02152 Concurrent Systems        Fall 2008
CP Mini Lab 2: Shared variables in Java

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Plan

Material

Purpose

To experience the race condition problem with shared variables in Java.

Prerequisites

See the note about processes and threads [Proc], section 4, for a description of threads in Java.

Instructions

1. Fetch the program Counting.java that is a small Java program with two concurrent threads that increments a variable x a large number of times.

    
   [The volatile-modifier on a variable declaration ensures that any change of the variable is actually written to the variable (and not just held in a register for optimization).]

2. Run the program a couple of times and see whether the result is always the same.

   If you get the same (correct) result every time, try the following:

   • Increase the number of iterations in T() with a factor of 10 until the program takes several seconds to execute. Now you may get varying results.
   • If you still get a single result the reason is probably that your Java Virtual Machine does not run threads with the same priority (as in our example) with preemption. Thus, one of the threads is run to completion before the other is executed. In this case, try the procedure at the bottom of the page.
3. As a last point, try to reduce the number of iterations with a factor of 10 at a time and run the program several times. When the number of iterations become small, the result may become stable (and correct). Why?

    
   Note that on a multiprocessor (such as in the G-databar or a dual-core machine), you may have to go very low and even then, the result may not become stable. Explain this.

Enjoy!

• Get the class VIP.java, compile it and insert the following in Counting.java right before the start of the two threads:

  new VIP().start();

  This will start a thread that will first give itself maximal priority and then repeatedly go to sleep for 10 ms. The thread declares itself a deamon such that the program will not wait for it at termination. Thus, there is no need to save a reference to the thread object.

• Compile Counting.java again (thereby compling VIP.java also) and run the program anew.
• Now, the VIP-thread at regular intervals will become activated and preempt the the increment threads due to its high priority. As a side effect the increment threads will now alternate the execution. What happens with the result?

Hans Henrik Løvengreen, Sep 5, 2008