8 - Avoidance synchronisation and the requeue facility

Summary

The models of synchronisation discussed in the previous four chapters have the common feature that they are based on avoidance synchronisation. Guards or barriers are used to prevent rendezvous and task-protected object interactions when the conditions are not appropriate for the communications event to start. Indeed, one of the key features of the tasking model is the consistent use of avoidance to control synchronisation. The use of guards and barriers represents a high-level abstract means of expressing and enforcing necessary synchronisations; and as such they can be compared favourably with the use of low-level primitives such as semaphores or monitor signals (see Chapter 3). This chapter starts by giving a more systematic assessment of avoidance synchronisation in order to motivate the requirement for ‘requeue’. It then describes the syntax and semantics of the requeue statement and gives examples of its use.

The need for requeue

Different language features are often compared in terms of their expressive power and ease of use (usability). Expressive power is the more objective criterion, and is concerned with the ability of language features to allow application requirements to be programmed directly. Ease of use is more subjective, and includes the ease with which the features under investigation interact with each other and with other language primitives.

In her evaluation of synchronisation primitives, Bloom (1979) used the following criteria to evaluate and compare the expressive power and usability of different language models.