example openmp

o/openmp/pi.c

+/*
+ * Calculate $\pi = 4 \int_0^1 \sqrt{1 - x^2} \, dx$
+ * using a rectangle rule $\sum_{i=1}^N f(x_0 + i h - h/2) h$
+ *
+ * compile as $> gcc -o hello pi.c -fopenmp -lm
+ * Run as       $> OMP_NUM_THREADS=2 ./a.out
+ */
+#include <stdio.h>
+#include <math.h>
+#include <omp.h>
+#include <stdint.h>
+
+int main()
+{
+    const int64_t  N = 100000000000;
+    const double L = 1.0;
+    const double h = L / N;
+    const double x_0 = 0.0;
+
+    double pi;
+    double t_1, t_2;
+
+    int64_t i;
+    double sum = 0.0;
+
+    t_1 = omp_get_wtime();
+
+#pragma omp parallel for reduction(+: sum) schedule(static)
+    for (i = 0; i < N; ++i)
+    {
+        double x = x_0 + i * h + h/2;
+        sum += sqrt(1 - x*x);
+    }
+
+    t_2 = omp_get_wtime();
+
+    pi = sum * h * 4.0;
+
+    printf("omp_get_max_threads(): %d\n", omp_get_max_threads());
+    printf("time: %f\n", t_2 - t_1);
+    printf("pi ~ %f\n", pi);
+
+    return 0;
+}

o/openmp/slurm.out

+Requested cpu_bind option outside of job step allocation for task[0]
+omp_get_max_threads(): 16
+time: 60.947409
+pi ~ 3.141593

o/openmp/testOpenMP.sh

+#!/bin/sh
+#SBATCH --job-name=test
+#SBATCH --partition=shared
+#SBATCH --output=slurm-%J.out
+#SBATCH --nodes 1
+#SBATCH --exclusive
+
+# facultatif
+#export OMP_NUM_THREADS=$SLURM_JOB_CPUS_PER_NODE
+
+# obligatoire si on ne spécifie pas le nombre de cpu par tâche.
+srun --cpu_bind=mask_cpu:0xffffffff hello
+
+# on peut également faire ainsi (max 1 noeud, pas top pour de l'hybride)
+#srun -c $SLURM_JOB_CPUS_PER_NODE hello