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OpenMP Home PagePARALLEL Directive
A Parallel Region is a block of code that is to be executed in parallel by a number of threads. Each thread executes the enclosed code separately.
Note that all code within a parallel region is executed by each thread unless other OpenMP directives specify otherwise. For instance, a DO loop that lies within a parallel region will be executed completely (and redundantly) by each thread unless a parallel DO directive is inserted before the loop. A DO or PARALLEL DO directive is necessary if you want the loop to be executed once with different threads performing different iterations of the loop in parallel.
It is illegal to branch out of a Parallel Region.
!$OMP PARALLEL [clause] code block !$OMP END PARALLEL
There are many possible values of [clause].
Examples
!Filename: parallel.f90
!
!This simply shows that code in a PARALLEL
!region is executed by each thread.
PROGRAM PARALLEL
IMPLICIT NONE
INTEGER I
I=1
!$OMP PARALLEL FIRSTPRIVATE(I)
PRINT *, I
!$OMP END PARALLEL
END PROGRAM PARALLEL
To run on franklin:
> cat parallel.pbs
#PBS -N parallel
#PBS -j oe
#PBS -o parallel.out
#PBS -q interactive
#PBS -S /bin/bash
#PBS -l mppwidth=1
#PBS -l mppnppn=1
#PBS -l mppdepth=2
#PBS -l walltime=00:05:00
#PBS -V
cd $PBS_O_WORKDIR
ftn -o parallel -mp=nonuma -Minfo=mp parallel.f90
export OMP_NUM_THREADS=2
aprun -n 1 -N 1 ./parallel
> qsub parallel.pbs
498022.nid00003
> cat parallel.out
/opt/xt-pe/2.0.44a2/bin/snos64/ftn: INFO: linux target is being used
parallel.f90:
parallel:
12, Parallel region activated
14, Parallel region terminated
1
1
Application 4673065 resources: utime 0, stime 0
The next example shows use of the REDUCTION clause. This simple example shows how the values of the variables are combined when leaving the parallel region when a REDUCTION clause is used. Also note that each thread executes the PRINT statement in the parallel region.
!Filename: reduction.f90
!
!This program shows the use of the REDUCTION clause.
PROGRAM REDUCTION
IMPLICIT NONE
INTEGER tnumber, OMP_GET_THREAD_NUM
INTEGER I,J,K
I=1
J=1
K=1
PRINT *, "Before Par Region: I=",I," J=", J," K=",K
PRINT *, ""
!$OMP PARALLEL DEFAULT(PRIVATE) REDUCTION(+:I)&
!$OMP REDUCTION(*:J) REDUCTION(MAX:K)
tnumber=OMP_GET_THREAD_NUM()
I = tnumber
J = tnumber
K = tnumber
PRINT *, "Thread ",tnumber, " I=",I," J=", J," K=",K
!$OMP END PARALLEL
PRINT *, ""
print *, "Operator + * MAX"
PRINT *, "After Par Region: I=",I," J=", J," K=",K
END PROGRAM REDUCTION
To run on franklin:
> cat reduction.pbs
#PBS -N reduction
#PBS -j oe
#PBS -o reduction.out
#PBS -q interactive
#PBS -S /bin/bash
#PBS -l mppwidth=1
#PBS -l mppnppn=1
#PBS -l mppdepth=2
#PBS -l walltime=00:05:00
#PBS -V
cd $PBS_O_WORKDIR
ftn -o reduction -mp=nonuma -Minfo=mp reduction.f90
export OMP_NUM_THREADS=2
aprun -n 1 -N 1 ./reduction
> qsub reduction.pbs
498041.nid00003
> cat reduction.out
/opt/xt-pe/2.0.44a2/bin/snos64/ftn: INFO: linux target is being used
reduction.f90:
reduction:
15, Parallel region activated
25, Begin critical section
End critical section
Parallel region terminated
Before Parallel Region: I= 1 J= 1 K= 1
Thread 0 I= 0 J= 0 K=
0
Thread 1 I= 1 J= 1 K=
1
Operator + * MAX
After Parallel Region: I= 2 J= 0 K= 1
Application 4673534 resources: utime 0, stime 0
