Pthreads in MPI

Question

Hi, I have written an MPI quicksort program which works like this:

In my cluster 'Master' will divide the integer data and send these to 'Slave nodes'. Upon receiving at the Slave nodes, each slave will perform individual sorting operations and send the sorted data back to Master. Now my problem is I'm interested in introducing hyper-threading for the slaves.

I have data coming from master

sub (which denotes the array)
count (size of an array)

Now I have initialized Pthreads as where

 num_threads=12.
    pthread_attr_init(&attr);
   pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_JOINABLE);
  for (i = 0; i < num_pthreads; i++) {
    if (pthread_create(&thread[i], &attr, new_thread, (void *) &sub[i])) 
    {
       printf("error creating a new thread \n");
       exit(1);
    }
    else
    {
        printf(" threading is successful %d  at node %d \n \t ",i,rank);
    }

and in a new thread function

void * new_thread(int *sub)
{

    quick_sort(sub,0, count-1);
    }
    return(0);
}

I don't understand whether my way is correct or not. Can anyone help me with this problem?

Austin Phillips Austin Phillips · Accepted Answer · 2012-09-10T13:25:27

Your basic idea is correct, except you also need to determine how you're going to get results back from the threads.

Normally you would want to pass all relevant information for the thread to complete its task through the *arg argument from pthread_create. In your new_thread() function, the count variable is not passed in to the function and so is global between all threads. A better approach is to pass a pointer to a struct through from pthread_create.

typedef struct {
    int *sub; /* Pointer to first element in array to sort, different for each thread. */
    int count; /* Number of elements in sub. */
    int done; /* Flag to indicate that sort is complete. */
} thread_params_t


void * new_thread(thread_params_t *params)
{
   quick_sort(params->sub, 0, params->count-1); 
   params->done = 1;
   return 0;
}

You would fill in a new thread_params_t for each thread that was spawned.

The other item that has to be managed is the sort results. It would be normal for the main thread to do a pthread_join on each thread which ensure that it has completed before continuing. Depending on your requirements you could either have each thread send results back to the master directly, of have the main function collect the results from each thread and send results back external to the worker threads.

Pthreads in MPI

2 Answers