3.6 Load Balance and Loop Scheduling

Loop scheduling is the process of distributing iterations of a parallel loop to multiple threads. In order to maximize the speedup, it is important that the work be distributed evenly among the threads while not imposing too much overhead. The compiler offers several types of scheduling for different situations.

3.6.1 Static or Chunk Scheduling

It is beneficial to divide the work evenly among the different threads on the system when the work performed by different iterations of a loop is the same. This approach is known as static scheduling.

Example 3-13 A Good Loop for Static Scheduling

for (i=1; i < 1000; i++) {
    sum += a[i]*b[i];       /* S1 */
}

Under static or chunk scheduling, each thread will get the same number of iterations. If there were 4 threads, then in the above example, each thread will get 250 iterations. Provided there are no interruptions and each thread progresses at the same rate, all the threads will complete at the same time.

3.6.2 Self Scheduling

Static scheduling will not achieve good load balance, in general, when the work performed by each iteration varies. In static scheduling, each thread grabs the same chunk of iterations. Each thread, except the master thread, upon completion of its chunk waits to participate in the next parallel loop execution. The master thread continues execution of the program. In self scheduling, each thread grabs a different small chunk of iteration and after completion of its assigned chunk, tries to acquire more chunks from the same loop.

3.6.3 Guided Self Scheduling

In guided self scheduling (GSS), each thread gets successively smaller number of chunks. In cases where the size of each iteration varies, GSS can help balance the load.