Job Dispatching Simulator

What are dispatching rules and how do they affect scheduling performance?

Dispatching rules are priority-based heuristics for sequencing jobs on machines in real time. Unlike optimal scheduling algorithms that require complete knowledge of all jobs upfront, dispatching rules make local decisions as each machine becomes available, making them practical for dynamic shop floor environments.

Common rules include SPT (Shortest Processing Time) which minimizes average flow time, EDD (Earliest Due Date) which minimizes maximum tardiness, and WSPT (Weighted Shortest Processing Time) which minimizes weighted flow time. No single rule dominates across all performance criteria — the best rule depends on the scheduling objective.

The Gantt chart visualization shows the resulting schedule timeline, while performance metrics like makespan (total completion time), total tardiness (sum of late deliveries), and average flow time quantify schedule quality. Comparing multiple rules on the same job set reveals which rule best fits the current objective.

Formula: SPT: Sort by ascending processing time (pⱼ) EDD: Sort by ascending due date (dⱼ) WSPT: Sort by descending wⱼ/pⱼ ratio CR: Sort by ascending (dⱼ - t) / pⱼ Tardiness: Tⱼ = max(0, Cⱼ - dⱼ)

Example Calculation

3 jobs: A(p=4, d=10), B(p=2, d=8), C(p=6, d=12). SPT order: B→A→C, completions: 2,6,12, tardiness: 0+0+0=0. EDD order: B→A→C (same), tardiness=0. LPT order: C→A→B, completions: 6,10,12, tardiness: 0+0+4=4. SPT is optimal here.

When to Use This Calculator

• When evaluating which dispatching rule minimizes total tardiness for your current job mix before implementing it on the shop floor
• During production planning meetings to simulate how different priority rules would sequence the current job queue
• When training shop floor supervisors on the trade-offs between different dispatching strategies and their impact on delivery performance
• After a schedule disruption (machine breakdown, rush order) to re-sequence remaining jobs and minimize delivery impact
• When comparing heuristic scheduling rules against each other to select the best rule for your recurring job shop characteristics

Common Mistakes to Avoid

• Using SPT to minimize tardiness — SPT minimizes average flow time, not tardiness; use EDD for minimizing maximum tardiness or weighted tardiness rules for due-date-focused objectives
• Applying FCFS by default because it seems 'fair' — FCFS often produces the worst performance on almost every metric; it should only be used when job priorities are truly equal and order sequence matters
• Not updating dispatching decisions when conditions change — dispatching rules should be re-evaluated when new jobs arrive, jobs are cancelled, or machine breakdowns occur
• Ignoring job weights when priorities differ — if some jobs are more important (higher-value customer, penalty clause), use WSPT or CR instead of simple SPT or EDD

How to Interpret Results

• On a single machine with all jobs released at time 0, makespan is the same for every rule (the machine is never idle) — judge rules by Total Tardiness, not makespan; makespan only differs when release times force idle gaps
• Total Tardiness of 0 means every job meets its due date under that rule — check the Gantt chart for how much slack remains before due dates to gauge robustness against disruptions
• In the Compare All Rules view, pick the rule with the lowest total tardiness for your job mix — when several rules tie, prefer the simpler one (e.g., EDD over CR) for shop-floor execution
• Use the per-job tardiness column to find which specific orders are late — if one dominant late job drives the total, renegotiating a single due date may beat changing the dispatching rule
• WSPT only differs from SPT when job weights vary, and CR only differs from EDD when processing times vary relative to due-date slack — if results look identical across rules, your job data may not exercise those inputs

Related Standards & References

• APICS CPIM Body of Knowledge — Production activity control module covering dispatching rules and shop floor scheduling
• ISA-95 (IEC 62264) — Enterprise-control system integration standard defining the scheduling and dispatching functional hierarchy

Frequently Asked Questions

Which dispatching rule should I use?

It depends on your objective. SPT minimizes average flow time and work-in-process inventory. EDD minimizes maximum lateness (best for meeting the tightest deadline). WSPT is optimal for minimizing weighted completion time when jobs have different priorities. CR (Critical Ratio) adapts dynamically to the current time. For most shops, try multiple rules and compare metrics.

Why not use optimal scheduling algorithms instead of dispatching rules?

Optimal scheduling (e.g., branch-and-bound, MIP) is NP-hard for most real problems — computation time grows exponentially with the number of jobs. Dispatching rules run in O(n log n) time and can handle dynamic arrivals, machine breakdowns, and priority changes in real time. In practice, sophisticated dispatching rules achieve within 5-15% of optimal for most problems.