---

A common parallel program structure

---

• A common structure for computational parallel program
  • The follow program fragment depicts a common motif of parallel programs:

    void *worker (void *arg) { .... (You will need to convert arg to the correct type before you can use the value passed) .... } int main(int argc, char *argv[]) { // SEQUENTIAL Section .... Prepare problem (setup shared variables) .... // PARALLEL Section // Start the worker threads for (i = 0; i < NUM_PROCESSORS; i = i + 1) { param[i] = ....; pthread_create(&tid[i], &attr, worker, & param[i]) } // Wait for all workers to finish for (i = 0; i < NUM_PROCESSORS; i = i + 1) pthread_join(tid[i], NULL); // SEQUENTIAL Section .... Post process results from workers.... (Or start another prepare section followed by a parallel + join section) }

    Note:

    You can pass a parameter of any type to the worker() function But inside the worker() function, you will have to cast the value to the correct type

  ---

  ---

  ---

  ---

• Use of global variables in multi-threaded programs
  • Fact:

    The main function often need to pass a huge amount of data to the worker( ) functions to be processed Example: sort an array The entire array need to be "passed" !!!

    ---

    ---

  • Passing a large amount of data between the main( ) function and the worker( ) function (that is executed by multiple threads) are usualy done through:

    Shared (memory) variable(s) !!!

    because:

    You will see in the demo programs that the data that need to be processed are stored in a global variabel

    You do this, so that:

    The main( ) function can initialize the variables (usually an array) The worker( ) function can access the variables !!! (Otherwise, the worker( ) function cannot work !!!)

    ---

    ---

  • Note:

    The worker( ) function sometimes will produce some output It is easier to use global variables to pass the output value to the main( ) function We will use global variables to pass the output from the worker( ) function to the main( ) function....