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OpenMP Home PageDO Directive
The DO directive specifies that the iterations of the immediately following DO loop must be executed in parallel. The DO directive must be enclosed in a parallel region; it creates no threads by itself. The following do loop can not be a DO WHILE.
!$OMP DO [clause[[,]clause ...] do_loop !$OMP END DO [NOWAIT]
The DO clause can have various values.
It is illegal to branch out of a DO loop associated with the DO directive.
Example
!Filename: dodir.f90
!
PROGRAM DODIR
IMPLICIT NONE
INTEGER I,L
INTEGER, PARAMETER:: DIM=16
REAL A(DIM),B(DIM),S
INTEGER nthreads,tnumber
INTEGER OMP_GET_NUM_THREADS,OMP_GET_THREAD_NUM
CALL RANDOM_NUMBER(A)
CALL RANDOM_NUMBER(B)
!$OMP PARALLEL DEFAULT(PRIVATE) SHARED(A,B)
!$OMP DO SCHEDULE(STATIC,2)
DO I=2,DIM
B(I) = ( A(I) - A(I-1) ) / 2.0
nthreads=OMP_GET_NUM_THREADS()
tnumber=OMP_GET_THREAD_NUM()
print *, "Thread",tnumber," of",nthreads," has I=",I
END DO
!$OMP END DO
!$OMP END PARALLEL
S=MAXVAL(B)
L=MAXLOC(B,1)
PRINT *, "Maximum gradient: ",S," at location:",L
END PROGRAM DODIR
Compiling and running on Franklin:
> cat dodir.pbs
#PBS -N dodir
#PBS -j oe
#PBS -o dodir.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 dodir -mp=nonuma -Minfo=mp dodir.f90
export OMP_NUM_THREADS=2
aprun -n 1 -N 1 ./dodir
> qsub dodir.pbs
500611.nid00003
> cat dodir.out
/opt/xt-pe/2.0.44a2/bin/snos64/ftn: INFO: linux target is being used
dodir.f90:
dodir:
15, Parallel region activated
17, Parallel loop activated; static block-cyclic iteration allocation
24, Barrier
Parallel region terminated
Thread 1 of 2 has I= 4
Thread 1 of 2 has I= 5
Thread 1 of 2 has I= 8
Thread 1 of 2 has I= 9
Thread 1 of 2 has I= 12
Thread 1 of 2 has I= 13
Thread 1 of 2 has I= 16
Thread 0 of 2 has I= 2
Thread 0 of 2 has I= 3
Thread 0 of 2 has I= 6
Thread 0 of 2 has I= 7
Thread 0 of 2 has I= 10
Thread 0 of 2 has I= 11
Thread 0 of 2 has I= 14
Thread 0 of 2 has I= 15
Maximum gradient: 0.6280164 at location: 1
Application 4733417 resources: utime 0, stime 0
Notice that the loop was divided among the 2 threads as we requested with the SCHEDULE(STATIC,2) clause. Also note that if had not enclosed the do loop in a DO/END DO directive block, the loop would not have been split, but would have been executed OMP_NUM_THREADS number of times.
