OpenMP

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OpenMP

• E. Bruce Pitman
• October, 2002

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Outline

• What is OpenMP
• Multi-threading
• How to use OpenMP
• Limitations
• OpenMP MPI
• References

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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

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Multi-threading
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Multi-threading
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How to use OpenMP
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How to use OpenMP
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How to use OpenMP
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How to use OpenMP
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How to use OpenMP
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How to use OpenMP
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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.

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How to use OpenMP

• schedule(runtime)
• Schedule and chunk size taken from the
  OMP_SCHEDULE environment variable.

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How to use OpenMP

• Heres an example of PRIVATE and FIRSTPRIVATE
• variables A,B, and C 1COMP PARALLEL
  PRIVATE(B) COMP FIRSTPRIVATE(C)
• Inside this parallel region A is shared by all
  threads and equals 1
• B and C are local to each thread - Bs
  initial value is undefined - Cs initial value
  equals 1
• Outside this parallel region the values of B
  and C are undefined.

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How to use OpenMP
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How to use OpenMP
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How to use OpenMP
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How to use OpenMP
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How to use OpenMP
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How to use OpenMP
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How to use OpenMP
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How to use OpenMP

• !/bin/csh -f
• PBS -l ncpus8
• PBS -V
• PBS -q medium_p
• PBS -M pitman_at_math.buffalo.edu

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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 gt outfile
• cp outfile /CCRSGI/home/pitman/fortran/sph/
• remove scratch directory
• cd /FCScratch
• \rm -r PBSTMPDIR

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How to use OpenMP
k 1 do while (k .le. maxit .and. error .gt.
tol) error 0.0 !omp parallel !omp do
do j1,m do i1,n uold(i,j) u(i,j)
enddo enddo
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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(i1,j))
ay(uold(i,j-1) uold(i,j1)) b
uold(i,j) - f(i,j))/b u(i,j) uold(i,j) -
omega error error residresid end do
enddo
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How to use OpenMP
!omp enddo nowait !omp end parallel omp end
parallel k k 1 error sqrt(error)/dble(nm
) enddo
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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

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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.

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OpenMP and MPI
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References

• www.openmp.org
• http//www.ccr.buffalo.edu/documents/CCR_openmp_pb
  s.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/