Threads Exercise

On path: Exercises	0: Exercises • 1: Pointer Exercise • 2: Arguments Exercise • 3: Malloc Exercise • 4: Structs Exercise • 5: Processes Exercise • 6: Shared memory Exercise • 7: Pipes Exercise • 8: Exceptions Exercise • 9: Synchronisation Exercise • 10: Files Exercise • 11: Threads Exercise • 12: Unix proc Exercise

Depends on	Threads

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Welcome to this exercise about threads. It’s all based on the classic dining philosophers, in which N diners are sat around a circular table and wish to alternate between thinking and eating. To eat, a diner needs to hold two forks, which can be picked up from the table, on their left and right respectively. The snag is, there are only N forks, each to be shared by the two diners on either side of it!

Good luck!

Thanks, John

Stage zero

Your first job is to read the code, making notes for your own memory, and any guesses at the few questions in comments.

I recommend reading the code in this order:

Logger.java
Fork.java
Diner.java
Dining.java

Stage one

Compile and run the program as is. Use:

javac Dining.java
java Dining | less

Explain the behaviour in ex11.txt.

Stage two

Uncomment the first synchronized in Diner.java, recompile, rerun and explain the change in behaviour.

Stage three

Now uncomment the second synchronized in Diner.java and recompile, Rerun without using less this time.

You might want to wonder how long to wait before hitting control-C. Explain the behaviour. In particular, why was stage two not sufficient to achieve this effect?

Stage four

Change Dining.AVOID_DEADLOCK to true and recompile. You will need to recompile Diner.java too.

Rerun through less again and explain the behaviour. Run it yet again – is it different? Try a few times. E.g.:

for n in `seq 1 5`
do
    java Dining > test-$n
done

(That should take about 5 minutes, so find something useful to do.)

Then maybe:

ls -l test-*

might show something.

Then, e.g.

diff test-[12]

etc… If you are lucky, then

tail -n 1 test-*

might show a significant difference (who was last to finish?).

If you are determined to see that change, you could try this:

for n in `seq 1 1000`
 	do
 	  java Dining > test-$n
 	  tail -n 1 test-*
 	  echo
 	done

which should show something every minute and you can control-C when you see what you are looking for, or want to give up waiting. In fact, better than control-C (which you would have to keep pressed down) would be:

control-Z
kill %1

which suspends the loop (to become job 1) and then kills that job (%1).

Stage five

Change Dining.AVOID_DEADLOCK back to false, Dining.NUMBER_OF_CYCLES to -1 and Dining.DELAY to false, then recompile. You will also need to recompile Diner.java.

Run the program several times (you will need control-C), and explain the behaviour.

Max. number:

Max. distance: