4.2.3 The Calendar Queue Scheduler

The calendar queue scheduler (Scheduler/Calendar../ns-2/scheduler.cc) uses a data structure analogous to a one-year desk calendar, in which events on the same month/day of multiple years can be recorded in one day. It is formally described in [6], and informally described in Jain (p. 410) [15]. The implementation of Calendar queues in ns v2 was contributed by David Wetherall (presently at MIT/LCS).

The calendar queue scheduler since ns v2.33 is improved by the following three algorithms:

• A heuristic improvement that changes the linear search direction in enqueue operations. The original implementation searches the events in a bucket in chronological order to find the in-order spot for the event that is being inserted. The new implementation searches the bucket in reverse chronological order because the event being inserted is usually later than most of the events that are already in the bucket.
• A new bucket width estimation that uses the average interval of dequeued events as the estimation of bucket width. It is stated in [6] that the optimal bucket width should be the average inverval of all events in the future. The original implementation uses the average interval of future events currently in the most crowded bucket as the estimation. This estimation is unstable because it is very likely that many future events will be inserted into the bucket after this estimation, significantly changing the averaged event interval in the bucket. The new implementation uses the observed event interval in the past, which will not change, to estimate the event interval in future.
• SNOOPy Calendar Queue: a Calendar queue variant that dynamically tunes the bucket width according to the cost trade-off between enqueue operation and dequeue operation. The SNOOPy queue improvement is described in [30]. In this implementation, there is one tcl parameter adjust_new_width_interval_ specifying the interval with which the SNOOPy queue should re-calculate the bucket width. Setting this parameter to 0 turns off the SNOOPy queue algorithm and degrades the scheduler back to the original Calendar Queue. In general, normal simulation users are not expected to change this parameter.

The details of these improvements are described in [33].

The implementation of these three improvements was contributed by Xiaoliang (David) Wei at Caltech/NetLab.