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OpenMP E. Bruce Pitman October, 2002
Outline • What is OpenMP • Multi-threading • How to use OpenMP • Limitations • OpenMP + MPI • References
What is OpenMP? • A portable standard for shared memory multi-processing • A library of compiler directives • Fine-grained (loop-level) • To varying degrees, Sun, SGI, IBM, HP, Intel
How to use OpenMP • schedule(static [,chunk]) • Deal-out blocks of iterations of size “chunk” to each thread. • schedule(dynamic[,chunk]) • Each thread grabs “chunk” iterations off a queue until all iterations have been handled. • uschedule(guided[,chunk]) • Threads dynamically grab blocks of iterations. The size of the block starts large and shrinks down to size “chunk” as the calculation proceeds.
How to use OpenMP • schedule(runtime) • Schedule and chunk size taken from the OMP_SCHEDULE environment variable.
How to use OpenMP Here’s an example of PRIVATE and FIRSTPRIVATE variables A,B, and C = 1C$OMP PARALLEL PRIVATE(B) C$OMP& FIRSTPRIVATE(C) Inside this parallel region “A” is shared by all threads and equals 1 “B” and “C” are local to each thread - B’s initial value is undefined - C’s initial value equals 1 Outside this parallel region the values of “B” and “C” are undefined.
How to use OpenMP • #!/bin/csh -f • #PBS -l ncpus=8 • #PBS -V • #PBS -q medium_p • #PBS -M email@example.com
How to use OpenMP • cp /CCRSGI/home/pitman/fortran/sph/sph* $PBSTMPDIR • cd $PBSTMPDIR • setenv OMP_NUM_THREADS 8 • f90 -O2 -LNO -IPA -n32 -mips4 -r12000 -mp -o sph sphomp.f • time ./sph > outfile • cp outfile /CCRSGI/home/pitman/fortran/sph/ • # remove scratch directory • cd /FCScratch • \rm -r $PBSTMPDIR
How to use OpenMP k = 1 do while (k .le. maxit .and. error .gt. tol) error = 0.0 !$omp parallel !$omp do do j=1,m do i=1,n uold(i,j) = u(i,j) enddo enddo
How to use OpenMP !$omp do private(resid) reduction(+:error) do j = 2,m-1 do i = 2,n-1 resid = (ax*(uold(i-1,j) + uold(i+1,j)) & + ay*(uold(i,j-1) + uold(i,j+1)) & + b * uold(i,j) - f(i,j))/b u(i,j) = uold(i,j) - omega error = error + resid*resid end do enddo
How to use OpenMP !$omp enddo nowait !$omp end parallel $omp end parallel k = k + 1 error = sqrt(error)/dble(n*m) enddo
Limitations • Easy to port serial code to OpenMP • OpenMP code can run in serial mode HOWEVER • Shared memory machines only • Limited scalability -- after ~8 processors, not much speed-up • Overhead of parallel do, parallel regions
Limitations • OpenMP currently does not specify or provide constructs for controlling the binding of threads to processors. • Processors can migrate, causing overhead. • This behavior is system-dependent. • System-dependent solutions may be available.
References • www.openmp.org • http://www.ccr.buffalo.edu/documents/CCR_openmp_pbs.PDF • http://www.epcc.ed.ac.uk/research/openmpbench/ • http://www.llnl.gov/computing/tutorials/workshops/workshop/openMP/MAIN.html • http://scv.bu.edu/SCV/Tutorials/OpenMP/