JAVA-BASED REAL-TIME PROGRAMMING

synch.
T1 entry
give
take
3.2. Resources and mutual exclusion – Semaphores
exit T2
take
give
Figure 3.4: Rendezvous accomplished by two (signaling) semaphores. Synchronization
takes place in the interval marked sync.
in Java. Still, rendezvous is useful when data is to be handed over from one
thread to another. That is, not only transferring data between the thread
objects (without buffering), but also making sure that the receiving thread of
execution has arrived in a state where the data has been accepted.
Figure 3.4 illustrates the rendezvous concept and how to implement it
using semaphores. Note that T1 is blocked during synchronization when only
T2 accesses the shared data. The ’sync.’ interval for T2 therefore corresponds
to a critical section, and T2 may not use the shared data before entry.take and
after exit.give. For T1 on the other hand, shared data may be manipulated
until entry.give is called (which is to be directly followed by exit.take) and after
(blocking ends after) exit.take. Thus, T1 is so to say in the critical section
all the time, but blocked when communication takes place. Since interaction
with T2 is required before T1 can proceed, it is easy to imagine situations
when the use of rendezvous as the only mean for synchronization gets clumsy.
Metaphors for semaphores
With the semaphore classes available, one has to keep in mind when to use
what class (semaphore use cases). It is then useful to have some mental picture
of how the different semaphore classes are to be used. Since semaphores (in
real life) historically come from railroad traffic, a train control example will be
used. Note, however, that this has very little to do with railroad semaphores
as such; it is only our semaphore classes used in a railroad context.
In Figure 3.5 there is a semaphore mutex used to having only one train at
a time on the track for unloading (shared resource), and there is a semaphore
signal used for signaling the availability of items in the buffer between the track
for unloading and loading. The (passive) buffer permit the (active) trains to
access the items independently (asynchronously) of each other, as long as the
buffer is not empty. The zone to the lower right, on the other hand, provides
synch.
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