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Environment variables specific to the Intel compiler start with <tt>KMP_</tt> whereas those specific to Gnu start with <tt>GOMP_</tt>. For optimal performance regarding memory access, it is important to set the | Environment variables specific to the Intel compiler start with <tt>KMP_</tt> whereas those specific to Gnu start with <tt>GOMP_</tt>. For optimal performance regarding memory access, it is important to set the | ||
<tt>OMP_PROC_BIND</tt> variable as well as the affinity variables, <tt>KMP_AFFINITY</tt> for Intel, and <tt>GOMP_CPU_AFFINITY</tt> for GNU compilers. This prevents the movement of OpenMP threads between processors by the operating system. This is particularly important in a [http://en.wikipedia.org/wiki/Non_Uniform_Memory_Access NUMA] architecture found in most modern computers. | <tt>OMP_PROC_BIND</tt> variable as well as the affinity variables, <tt>KMP_AFFINITY</tt> for Intel, and <tt>GOMP_CPU_AFFINITY</tt> for GNU compilers. This prevents the movement of OpenMP threads between processors by the operating system. This is particularly important in a [http://en.wikipedia.org/wiki/Non_Uniform_Memory_Access NUMA] architecture found in most modern computers. | ||
== Example == | |||
Here is a ''hello world'' example that shows the use of openMP. | |||
{| border="0" cellpadding="5" cellspacing="0" align="center" | |||
! style="background:#8AA8E5;" | '''C CODE''': <tt>ompHello.c</tt> | |||
! style="background:#ECCF98;" | '''FORTRAN CODE''': <tt>ompHello.f90</tt> | |||
|-valign="top" | |||
|<source lang="c"> | |||
#include <stdio.h> | |||
#include <omp.h> | |||
int main() { | |||
#pragma omp parallel | |||
{ | |||
printf("Hello world from thread %d out of %d\n", | |||
omp_get_thread_num(),omp_get_num_threads()); | |||
} | |||
return 0; | |||
} | |||
</source> | |||
|<source lang="fortran"> | |||
program omphello | |||
implicit none | |||
integer omp_get_thread_num,omp_get_num_threads | |||
!$omp parallel | |||
print *, 'Hello world from thread',omp_get_thread_num(), & | |||
'out of',omp_get_num_threads() | |||
!$omp end parallel | |||
end | |||
</source> | |||
|} | |||
Compiling and running the C code goes as follows: | |||
litai10:~$ gcc -O3 -fopenmp ompHello.c -o ompHello | |||
litai10:~$ export OMP_NUM_THREADS=4 | |||
litai10:~$ ./ompHello | |||
Hello world from thread 0 out of 4 | |||
Hello world from thread 2 out of 4 | |||
Hello world from thread 1 out of 4 | |||
Hello world from thread 3 out of 4 | |||
Compiling and running the fortran 90 code is as follows: | |||
litai10:~$ gfortran -O3 -fopenmp ompHello.f90 -o fomphello | |||
litai10:~$ export OMP_NUM_THREADS=4 | |||
litai10:~$ ./fomphello | |||
Hello world from thread 0 out of 4 | |||
Hello world from thread 2 out of 4 | |||
Hello world from thread 1 out of 4 | |||
Hello world from thread 3 out of 4 | |||