Class Scheduler

java.lang.Object
  |
  +--Scheduler

Direct Known Subclasses:

DiskScheduler, RRScheduler

public abstract class Scheduler

extends java.lang.Object

A Scheduler schedules jobs seeking service from a Device. The class Scheduler itself is "abstract", meaning that there are no instance of Scheduler, only of its subclasses. This simulation has two Schedulers: DiskScheduler for scheduling the disk and RRScheduler for scheduling the CPU. As part of the assignment, students will write other schedulers for the CPU. The base class takes care of maintaining statistics on queue lengths. The subclasses do all the actual work.

See Also:

DiskScheduler, RRScheduler

Constructor Summary

Scheduler()

Method Summary

abstract boolean	add(Job newJob, Job currentJob) A new job has just requested service for the device.
void	endBurst(Job job) This method is called when a job finishes a burst.
abstract void	printQueue() For debugging: print the queue of waiting jobs
void	printStats() Print statistics about the history of this queue.
protected void	queueChanged(int amount) This method should be called by a subclass whenever the queue length changes.
abstract Job	remove() Retrieve (and remove) the next job to be served.
abstract boolean	reschedule(Job j) This method is called when there is a clock interrupt.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail

Scheduler

public Scheduler()

Method Detail

add

public abstract boolean add(Job newJob,
                            Job currentJob)

A new job has just requested service for the device. The scheduler should add it to the queue. This method should return true if this arrival warrents preempting the current job using the device. Otherwise, it should return false. Note that the sheduler doesn't actually preempt the current job, it just indicates its opinion whether the job should be preempted. The main simulator loop will stop the job and call this method again to add that job to the queue. It will then call the remove method to select the next job to use the device.

Parameters:

newJob - the job newly awaiting service for the device.

currentJob - the job currently using the device, if any (may be null).

Returns:

true if this scheduler would like to preempt the current job.

remove

public abstract Job remove()

Retrieve (and remove) the next job to be served.

Returns:

the next job (null if there is no such job).

reschedule

public abstract boolean reschedule(Job j)

This method is called when there is a clock interrupt. It should decide whether to preempt the current job. If it returns true, the job will be stopped and added to the queue (by a call to the add method), and then another job will be selected to use the device (by a call to the remove method).

Parameters:

j - the job currently being serviced by the corresponding device.

Returns:

true if the scheduler wants to preempt the current job.

endBurst

public void endBurst(Job job)

This method is called when a job finishes a burst. At this point the job's burstUsed() method returns the total length (in ms) of the burst just completed. The default implementation of this method does nothing. A subclass of Scheduler can override it to perform any bookkeeping necessary for a specific scheduling algorithm.

Parameters:

job - the job that just finished its burst.

printQueue

public abstract void printQueue()

For debugging: print the queue of waiting jobs

queueChanged

protected void queueChanged(int amount)

This method should be called by a subclass whenever the queue length changes.

printStats

public void printStats()

Print statistics about the history of this queue.