COMP5126 Distributed System Programming
Lab 7: Usage of Monitors
1. The Roller Coaster Problem. Suppose there are n passenger processes and one car
process. The passengers repeatedly wait to take rides in the car, which can hold C
passengers, C < n. However, the car can go around the tracks only when it is full.
Develop a monitor (i.e. pseudo-code) to synchronise the passenger and car processes.
The monitor should have three operations: takeRide, which is called by the
passengers, and load and unload, which are called by the car process. Use the
Signal and Continue discipline.
2.
a)
Write an MPD program to simulate the Dining Philosophers problem described
in Section 4.3 of the text. There should be five processes, one per philosopher
and one “monitor” for the table to synchronise the philosophers. The monitor
should have two methods: getforks(id) and relforks(id), where id
is an integer between 1 and 5 representing the process id.
Your program should have one command-line argument rounds that specifies
the number of rounds of thinking and eating that each philosopher should
execute. The philosophers should eat and think random amounts of time. Add
print statements to your program to generate a trace of activity of the program.
Your program should print a trace of interesting events in the program. These
events include when a process call getforks, start to eat, calls relforks
and start to think. For each event, print a line containing a timestamp (from
age() function), a process id and a string describing the event. For example, a
line of the trace might look like:
12345:
b)
Philosopher 4 calls getforks()
Your program for part (a) should allow a philosopher to start eating as long as
neither neighbour is eating. This can, however lead to starvation if the processes
were to execute forever: One philosopher could be unlucky enough that at all
times one of the other of its neighbours is eating, even though each neighbour
repeatedly eats and thinks.
Modify your program so that philosophers get to start eating in FIFO (or FCFS)
order. This means that if one philosopher has to wait to get permission to eat
(because a neighbour is eating) and a second philosopher calls getforks, the
second philosopher must wait, even if neither of its neighbours is eating. Hint:
Use the Ticket algorithm (Section 3.3.2) to order the waiting philosophers in
getforks.
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