Xeon Phi Performance / Energy Tradeoff Issues

Hi

I have been doing experiments on Xeon Phi and ran an fibonacci(40) application with varied number of core allocations (i.e. number of cores). The energy consumption was measured through /sys/class/micras/power and performance (i.e. execution time) was measured using elapsed time.

I get the following trade-off:

Cores Execution_time(ms) Energy(J)
1    142450.897    11123.35
2    312780.938    24676.25
4    172721.295    13849
8    104500.9    8527.75
16    60941.152    5176.8
24    43881.031    3854.75
32    31231.054    2828.1
40    25801.364    2450.9
48    23231.324    2267.2
56    19880.978    1997.7
61    17380.948    1784.5

As you can see there is a sudden jump in the execution time from core allocation of 1 to 2, which is also causing the energy to be much higher. Other number of core allocations are I suppose normal. But I cannot explain this trade-off when the core allocation increases from 1 to 2.

Here is the application if you'd like to try it out:

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int fib(int n)

{
  int i, j;
  if (n<2)
    return n;
  else
    {
      //omp_set_num_threads(NUM_CPUS);

       #pragma omp task shared(i) firstprivate(n)
       i=fib(n-1);

       #pragma omp task shared(j) firstprivate(n)
       j=fib(n-2);

       #pragma omp taskwait
       return i+j;
    }
}

int main()
{
  int n = 40;
  omp_set_dynamic(1);
  #pragma omp parallel shared(n)
  {
    #pragma omp single
    printf ("\033[37;1mfib(%d) = %d\033[0m   \n", n, fib(n));
    #pragma omp single
    printf ("CPUs in Parallel = %d \033\n", omp_get_num_threads());
  }
  return 0;
}

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