OpenMP with 1 thread slower than sequential version

Question

I have implemented knapsack using OpenMP (gcc version 4.6.3)

#define MAX(x,y)   ((x)>(y) ? (x) : (y))
#define table(i,j)    table[(i)*(C+1)+(j)]

   for(i=1;          


        

Answer 1

Enabling OpenMP inhibits certain compiler optimisations, e.g. it could prevent loops from being vectorised or shared variables from being kept in registers. Therefore OpenMP-enabled code is usually slower than the serial and one has to utilise the available parallelism to offset this.

That being said, your code contains a parallel region nested inside the outer loop. This means that the overhead of entering and exiting the parallel region is multiplied N times. This only makes sense if N is relatively small and C is significantly larger (like orders of magnitude larger) than N, therefore the work being done inside the region greatly outweighs the OpenMP overhead.