Bubble Sort slower in Open MP

Question

the bubble sorting program i wrote in openMP is slower than the serial version of this, why is this, i cant understand whats the problem.

On serial version it takes around 0.07 and on parallel it takes around 0.9 for an array of 5000 elements

#include <omp.h>
#include <stdio.h>
#include <stdlib.h>

void swap(int *num1, int *num2)
{
    
    int temp = *num1;
    *num1 =  *num2;
    *num2 = temp;
}

int main (int argc, char *argv[]) {
    int SIZE =5000;
    int A[SIZE];
    for(int i=0;i<SIZE;i++)
    {
        A[i]=rand()%SIZE;
    }
    int N = SIZE;
    int i=0, j=0; 
    int first;
    double start,end;
    start=omp_get_wtime();
        
    #pragma omp parallel reduction( +:first )

    for( i = 0; i < N; i++ )
    {

        first = i % 2; 
        
        #pragma omp parallel for default(none),shared(A,first,N)

        for( j = first; j < N-1; j += 2 )
        {
            
            if( A[ j ] > A[ j+1 ] )
            {
                swap( &A[ j ], &A[ j+1 ] );
            }
            
            
        }
            
    }
    
    
end=omp_get_wtime();
    for(i=0;i<N;i++)
    {
        printf(" %d",A[i]);
    }

printf("\n-------------------------\n Time Parallel= %f",(end-start));
}

Answer 1

You just have to delete the line

 #pragma omp parallel reduction( +:first )

This line is a mistake as it runs the algorithm several times, but if you skip this line the algorithm runs correctly in parallel and will be faster using openMP, please check these slides .