Lecture 8:
Dispatch Rules
© J. Christopher Beck 2008
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Outline
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A Scheduling Problem
What are Heuristic Methods?
What is a Dispatch Rule?
Trying them out
© J. Christopher Beck 2008
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Readings
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P Ch 5.2, C.2, C.3
© J. Christopher Beck 2008
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A Scheduling Problem
Jobs
Processing times
0
J0R0[15]  J0R1[50]  J0R2[60]
1
J1R1[50]  J1R0[50]  J1R2[15]
2
J2R0[30]  J2R1[15]  J2R2[20]
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Find a schedule that minimizes the
makespan (saw this in last lecture, too)
© J. Christopher Beck 2008
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Heuristic Methods
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Fast algorithms that result in a schedule
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usually polynomial run-time
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Why would
you use a
heuristic
method?
What does this mean?
Usually no quality guarantee
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except in very restricted cases
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e.g., 1-machine
sometimes good quality, but can be
arbitrarily bad depending on the data for
the specific problem
© J. Christopher Beck 2008
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Dispatch Rules
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Create a queue of operations for each
resource
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include all operations that can execute at
the current time
Whenever a machine becomes free:
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update queues
rank activities (based on a [simple] rule)
schedule highest priority activity
© J. Christopher Beck 2008
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Shortest Processing Time
(SPT)
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Always choose the to execute the
activity in the queue with the shortest
processing time
There are many other rules
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we will look at some of these later in the
course
© J. Christopher Beck 2008
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Weighted Shortest
Processing Time (WSPT)
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Always choose the to execute the
activity in the queue with the greatest
ratio of weight to processing time
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Only makes sense if activities have weight
© J. Christopher Beck 2008
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Dispatch Rules
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Except in special cases, dispatch rules
are not guaranteed to find the optimal
solution
Often used for very hard (or large)
problems were a good solution is “good
enough”
© J. Christopher Beck 2008
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Another Scheduling
Problem
Jobs
0
Processing times
J0R0[15]  J0R1[50]  J0R2[60]
w0 = 2
1
J1R1[50]  J1R0[50]  J1R2[15]
w1 = 3
2
J2R0[30]  J2R1[15]  J2R2[20]
w2 = 5
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Find a schedule that minimizes the total
weighted completion time
C is the end
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Weights
w0C0 + w1C1 + w2C2
Use WSPT
© J. Christopher Beck 2008
i
time of job i
There is a slight complication here …
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