A manufacturer has the following jobs waiting on a single work center. The firm has not decided which dispatching rule to apply in order to prioritize the jobs and fix them into the schedule
Processing time in 15-minute time blocks and due date for each job are in the table below.
Time Due Date
Job 1 5 17
Job 2 4 20
Job 3 3 16
Job 4 7 9
Job 5 3 6
Job 6 5 7
a) Complete the following table. Show your supporting calculations below.
b) Which dispatching rule has the best score for flow time? for work in process (jobs in the system)? for lateness?
c) Is there ANY sequence that can avoid all lateness? Explain or provide an example.
Dispatching Rule
Job Sequence Average
Flow Time Average Number of Jobs
Average Lateness
EDD
SPT
LPT
(a) The results appear in the table below. A supporting table for each dispatch rule also follows.
Dispatching Rule
Job Sequence Average
Flow Time Average Number of Jobs Average Lateness
EDD 5-6-4-3-1-2 15.67 3.48 3.67
SPT 3-5-2-1-6-4 13.50 3.00 5.17
LPT 4-6-1-2-5-3 18.00 4.00 5.83
(b, c, d) SPT is best for flow time, and for average jobs in system; EDD is best for lateness.
(e) Lateness is unavoidable with any dispatch rule; the sum of processing times is 27 but the longest due date is only 20 days away.
EDD Time Due Date Slack Flow time Late days
Job 5 3 6 3 3 0
Job 6 5 7 2 8 1
Job 4 7 9 2 15 6
Job 3 3 16 13 18 2
Job 1 5 17 12 23 6
Job 2 4 20 16 27 7
Total 94 22
Average 15.67 3.667
Average number of jobs in system: 3.481
SPT Time Due Date Slack Flow time Late days
Job 3 3 16 13 3 0
Job 5 3 6 3 6 0
Job 2 4 20 16 10 0
Job 1 5 17 12 15 0
Job 6 5 7 2 20 13
Job 4 7 9 2 27 18
Total 81 31
Average 13.5 5.167
Average number of jobs in system: 3
LPT Time Due Date Slack Flow time Late days
Job 4 7 9 2 7 0
Job 6 5 7 2 12 5
Job 1 5 17 12 17 0
Job 2 4 20 16 21 1
Job 5 3 6 3 24 18
Job 3 3 16 13 27 11
Total 108 35
Average 18 5.83
Average number of jobs in system: 4
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