15.5 Service Shop Scheduling
Unlike manufacturing environments that often rely on special-purpose machines, service shops typically use multi-purpose equipment and face more dynamic scheduling challenges. Jobs in service settings vary in nature, arrive at different times, require different processing durations, and have distinct due dates. These complexities make scheduling in service shops both critical and nuanced.
Key performance considerations in service scheduling include:
- Flow Time: Total time required to complete a job from start to finish.
- Work in Progress (WIP): Number of jobs currently being processed.
- Lateness: The extent to which a job is completed after its due date.
To manage these challenges, service providers must choose appropriate job sequencing strategies. Common strategies include:
First Come, First Served (FCFS)
- Jobs are processed in the order they arrive.
- Widely used in public service settings (e.g., medical clinics, government offices).
- Simple and fair, but may not optimize flow time or minimize lateness.
Shortest Processing Time (SPT)
- Jobs with the shortest processing time are completed first.
- Helps reduce average flow time and WIP.
- May delay longer jobs, increasing their lateness.
Earliest Due Date (EDD)
- Jobs are scheduled based on their due dates, with the earliest due date processed first.
- Minimizes maximum lateness.
- Useful when meeting delivery deadlines is a priority.
Critical Ratio (CR)
- A dynamic priority rule that considers both due date and remaining work.
- Formula:
[latex]\text{CR}= \frac{\text{Remaining Time to Delivery}}{\text{Work Remaining}}= \frac{\text{Due Date-Today's Date}}{\text{Work Time Remaining}}[/latex]
- Jobs with CR 1 are at risk of being late and should be prioritized.
- Balances urgency and workload.
Longest Processing Time (LPT)
- Opposite of SPT: jobs with the longest processing time are completed first.
- Useful when longer jobs risk being delayed due to time constraints later in the schedule.
Example: Jimmy Auto Repair
Jimmy Auto Repair receives five service jobs, each with different processing times and due dates. The processing time and delivery time of these jobs are summarized in the following table.
| Jobs | Process time (hours) | Due time (hours) |
|---|---|---|
| A | 16 | 36 |
| B | 8 | 30 |
| C | 40 | 100 |
| D | 10 | 39 |
| E | 35 | 90 |
The impact of different sequencing strategies is as follows:
FCFS Strategy
- Jobs are processed in arrival order.
- Results in delays for Jobs D and E.
| Jobs | Process time (hours) | Due time (hours) | Flow time | Lateness |
|---|---|---|---|---|
| A | 16 | 36 | 16 | 0 |
| B | 8 | 30 | 24 | 0 |
| C | 40 | 100 | 64 | 0 |
| D | 10 | 39 | 74 | 35 |
| E | 35 | 90 | 109 | 19 |
SPT Strategy
- Jobs with the shortest processing times are completed first.
- Only Job C is delayed (by 9 hours).
| Jobs | Process time (hours) | Due time (hours) | Flow time | Lateness |
|---|---|---|---|---|
| B | 8 | 30 | 8 | 0 |
| D | 10 | 39 | 18 | 0 |
| A | 16 | 36 | 34 | 0 |
| E | 35 | 90 | 69 | 0 |
| C | 40 | 100 | 109 | 9 |
EDD Strategy
- Jobs are scheduled by due date.
- Only Job C is delayed (by 19 hours).
| Jobs | Process time (hours) | Due time (hours) | Flow time | Lateness |
|---|---|---|---|---|
| B | 8 | 30 | 8 | 0 |
| A | 16 | 36 | 34 | 0 |
| D | 10 | 39 | 44 | 0 |
| E | 35 | 90 | 79 | 0 |
| C | 40 | 100 | 119 | 19 |
CR Strategy
- Jobs are prioritized based on urgency and workload.
- Jobs E, B, and D are delayed by 1, 68, and 69 hours, respectively.
| Jobs | Process time (hours) | Due time (hours) | Flow time | Lateness | CR |
|---|---|---|---|---|---|
| A | 16 | 36 | 16 | 0 | 2.25 |
| C | 40 | 100 | 56 | 0 | 2.5 |
| E | 35 | 90 | 91 | 1 | 2.57 |
| B | 8 | 30 | 98 | 68 | 3.75 |
| D | 10 | 39 | 108 | 69 | 3.9 |
LPT Strategy
- Longer jobs are processed first.
- Results in delays for Jobs A, D, and B (55, 62, and 79 hours, respectively).
| Jobs | Process time (hours) | Due time (hours) | Flow time | Lateness |
|---|---|---|---|---|
| C | 40 | 100 | 40 | 0 |
| E | 35 | 90 | 75 | 0 |
| A | 16 | 36 | 91 | 55 |
| D | 10 | 39 | 101 | 62 |
| B | 8 | 30 | 109 | 79 |
The choice of sequencing strategy has a significant impact on lateness, flow time, and workload distribution. While FCFS is simple and fair, strategies like SPT, EDD, and CR offer more control over performance metrics. In service environments where customer satisfaction and timely delivery are critical, selecting the right scheduling approach can enhance both operational efficiency and service quality.
