Lesson 12

Date: 4/16/2014
High Performance Computing (part I)
Linux for Engineering and IT applications


Synchronization Constructs

Exercise
  • The CRITICAL directive specifies a region of code that must be executed by only one thread at a time. All threads in the team will attempt to execute in parallel, however, because of the CRITICAL construct surrounding the increment of sum, only one thread will be able to read/increment/write sum at any time.
    C / C++ - critical Directive Example 
    #include <omp.h>
#include <stdio.h>

int main() {
    double a[1000000];
    int i;
    #pragma omp parallel for 
    for (i=0; i<1000000; i++) a[i]=i;
    double sum = 0;
    #pragma omp parallel for shared (sum) private (i) 
    for ( i=0; i < 1000000; i++) {
       #pragma omp critical   
        sum = sum + a[i];
    }
    printf("sum=%lf\n",sum);
}
    Copy the content of the code above into file sum.c, then compile and run it as follows: 
    gcc -fopenmp -o sum.x sum.c
export OMP_NUM_THREADS=4
./sum.x



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