’ [C 94]. Many other methods
have the same essential restriction as Fusion, although they are less explicit about their
(in)abilities to describe parallel systems.
A well defined non-object-oriented analysis method that offers synchronous concur-