Fork-Join Programming

This approach is the simplest method of coordinating your threads. As in the earlier examples in this chapter, a master thread sets up some global data structures that describe the tasks each thread is to perform and then use the pthread_create( ) function to activate the proper number of threads. Each thread checks the global data structure using its thread-id as an index to find its task. The thread then performs the task and completes. The master thread waits at a pthread_join( ) point, and when a thread has completed, it updates the global data structure and creates a new thread. These steps are repeated for each major iteration (such as a time-step) for the duration of the program:

      for(ts=0;ts<10000;ts++) { /* Time Step Loop */
         /* Setup tasks */
         for (ith=0;ith<NUM_THREADS;ith++) pthread_create(..,work_routine,..)
         for (ith=0;ith<NUM_THREADS;ith++) pthread_join(...)
      }
      work_routine() {
         /* Perform Task */
         return;
      }

The shortcoming of this approach is the overhead cost associated with creating and destroying an operating system thread for a potentially very short task.

The other approach is to have the threads created at the beginning of the program and to have them communicate amongst themselves throughout the duration of the application. To do this, they use such techniques as critical sections or barriers.

Synchronization

Synchronization is needed when there is a particular operation to a shared variable that can only be performed by one processor at a time. For example, in previous SpinFunc( ) examples, consider the line:

globvar++;
    

In assembly language, this takes at least three instructions:

      LOAD   R1,globvar
      ADD    R1,1
      STORE  R1,globvar
    

What if globvar contained 0, Thread 1 was running, and, at the precise moment it completed the LOAD into Register R1 and before it had completed the ADD or STORE instructions, the operating system interrupted the thread and switched to Thread 2? Thread 2 catches up and executes all three instructions using its registers: loading 0, adding 1 and storing the 1 back into globvar. Now Thread 2 goes to sleep and Thread 1 is restarted at the ADD instruction. Register R1 for Thread 1 contains the previously loaded value of 0; Thread 1 adds 1 and then stores 1 into globvar. What is wrong with this picture? We meant to use this code to count the number of threads that have passed this point. Two threads passed the point, but because of a bad case of bad timing, our variable indicates only that one thread passed. This is because the increment of a variable in memory is not atomic. That is, halfway through the increment, something else can happen.

Another way we can have a problem is on a multiprocessor when two processors execute these instructions simultaneously. They both do the LOAD, getting 0. Then they both add 1 and store 1 back to memory.1 Which processor actually got the honor of storing their 1 back to memory is simply a race.

We must have some way of guaranteeing that only one thread can be in these three instructions at the same time. If one thread has started these instructions, all other threads must wait to enter until the first thread has exited. These areas are called critical sections. On single-CPU systems, there was a simple solution to critical sections: you could turn off interrupts for a few instructions and then turn them back on. This way you could guarantee that you would get all the way through before a timer or other interrupt occurred:

      INTOFF                // Turn off Interrupts
      LOAD    R1,globvar
      ADD     R1,1
      STORE   R1,globvar
      INTON                 // Turn on Interrupts
    

However, this technique does not work for longer critical sections or when there is more than one CPU. In these cases, you need a lock, a semaphore, or a mutex. Most thread libraries provide this type of routine. To use a mutex, we have to make some modifications to our example code:

...
      pthread_mutex_t my_mutex; /* MUTEX data structure */
        ...

      main() {
        ...
        pthread_attr_init(&attr); /* Initialize attr with defaults */
        pthread_mutex_init (&my_mutex, NULL);
         .... pthread_create( ... )
          ...
      }
      void *SpinFunc(void *parm)    {
        ...
        pthread_mutex_lock (&my_mutex);
        globvar ++;
        pthread_mutex_unlock (&my_mutex);
        while(globvar < THREAD_COUNT ) ;
        printf("SpinFunc me=%d – done globvar=%d...\n", me, globvar);
        ...
      }

The mutex data structure must be declared in the shared area of the program. Before the threads are created, pthread_mutex_init must be called to initialize the mutex. Before globvar is incremented, we must lock the mutex and after we finish updating globvar (three instructions later), we unlock the mutex. With the code as shown above, there will never be more than one processor executing the globvar++ line of code, and the code will never hang because an increment was missed. Semaphores and locks are used in a similar way.

Interestingly, when using user space threads, an attempt to lock an already locked mutex, semaphore, or lock can cause a thread context switch. This allows the thread that “owns” the lock a better chance to make progress toward the point where they will unlock the critical section. Also, the act of unlocking a mutex can cause the thread waiting for the mutex to be dispatched by the thread library.

Barriers

Barriers are different than critical sections. Sometimes in a multithreaded application, you need to have all threads arrive at a point before allowing any threads to execute beyond that point. An example of this is a time-based simulation. Each task processes its portion of the simulation but must wait until all of the threads have completed the current time step before any thread can begin the next time step. Typically threads are created, and then each thread executes a loop with one or more barriers in the loop. The rough pseudocode for this type of approach is as follows:

main() {
        for (ith=0;ith<NUM_THREADS;ith++) pthread_create(..,work_routine,..)
        for (ith=0;ith<NUM_THREADS;ith++) pthread_join(...) /* Wait a long time */
        exit()
      }

      work_routine() {

        for(ts=0;ts<10000;ts++) { /* Time Step Loop */
               /* Compute total forces on particles */
          wait_barrier();
               /* Update particle positions based on the forces */
          wait_barrier();
        }
        return;
      }    

In a sense, our SpinFunc( ) function implements a barrier. It sets a variable initially to 0. Then as threads arrive, the variable is incremented in a critical section. Immediately after the critical section, the thread spins until the precise moment that all the threads are in the spin loop, at which time all threads exit the spin loop and continue on.

For a critical section, only one processor can be executing in the critical section at the same time. For a barrier, all processors must arrive at the barrier before any of the processors can leave.