Objectives

Notes

• Start a parallel program.
• #pragma omp parallel
• #include < omp.h>
• link with -fopenmp
• As we said before once in the parallel pragma block each thread runs the code.
• Task: Write hello world in parallel.
  • To clean printing,
  • #pragma omp critical
  • Only one thread at a time can execute the next code block.
• omp_set_num_threads(), to change the number of threads.
• omp_get_thread_num() gives us a unique id for each thread.
  • Clean up your code so you can get a hello from each tread,
• Can we nest parallel, yes we can.
  • Declare a parallel block inside the parallel block.
  • Get a new id for each thread, and print each out.
  • Not so impressive, everyone just runs one thread.
• omp_set_max_active_levels(int n ); will let you have n levels of parallelism
• Try the last code again.
• If we want, we can limit the threads in a parallel section
  • #pragma omp parallel num_threads(int n)
• There are more directives we can put there, but this will do for now.
• #pragma omp barrier
  • Will stop all threads until the barrier is reached.
• Remember, the for loop.
  • #pragma omp for
    • This uses the active threads.
    • If the loop runs k times with b threads, n = k/b
      • Thread 0 gets i = 0, k/b-1
      • Thread 1 get i = k/b to 2k/b -1
      • Thread n-1 gets i = k/b(n-1) to nk/b -1
• Rules parallel for loops
  • OMP needs to be able to compute the number of iterations.
  • No break, return, exit or goto
  • index update must be by a fixed amount
  • Loop variable is private, no changes inside the loop, only for increment..
• Remember omp_get_wtime
• One way to compute pi is by throwing 'uniform' darts at a dart board.
  • If the probability of hitting the box the dartboard in which the dartboard is inscribed is 100%, the probability of hitting the board is the area of the board
  • Look at a r=1 dartboard, only 1/4 of the board (why)
  • Centered at 0,0
  • Use the random number generator to "throw a dart"
  • Seed with threadID * time
  • Record the hits
  • Pi = 4 * hits/trials