Shortest processing time, abbreviated SPT, is a priority rule that runs the job with the least remaining work first and the longest job last. It is the scheduling equivalent of clearing the quick emails before the long ones so the backlog shrinks fast. SPT is famous for minimizing average flow time and work in process, because short jobs no longer wait behind long ones, and it is one of the ordering heuristics EDGEBIC can apply when sequencing a queue.
This entry is part of the EDGEBIC by User Solutions glossary series; for the broader vocabulary of planning, see the manufacturing glossary.
How Shortest Processing Time Works
SPT sorts the waiting jobs by total work content, smallest first, and runs them in that order. The rule cares only about how long each job takes, not when it is due or how important it is. Its headline property, proven on a single machine, is that it produces the smallest possible average flow time, the average of how long each job spends in the system from arrival to completion.
The intuition is simple. Every job behind the one currently running is waiting. If you run a long job first, all of those jobs absorb its full duration before they even start. If you run the short jobs first, each finishes quickly and stops contributing to the total wait, so the sum of everyone's time in the system falls. That is also why SPT tends to lower work in process: fewer jobs are open at any moment.
The cost is fairness. A large job can be pushed back indefinitely as shorter jobs keep arriving, so SPT alone risks leaving a big order badly late. It is also deaf to deadlines. For those reasons SPT is usually one candidate among several rather than a shop's only rule.
A Concrete Example
Five jobs wait for one machine with these work contents:
| Job | Work content (hours) |
|---|---|
| A | 8 |
| B | 2 |
| C | 5 |
| D | 1 |
| E | 4 |
SPT sequences them D, B, E, C, A: one hour, then two, then four, five, and eight. Compare the average time in system against running them in arrival order A, B, C, D, E. Under SPT the small jobs clear almost immediately, so most jobs report a short flow time and only Job A waits through everything. Under the arrival order, jobs B, C, D, and E all wait behind the eight-hour Job A, inflating the average. Same total hours worked, very different average wait, which is the whole point of SPT.
How EDGEBIC Uses It
EDGEBIC exposes shortest processing time as one of the named ordering heuristics inside its multi-run schedule optimizer, described as shortest total work content first. Its mirror image, longest processing time, is also available for cases where you deliberately want the big jobs anchored early.
When the optimizer runs, it feeds each candidate ordering, including SPT, through the unmodified scheduling engine to produce a full finite-capacity schedule, then scores every schedule against your goals and keeps the best. A never-worse clamp guarantees the answer is never inferior to the engine's default greedy plan.
That means SPT is a contender, not a fixed dial. The engine adopts the SPT sequence only when the complete schedule it produces, with real shift hours and setups applied, scores best on the objective you have chosen. If your priority is average flow time or trimming work in process, SPT often wins; if it is on-time delivery, a due-date rule may edge it out, and the comparison decides rather than a rule of thumb. The optimizer goals and presets guide explains how those objectives are set.
Related Reading
SPT has an opposite twin in longest processing time, and it is often weighed against earliest due date when deadlines matter. The generic priority rules glossary covers the full dispatching family, and what production scheduling is sets the wider context.
Expert Q&A: Deep Dive
Q: Will running short jobs first actually get more work out the door?
A: It gets more jobs completed sooner and shrinks the queue faster, which lowers average flow time and work in process, but it does not increase total output for the shift. The same hours are worked either way. What changes is that more jobs are done earlier, so fewer things are waiting at any moment. If your real problem is on-time delivery rather than queue length, a due-date rule may serve you better, which is why EDGEBIC compares both.
Q: How does EDGEBIC decide between SPT and a due-date rule?
A: It does not pick in advance. The multi-run optimizer builds a complete finite-capacity schedule for each ordering, including shortest processing time, longest processing time, earliest due date, and critical ratio, then scores each against your goals and keeps the best, never worse than the baseline. So SPT is adopted only when the full schedule it produces genuinely wins on the objective you have chosen, such as flow time or on-time performance.
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