Touch Dreams

The C10K problem refers to the problem of serving ten thousand clients simultaneously on a web server. This article written by Daniel Kegel contains some history and background information (and lots links) about threading on Linux, Solaris, BSD, MAC OS X, etc.

There is no synchronization between the threads in a team when they enter a worksharing construct. Many people assume there is a barrier before the threads enter a worksharing construct, especially when there is a FIRSTPRIVATE used in the worksharing construct. This is a common mistake.

For example, in the following code, assume two threads - thread 1 and thread 2 are in the team, and Read1 is executed by thread 1 and Read2 is executed by thread 2.

  #pragma omp parallel
  {
     if (omp_get_thread_num()==0)
        z = 1;
     else
        z = 2;
     #pragma omp sections firstprivate(z)
     {
       #pragma omp section
       {
          ... = z;      // Read1
       }
       #pragma omp section
       {
          ... = z;      // Read2
       }
     }
  }

What are the values of z at Read1 and Read2? All the following three combinations are possible,

1. Read1:1 Read2:1
2. Read1:1 Read2:2
3. Read1:2 Read2:2

If there were a synchronization before the worksharing construct, then the above (Read1:1, Read2:2) is not possible.

Now, look at the following example which has both FIRSTPRIVATE and LASTPRIVATE,

  #pragma omp parallel
  {
     z = 1;
     #pragma omp for firstprivate(z) lastprivate(z) nowait
     for (i=0; i<n; i++) {
          ... = z;      // Read1
          z = 2;        // Write1
     }
  }

What could be the value of z at Read1? Would it be 2? OpenMP 3.0 Draft has clarified this situation. It says

If a list item appears in both firstprivate and lastprivate clauses, the update
required for lastprivate occurs after all initializations for firstprivate.

So, the value of z at Read1 cannot be 2.