FIVE PRIORITY SEQUENCING RULES ARE:
First come, first served (FCFS); or First in, first out (FIFO): Jobs are sequenced in the order in which they arrive at the workstation.
Earliest due date (EDD): Jobs are sequenced in the order in which they are due for delivery to the customer.
Shortest processing time (SPT): Jobs are sequenced in order of the processing time required at the workstation, with the job requiring the least processing time at the workstation scheduled first.
Longest processing time (LPT): Jobs are sequenced in order of the processing time required at the workstation, with the job requiring the longest processing time at the workstation scheduled first.
Critical ratio (CR): Jobs are sequenced in order of increasing critical ratio (the ratio of time required by work left to be done to time left to do the work)
Johnson’s rule is used to sequence several jobs through two or three work centers.
The are a number of criteria for evaluating job sequencing rules. Criteria discussed in the text include:
Average job completion time
Average number of jobs in the system
Average job lateness.
Additional criteria include: average wait time, average total completion time, and variance in completion times.
|15.10 |JOB |DUE DATE |DURATION (DAYS) | | |A |313 | 8 | | |B |312 |16 | | |C |325 |40 | | |D |314 | 5 | | |E |314 | 3 |
FCFS (first come, first served):
Because all jobs arrived on day 275, and presumably in the order given, the FCFS sequence is:
A, B, C, D, E
EDD (earliest due date):
| | |Job Sequence |Due Date |
| | |B |312 |
| | |A |313 |
| | |D |314 |
| | |E |314 |
| | |C |325 |
SPT (shortest processing time)
| | |Job Sequence |Processing Time | | | |E | 3 |
| | |D | 5 |
| | |A | 8 |
| | |B |16 |
| | |C |40 |
LPT (longest processing time)
| | |Job Sequence |Processing Time | | | |C |40 | | | |B |16 | | | |A | 8 | | | |D | 5 | | | |E | 3 |
| | |Scheduling |Average |Average |Average Number | | | |Rule |Tardiness |Flow Time |of Jobs in System | | | |FCFS |14.8 |47.4 |3.3 | | | |EDD | 4.2 |34.6 |2.4 | | | |SPT | 4.2 |26.2 |1.8 | | | |LPT |20.8 |60.2 |4.2 |
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