OpenMP fundamentials

OpenMP for directive

#pragma omp for [ clause [ clause ] ...

Following loop will be executed in parallel (the iterations will be divided by the execution threads)

#pragma omp parallel private(f)
{
  f=7;
#pragma omp for
   for (i=0; i<20; i++)
   a[i] = b[i] + f * (i+1);
} /* omp end parallel */

Рис. 7.3.

Available definitions:

  private( list ) 
  reduction( operator: list ) 
  schedule( type [ , chunk ] )
  nowait (для #pragma omp for)

At the end of the loop all threads will be synchronized unless"nowait" directive is mentioned

schedule defines iteration space scattering method (default behaviour depend on OpenMP version)

OpenMP variables

• private ( list ) Each of the listed variables will have the local copy for each exection thread
• shared ( list ) All the thread will share the same instance of the variable
• firstprivate ( list ) All the local copies will be initialized by master thread value
• lastprivate ( list ) The resulting master thread value will be taken from the last thread executed
• …

Рис. 7.4.

All the variables are shared by default, except the local variables inside a function calls and the loop iterators

Example

int x;
x = 0;                           // Initialize x to zero
#pragma omp parallel for firstprivate(x)   // Copy value
       // of x
                                 // from master
for (i = 0; i < 10000; i++) {
     x = x + i;
}
printf( "x is %d\n", x );     // Print out value of x
/* Actually needs lastprivate(x) to copy value back out to master */

OpenMP schedule clause

schedule( type [ , chunk ] )

• static: Every thread gets fixed amount of data
• dynamic: Amount of data will depend on the thread execution speed
• guided: Threads will get decreased amounts of data dynamically
• runtime: Schedule type will be defined at runtime

Рис. 7.5. Loop scheduling

Main OpenMP functions

int omp_get_num_threads(void);
int omp_get_thread_num(void); 
…

http://www.openmp.org/

OpenMP synchronization

Implicit sunchrionization is performed at the end of any parallel section (unless nowait option is mentioned)

• сritical – can be executed only by one thread at a time.
• atomic – Special critical section version for the atomic operations
• barrier – synchronization point
• ordered – sequential execution
• master – only the main thread will execute the following code
• …

OpenMP critical

cnt = 0;
f=7;
#pragma omp parallel
{
#pragma omp for
    for (i=0; i<20; i++) {
      if (b[i] == 0) {
          #pragma omp critical
         cnt ++;
     } /* endif */
  a[i] = b[i] + f * (i+1);
  } /* end for */
} /*omp end parallel */

Рис. 7.6.

More information

OpenMP Homepage: http://www.openmp.org/

Introduction to OpenMP - tutorial from WOMPEI 2000 http://www.llnl.gov/computing/tutorials/workshops/workshop/openMP

Writing and Tuning OpenMP Programs on Distributed Shared Memory Machines http://www.sao.nrc.ca/~gabriel/openmp

R.Chandra, L. Dagum, D. Kohr, D. Maydan, J. McDonald, R. Menon:

Parallel programming in OpenMP.

Academic Press, San Diego, USA, 2000, ISBN 1-55860-671-8

R. Eigenmann, Michael J. Voss (Eds):

OpenMP Shared Memory Parallel Programming.

Springer LNCS 2104, Berlin, 2001, ISBN 3-540-42346-X