SECOND Food-Passing Problem

(Last revision 11/12/2002)

CS 3750 SECOND C++ Concurrent Programming Assignment (prog #2)

This assignment is much like the previous one. The difference is that now, in the spaces between two trivets, there are TWO diners (sitting opposite each other) instead of just one. The layout is like this:


                               EAST

                        diner        diner
                          1            3
(NORTH) Server   T               T              T    Busser (SOUTH)
                        diner        diner          
                          0            2

                                WEST

(The T's in the diagram are trivets.)

Make a copy of the foodPass2.cpp program. Read the program header comments in order to figure out what the requirements of a solution are.
Insert declarations and instructions into a copy of the program so that it properly performs the required job.

You may use (only) ordinary variables and queuing semaphores as implemented by sem.cpp and sem.h to achieve the synchronization of threads.

Errors in concurrent programs tend to be intermittant. They tend to be manifested only under certain timings of events. The random delays in the program will tend to make the timing of events change considerably each time the program is executed. If you run the program several times and check the output you are more likely to find out about possible errors than if you just run the program one time and check the output.

For this assignment, you may work in teams of two (2) people. If you work in a team, make sure that both names are in your comments!

More Discussion of the Assignment

When you think about doing this assignment, imagine that all the threads (the server, busser, and all the diners) will run on a separate CPU of a tightly-coupled multiprocessor. Of course, our Sun Ultra computing systems are not tightly-coupled multiprocessors. Nevertheless, the point of this exercise is for you to write a program that will work on *any* computing system on which multiple processes can execute concurrently while sharing variables.

A tightly-coupled multiprocessing system can be diagrammed this way:

  CPU     CPU     CPU      CPU     CPU     CPU   CPU    MEMORY
    |       |       |        |       |       |     |    |
 ------------------------------------------------------------ BUS
       |      |       |     |     |     |     |     |     |
     CPU    CPU     CPU   CPU   CPU   CPU   CPU   CPU   CPU

Here we have a RAM memory being shared over a common bus by several CPU's. It is possible that the different CPU's have radically different speeds. Speed can be affected by the inherent power of the CPU, or by the current load on the CPU. Besides that, it is impossible to tell which CPU will be assigned to which thread. And also, each time we run the program, the assignment can be different.

In your program, the mother process creates child threads to be the diners and the food server. Then the mother thread plays the role of the busser. The mother does nothing to direct the server and diners after she creates them.

In general it is a bad idea to create "traffic cop" threads, because the traffic cop too often becomes a bottleneck. When every action has to be approved by a master, events can only happen as fast as the master can approve them. This would defeat the purpose of parallel processing.

The general rules for submitting assignments by e-mail are:

Always send me e-mail as plain text in the main message body. Never send me attachments.
When you send a program source file, you also send a script of your tests. Combine the source file and the script into one shell archive (shar) file.
I will tell you what subject line to use with each message, and I need you to use exactly the subject lines I give you. (I get hundreds of e-mail messages at a time and your subject line allows me to sort messages.)
Send each submission before midnight on the due date.

Here is what you have to turn in:

By the first due date e-mail me a pseudo-code copy of your solution to the problem -- a copy of foodPass2.cpp with the appropriate pseudo-code inserted.

Use this subject line: CS3750,prog2,pseudo
By the second due date e-mail me a shar file containing:
1. a completed copy of the foodPass2.cpp source file,
  
  (Do not send me copies of sem.h or sem.cpp. I will compile your copy of foodPass2.cpp on a Sun Ultra with my own copies of sem.h and sem.cpp. To do the compile, I will use a "make" file just like the one I am giving you. Your program should work for me if it worked for you.)
2. script(s) showing several successful test runs
Use this subject line: CS3750,prog2,final

Note that there are no spaces in the subject lines given above. It is important that you do not insert any spaces. My e-mail address is: john@ishi.csustan.edu.

DUE DATES:

For due dates, see the class schedule.