SCHEDULE_OPENMP
Scheduling Loop Iterations in OpenMP


SCHEDULE_OPENMP is a C++ program which demonstrates the use of default, static and dynamic scheduling of loop iterations in OpenMP.

By default, when OpenMP, executing with T threads, encounters a parallel loop of N iterations, it assigns the first N/T iterations to thread 0, the second chunk of N/T iterations to thread 1 and so on.

Sometimes this simple, default "scheduling" of the iterations is not ideal. It may be the case that iterations of high index represent more work. In that case, the lower numbered threads will finish early and have nothing to do.

The static schedule clause modifies the iteration assignment procedure by essentially "dealing out" the iterations. The clause schedule(static,5), for instance, indicates that the N iterations are to be dealt out in groups of 5, until all are assigned. This schedule might divide up the work more evenly.

In more complicated situations, where the work involved with each iteration can vary dramatically, the dynamic schedule clause allows the user to parcel out a small number of iterations initially, and then to wait until a thread is finished that work, at which point it is given another group of iterations. The format of this clause is schedule(dynamic,7) where here 7 iterations are assigned to each thread initially, and the remaining work is assigned, 7 iterations at a time, to threads that finish what they have already been assigned.

Example:

For simplicity, we assume that we have a loop of 16 iterations, which has been parallelized by OpenMP, and that we are about to execute that loop using 2 threads.

In default scheduling

In static scheduling, using a "chunksize" of 4:

In dynamic scheduling, using a "chunksize" of 3:

The next chunk is iterations 7 to 9, and will be assigned to whichever thread finishes its current work first, and so on until all work is completed.

Usage:

In the BASH shell, the program could be run with 2 threads using the commands:

        export OMP_NUM_THREADS=2
        ./schedule_openmp
      

Licensing:

The computer code and data files described and made available on this web page are distributed under the GNU LGPL license.

Languages:

SCHEDULE_OPENMP is available in a C version and a C++ version and a FORTRAN77 version and a FORTRAN90 version.

Related Data and Programs:

DIJKSTRA_OPENMP, a C++ program which uses OpenMP to parallelize a simple example of Dijkstra's minimum distance algorithm for graphs.

FFT_OPENMP, a C++ program which demonstrates the computation of a Fast Fourier Transform in parallel, using OpenMP.

HEATED_PLATE_OPENMP, a C++ program which solves the steady (time independent) heat equation in a 2D rectangular region, using OpenMP to run in parallel.

HELLO_OPENMP, a C++ program which prints out "Hello, world!" using the OpenMP parallel programming environment.

MD_OPENMP, a C++ program which carries out a molecular dynamics simulation in parallel, using OpenMP.

MULTITASK_OPENMP, a C++ program which demonstrates how to "multitask", that is, to execute several unrelated and distinct tasks simultaneously, using OpenMP for parallel execution.

MXM_OPENMP, a C++ program which computes a dense matrix product C=A*B, using OpenMP for parallel execution.

OPENMP, C++ programs which illustrate the use of the OpenMP application program interface for carrying out parallel computations in a shared memory environment.

OPENMP_RCC, C++ programs which illustrate how a C program, using OpenMP, can be compiled and run in batch mode on the FSU High Performance Computing (HPC) cluster operated by the Research Computing Center (RCC).

POISSON_OPENMP, a C++ program which computes an approximate solution to the Poisson equation in a rectangle, using the Jacobi iteration to solve the linear system, and OpenMP to carry out the Jacobi iteration in parallel.

PRIME_OPENMP, a C++ program which counts the number of primes between 1 and N, using OpenMP for parallel execution.

QUAD_OPENMP, a C++ program which approximates an integral using a quadrature rule, and carries out the computation in parallel using OpenMP.

RANDOM_OPENMP, a C++ program which illustrates how a parallel program using OpenMP can generate multiple distinct streams of random numbers.

SATISFY_OPENMP, a C++ program which demonstrates, for a particular circuit, an exhaustive search for solutions of the circuit satisfy problem, using OpenMP for parallel execution.

SGEFA_OPENMP, a C++ program which reimplements the SGEFA/SGESL linear algebra routines from LINPACK for use with OpenMP.

TIMER, C++ programs which demonstrate how to compute CPU time or elapsed time.

ZIGGURAT_OPENMP, a C++ program which demonstrates how the ZIGGURAT library can be used to generate random numbers in an OpenMP parallel program.

Reference:

  1. Peter Arbenz, Wesley Petersen,
    Introduction to Parallel Computing - A practical guide with examples in C,
    Oxford University Press,
    ISBN: 0-19-851576-6,
    LC: QA76.58.P47.
  2. Rohit Chandra, Leonardo Dagum, Dave Kohr, Dror Maydan, Jeff McDonald, Ramesh Menon,
    Parallel Programming in OpenMP,
    Morgan Kaufmann, 2001,
    ISBN: 1-55860-671-8,
    LC: QA76.642.P32.
  3. Barbara Chapman, Gabriele Jost, Ruud vanderPas, David Kuck,
    Using OpenMP: Portable Shared Memory Parallel Processing,
    MIT Press, 2007,
    ISBN13: 978-0262533027.
  4. The OpenMP web site
  5. OpenMP Architecture Review Board,
    OpenMP Application Program Interface,
    Version 3.0,
    May 2008.

Source Code:

Examples and Tests:

SCHEDULE_LOCAL_G++ compiles, links, loads and runs the program using 2 threads, the local interactive environment, and the G++ C++ compiler.

SCHEDULE_LOCAL_ICPC compiles, links, loads and runs the program using 2 threads, the local interactive environment, and the ICPC C++ compiler.

SCHEDULE_ITHACA_G++ compiles, links, loads and runs the program using 2 threads, using the PBS queueing system on Virginia Tech's Ithaca cluster, and the G++ C++ compiler.

SCHEDULE_ITHACA_ICPC compiles, links, loads and runs the program using 2 threads, using the PBS queueing system on Virginia Tech's Ithaca cluster, and the ICPC C++ compiler.

You can go up one level to the C++ source codes.


Last revised on 10 July 2010.