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Broadly speaking the primitives (or perhaps the applications) can be divided into two categories. The first enables common data or common code to be protected from multiple usage. The second enables one task to send a message to another; this includes the degenerate case of a signal, which can be thought of as a message with no content.
One of the oldest and best known primitive sets is the binary semaphore described by Dijkstra [Di 68]. This consists of the two operators P and V, acting on a semaphore S which takes two values, busy and free (or equivalently true and false). The behavior of the operations is:
|P(S)||If S is busy the task is suspended until S becomes free. If S is free then it is set busy and the task proceeds.|
|V(S)||S is set free. If there are tasks held up on a P(S) operation then one of them is allowed to proceed.|
Semaphores can be used to protect data by embedding the code that accesses the data between matched calls thus:
P(S) -- access data V(S) task A
P(S) -- access data V(S) task B
Semaphores can also be used to signal happenings. One task waits by calling P, the other signals by calling V.
P(S) -- wait for B task A
V(S) -- signal to A task B
Semaphores can therefore be used both for protection and for signalling. They also have the merit of being primitives that are both simple to describe and easy to understand. What then are their disadvantages? Briefly the problem is that for all but the simplest applications, the programming of semaphores is difficult. Programs using semaphores exhibit similar symptoms to unstructured programs using gotos. They are hard to write, understand, prove, and maintain. More specifically, typical problems are:
Closely related to the semaphore is the signal or event. There are variations, but a typical definition would be that an event E has two states, set and unset, and the following operations upon it:
|WAIT(E)||If E has not been set then the task is suspended until the event is set. If E has been set then it is unset and the task is allowed to proceed.|
|SEND(E)||E is set. If there are tasks waiting for E then one of them is allowed to proceed.|
Clearly such an event is isomorphic to the binary semaphore. The difference lies perhaps in the intended use. Semaphores are associated with data protection, and events with indicating that something has happened. There are variations in which several events are remembered. But in all forms, events suffer from the same structuring problems as semaphores.
Various other primitives have been proposed in order to overcome the structuring difficulties of semaphores and events. However, they usually tackle only one of the application areas distinguished above (data protection and signalling). In this respect they are somewhat unbalanced.
The critical section has been proposed as a syntactic form equivalent to a bracketed pair of P and V operations. This prevents goto statements from bypassing one of the operations and hence overcomes some of the difficulties of semaphores. A further form, the conditional critical section, allows an alternative action to be performed if the resource represented is busy.
Critical sections do not seem to have been successful. They solve only the exclusion problem and need to be complemented with a signalling mechanism; this does not lead to the unification sought by language designers.
Many forms of message switching system have been implemented in order to give improved solutions to the signalling problem (see [BH 70, 73]). Typically they enable messages to be sent between tasks, and allow the source or destination of the message to be optionally specified. They therefore give added protection by preventing unauthorized access to messages.
Perhaps the biggest disadvantage of message systems is the need for a sizeable message controller. Message systems also seem to be of an ad- hoc nature with an apparently arbitrary set of parameters. Moreover they do not easily solve exclusion problems because of the high overhead involved.
A significant step forward was the monitor, first described by Brinch Hansen [BH 73,75] and by Hoare [Ho 74]. This includes the facilities of the critical section, and when combined with events (as in Modula), gives a reasonable solution to problems such as the bounded buffer. The monitor solves the exclusion problem but not the message problem. Indeed the signals in Modula still suffer from the structuring problems of semaphores.
The difficult problem that arises here is one of making tasks known to each other. Tasks have names that identify them unambiguously. Should these names be used by tasks to synchronize with each other, or should there exist a further entity that makes both candidates for synchronization known to each other by reference to some common channel? These two solutions are extreme forms of symmetric communication; either each communicating task has full knowledge of its colleague, or it has no information at all. Both solutions appear in the literature: [Ho 78] and [Ka 74].
We rejected the channel solution in this design in order to avoid an additional language concept and the dual connection mechanism that it requires. The solution adopted in Ada, although closer to the one proposed by Hoare, is asymmetric: one of the two communicating tasks knows the name of the other and names it explicitly; the second task knows only that it expects some external interaction.
In order to justify the asymmetry, let us first summarize the symmetric proposal developed by Hoare and embedded in a language which has become known as CSP (Communicating Sequential Processes). In CSP, communication between tasks is seen as synchronized input-output: one task outputs data which the other inputs, and both tasks rendezvous during the transfer - that is, the first to arrive at its input or output statement waits for the other and they both then execute the I/O statements together (or apparently together) before proceeding independently. Each task names the other in the transfer. The transfer can be thought of as an assignment split into two parts with the left side in one task and the right side in the other.
As an example we shall consider a task BUFFER, to smooth variations in the speed of output of items by a producer task and input by a consumer task (given in section 13.2.5). The program is as follows:
BUFFER :: pool : (1 .. 10) item; inindex, outindex, count : integer; inindex := 1; outindex := 1; count := 0; * [count < 10; producer?pool(inindex) -> inindex := inindex mod 10 + 1; count := count + 1  count > 0; consumer?more() -> consumer!pool(outindex); outindex := outindex mod 10 + 1; count := count - 1 ]
The key language statements in this example are:
X ? Y Input Y from task X X ! Y Output Y to task X
On each iteration the guards "count < 10" and "count > 0" are evaluated. If both guards are true then calls from either the consumer or producer are acceptable and the first such call will be waited for; if both have already made such a call and are therefore themselves waiting then a nondeterministic choice will be made; if only one has made a call then obviously that call is taken. If, however, only one guard is true then only the corresponding call can be accepted, and the other task will wait until the buffer is partially filled or emptied as the case may be. In this example both guards cannot be false and so the iterative process continues indefinitely.
In the producer case the statement
producer ? pool(inindex)
moves the item into the buffer directly. In the consumer case the statement
consumer ? more()
indicates that the consumer is ready and a subsequent
consumer ! pool(outindex)
actually does the transfer. The producer task therefore contains statements such as
BUFFER ! X
whereas the consumer task has pairs such as
BUFFER ! more(); BUFFER ? X
Note that more() denotes a structured value with no components and is used here as a signal.
As can be seen the program is readable, although perhaps presented in a terse style by traditional high-level language standards. However, there are two problems with CSP. One is that a (one-to-one) named correspondence is required; because of this symmetry, it is not possible to program a library routine to provide resources to arbitrary users. The other problem is that a double interaction is required for the consumer; this means that the two calls really need to be encapsulated by a single procedure in order to give a clean interface.
In Ada, as we have seen, naming is one-sided. Tasks can be characterized as services or as users. A user certainly needs to know the name of the service it is requesting. On the other hand, a server need not know the names of its users. Because of this asymmetry it possible to program the above library routine. As a consequence there can be queues of waiting tasks associated with each request. On each successful rendezvous just one waiting task is served.
The other important concept introduced in Ada is the notion of the extended rendezvous. This notion is a major breakthrough to a higher level of abstraction. In the case of the task BUFFER this overcomes the need for the double rendezvous with the consumer. This is seen by comparing the example in section 13.2.5 with that above. Thus we now have
rather than the two statements of CSP. This also illustrates the procedural form of entry call as opposed to some specialized statements. As we have seen, this enables a similar external interface to be presented, even if a change of solution demands that a procedure be replaced by an entry or vice versa.
The extended rendezvous is more disciplined since it ensures that the same task performs the interaction throughout. It is also instructive to consider the same example written in Modula using monitors as follows:
interface module buffer; define put, get; const poolsize = 10; var pool : array [1 .. poolsize] of item; inindex, outindex, count : integer; nonfull, nonempty : signal; procedure put (x : item); begin if count = poolsize then wait(nonfull) end; pool [inindex] := x; inindex := inindex mod poolsize + 1; count := count + 1; send(nonempty) end put; procedure get (var x : item); begin if count = 0 then wait(nonempty) end; x := pool [outindex]; outindex := outindex mod poolsize + 1; count := count - 1; send(nonfull) end get; begin inindex := 1; outindex := 1; count := 0 end buffer;
The producer and consumer processes move the items by calls such as
put(x) and get(x)
respectively. This is satisfactory, but the internal behavior of the monitor is not nearly as clear as in CSP and Ada. The rendezvous mechanism is more disciplined than a monitor, since the accept statements appear inside a context (for example following a guard) from which information can be deduced, thereby facilitating both understanding and proof.
Perhaps the most important point about both CSP and Ada is that they offer mechanisms that are applicable to both data protection and signalling. Earlier attempts to develop features at a higher level than semaphores or events (such as message systems and monitors) seemed to solve only one problem, and by offering an unbalanced solution were not clearly better than the original simple primitives.
The rationale behind the accept statement and entry call is simply to provide a rendezvous. In some applications it is necessary that a rendezvous be achieved, whereas in others it is important for the caller not to be held up. It is much more difficult to program a rendezvous in terms of non-rendezvous primitives than vice versa. Hence the rendezvous has been chosen as the natural primitive.
It is noted that calls are accepted in simple order of arrival. The alternative of making the order depend on some parameter of the call was considered and rejected because of the difficulty of implementation, which could severely penalize the simple user. As has been demonstrated in the examples, it is possible to program different strategies when necessary.
The introduction of entries leads naturally to the unification of tasks and packages. A task encapsulates entries in the same way as a package encapsulates procedures. Moreover there is a strong analogy between on the one hand the specification, in which the entries are specified, and the body, containing the sequence that controls the critical actions, and on the other hand the corresponding subdivisions of a package with respect to the specification and bodies of procedures.
However, this unification has its limits. It proved necessary to disallow entities other than entries in the specification. As mentioned earlier, this was partly for methodological reasons and partly because of the cost of preventing access to variables of an inactive task and of implementing access on a distributed system.
The general applicability of the rendezvous concept has been confirmed by its use in other examples. This concept is well adapted to distributed systems - communication is achieved by entry calls, exchanged data is passed via parameters. From a more theoretical viewpoint, it is interesting to note that path expressions [CH 74] can be shown to be easily expressible in terms of rendezvous primitives.