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Scheduler: Program Schedulers

abstract class Scheduler
{
  int getNumber()
  boolean isReady(int i)
  int getTime(int i)
  abstract int nextProgram()
}

A scheduler selects the next program for execution among those node programs that are in the ready state. A scheduler must redefine the abstract function nextProgram() such that this function returns some i with 0 <= i < getNumber() and isReady(i) (i.e. it must select one ready node program for execution). If this condition cannot be fulfilled, the scheduler must return -1 (denoting deadlock). The function getTime(i) returns the (simulation) time elapsed for program i which may be used for the scheduling decision.

The default scheduler selects in a round robin mechanism the next program ready for execution; it is an object of type

class SchedulerDefault extends Scheduler
{
  int last = -1;
  int nextProgram()
  {
    int n = getNumber();
    boolean reset = false;
    do
      {
        last++;
        if (last == n)
          {
            last = 0;
            if (reset) return -1;
            reset = true;
          }
      }
    while (!isReady(last));
    return last;
  }
}

The default scheduler may be redefined to implement different kinds of network models (synchronized networks, asynchronous networks, ...).

The following schedulers are currently predefined:

• SchedulerMinTime: Selects the ready program with the minimum elapsed time.
• SchedulerMaxTime: Selects the ready program with the maximum elapsed time.