JAVA-BASED REAL-TIME PROGRAMMING

3. Multi-Threaded Programming
mutex
Track for loading
Track for unloading (incoming trains mutual exclusive)
Buffer signal
Industrial track (resources handled along the line)
Wagon to wagon zone
entry exit
Figure 3.5: Railroad tracks seen from above, illustrating semaphore usage.
The name of the semaphores refer to the semaphore figures earlier in this
section. The Buffer is a place for storage, permitting one train to unload and
continue before the next train arrives for loading. In the lower right zone
there is no such buffer (or intermediate passive object) so loading has to be
done directly from one train to another, which corresponds to the synchronous
mechanism of rendezvous.
no such intermediate storage; both trains have to be there to permit items
to be transferred from one train to another, which corresponds to rendezvous
implemented by the semaphores entry and exit. In more detail, trains on the
industrial track have access to resources along the track but when reaching
the transfer zone it gets blocked until the other train is on the adjacent track.
The train entering the zone for unloading (coming from the track for loading)
also has to wait until the other train is there.
Hence, in terms of software again, with the use of rendezvous you know that
the passed data is actually in use by the other thread when the calling thread
continues execution, whereas using ordinary method calls for data transfer
(even if the method belongs to an active object) the executing thread using
the provided data can be unaffected (for some time after the call).
Disadvantages
When using semaphores, one should also be aware of the drawbacks (both in
general and compared to other techniques introduced later). If semaphores
are inlined in the code wherever mutual exclusion is to be accomplished, it
gets hard to read and maintain the program. If it is not clear where the critical
sections actually are, the risk for shared data not being locked increases.
Another case is statements like if, throw, and return unintentionally results in
a resource being left in a locked state, and the entire application gets locked
up or does not work anymore. A first good rule therefore is to place critical
sections as methods in separate classes.
62 2012-08-29 16:05