Experiment 2

Process Synchronization

1. OBJECTIVES

After completing this lab, you will be able to:

• Understand how to synchronize processes/threads.
• Understand interleavings and race conditions, and master some way of controlling them.
• Know how to use locks/semaphores to solve a critical section problem.

2. LABORATORY

Software Lab 1B (N4-01a-02)

3. EQUIPMENT

Pentium IV PC with Nachos 3.4.

4. MODE OF WORKING

You should be working alone. No group effort.

5. ASSESSMENT

The assessment of this lab will be based on oral examination.

6. THREAD OPERATIONS

The thread is a light-weight process in Nachos. In the first experiment, we have learnt the basic feature of thread execution in Nachos. In this experiment, we need to use the following thread operations to finish the exercises. Please read the source code under the directory (particularly for thread.h and thread.cc).

void Fork(VoidFunctionPtr func, int arg, int joinP);
// Make thread run (*func)(arg) and
 // know if a Join is going to happen.

Fork creates a new thread of control executing in the calling user thread address space. The function argument is a procedure pointer. The new thread will begin executing in the same address space as the thread calling Fork. The new thread must have its own user stack in the user address space, separate from all other threads. It must be able to make system calls and block independently of other threads in the same thread.

void Join(Thread *forked);     // Waits for specified thread to
                                // finish before continuing

A parent thread may call Join to wait for a child thread to complete (e.g., to Exit).

void Yield();                 // Relinquish the CPU if any
                                    // other thread is runnable

A thread gives up the CPU ownership and allows another thread to execute. Its state shifts from running to ready, and it is put to the ready queue.

void Sleep();                 // Put the thread to sleep and
                                    // relinquish the processor.

A thread gives up the CPU ownership and allows another thread to execute. Its state shifts from running to waiting (waiting for the wake-up event). When the wake-up event is triggered, the thread is put to the ready queue.

7. CONTEXT SWITHCHES

On a multiprocessor, the executions of threads running on different processors may be arbitrarily interleaved, and proper synchronization is even more important. In Nachos, which is uniprocessor-based, interleavings are determined by the timing of context switches from one thread to another. On a uniprocessor, properly synchronized code should work no matter when and in what order the system chooses to run the threads. The best way to find out if your code is “properly synchronized” is to see if it breaks when you run it repeatedly in a way that exhaustively forces all possible interleavings to occur. This is the basic idea to check whether multiple threads have the problem of race condition. To experiment with different interleavings, you must somehow control when the executing program makes context switches.

Context switches can be either voluntary or involuntary. Voluntary context switches occur when the thread that is running explicitly calls a yield or sleep primitive (Thread::Yield or Thread::Sleep) to cause the system to switch to another thread. A thread running in Nachos might initiate a voluntary switch for any of a number of reasons, perhaps in the implementation of a synchronization facility such as a semaphore.

In contrast, involuntary context switches occur when the inner Nachos modules (Machine and Thread) decide to switch to another thread all by themselves. In a real system, this might happen when a timer interrupt signals that the current thread is hogging the CPU.

8. EXERCISES

We have two exercises in this experiment.

1. In this exercise, we will conduct the following steps to understand race condition problem in Nachos.

   1. Change your working directory to lab2 by typing cd ~/nachos-3.4/lab2

   2. Read the Nachos thread test program threadtest.cc carefully. There is a shared variable named value (initially zero). There are two functions, namely void Inc(_int which) and void Dec(_int which) , where increases and decreases value by one, respectively. In this exercise, you need to consider different interleaving executions of Inc and Dec so that the shared variable value is equal to a predefined value after threads complete.

   3. You need to implement the following three functions. When all the threads (two Inc_v1 threads and two Dec_v1 threads) complete in TestValueOne(), value=1.

      void Inc_v1(_int which)
      void Dec_v1(_int which)
      void TestValueOne()
      

      In Inc_v1 and Dec_v1, you need to use Yield primitive in Nachos to induce context switch. Inc_v1 and Dec_v1 should have the same logic as Inc and Dec, respectively. You are only allowed to add Yield into those two functions. You need to implement ThreadValueOne() by creating two threads with Inc_v1 and two threads with Dec_v1. The current thread should wait for all those threads to complete. At the end of TestValueOne(), a checking is performed on whether the value is 1. If the checking is passed, you should get the message "congratulations! passed.". Otherwise, an error message is printed.

   4. After you finish implementing the above-mentioned functions, you can demonstrate the result of TestValueOne(), by commenting other test functions in ThreadTest() like below.

      //for exercise 1.
      TestValueOne();
      //TestValueMinusOne();
      //for exercise 2.
      //TestConsistency();
      

   5. Compile Nachos by typing make. If you see "ln -sf arch/intel-i386-linux/bin/nachos nachos" at the end of the compiling output, your compilation is successful. If you encounter any anomalies, type make clean to remove all object and executable files and then type make again for a clean compilation.

   6. Test this program by typing ./nachos. If you see “congratulations! passed.” at the end of the debugging messages, your program is successful. Otherwise, “failed.” will be displayed.

   7. Repeat Steps 3)–6), and implement the following three functions. When all the threads (two Inc_v2 threads and two Dec_v2 threads) complete in TestValueMinusOne(), value=-1. At Step 4), you need to test TestValueMinusOne().

      void Inc_v2(_int which)
      void Dec_v2(_int which)
      void TestValueMinusOne()
      

2. In this exercise, we will conduct the following steps to understand process synchronization problem in Nachos.

   1. Change your working directory to lab2 by typing cd ~/nachos-3.4/lab2

   2. You need to implement the following three functions. When all the four threads (two Inc_Consistent threads and two Dec_Consistent threads) complete in TestConsistency(), value=0. You need to achieve consistent result (value=0), regardless of different interleaving execution orders in Inc_Consistent and Dec_Consistent as well as different thread fork orders in TestConsistency().

      void Inc_Consistent (_int which)
      void Dec_Consistent (_int which)
      void TestConsistency ()
      

      In Inc_Consistent and Dec_Consistent, you use Yield interface in Nachos to induce context switch. You need to implement TestConsistency() by creating two threads with Inc_Consistent and two threads with Dec_Consistent. The current thread should wait for all those threads to complete. At the end of TestConsistency(), a checking is performed on whether the value is 0. If the checking is passed, you should get the message "congratulations! passed.". Otherwise, an error message is printed.

   3. After you finish implementing the above-mentioned functions, you can demonstrate the result of TestConsistency(), by commenting other test functions in ThreadTest() like below.

      //for exercise 1.
      //TestValueOne();
      //TestValueMinusOne();
      //for exercise 2.
      TestConsistency();
      

   4. Compile Nachos by typing make. If you see "ln -sf arch/intel-i386-linux/bin/nachos nachos" at the end of the compiling output, your compilation is successful. If you encounter any anomalies, type make clean to remove all object and executable files and then type make again for a clean compilation.

   5. Test this program by typing ./nachos. If you see “congratulations! passed.” at the end of the debugging messages, your program is successful. Otherwise, “failed.” will be displayed. In the oral exam, you need to demonstrate your testing with different interleaving execution orders in Inc_Consistent and Dec_Consistent as well as different thread fork orders in TestConsistency().