Sequencing Rules (Part I)

(I) OVERVIEW

Scheduling is a ‘decision making’ process, which forms the core of any manufacturing and service industry. The objective of scheduling is to manage resources and task in the best possible manner. Any problem comprising of following four parameters can be identified as a valid problem for scheduling:

• Task
• Time
• Resource
• Constraint

Better the usage and utilization of resources is, better would be the end result w. r. t. standard manufacturing norms. Scheduling not only gives a good way of task execution but also determines practical delivery dates.

Sequencing rules are one of the key measures to ensure that resource usage is in line with the objectives to be achieved. Sequencing rules sometimes also referred as dispatching rules as well.

Sequencing rule is a rule that prioritizes the jobs waiting for processing on a particular machine. The prioritization scheme normally takes into account the attributes of the job , machine and current time as well.

We can classify the sequencing rule as Static rule and Dynamic rule.

Static Rules are those, which does not change with time and determined by job attributes and / or machine attributes. Dynamic rules are those, which is time dependent.

Another way of defining the classification is Local sequencing rules and Global sequencing rules.

Local rules concern information that is related to the queue where the jobs are waiting or the machine where the job is queued.

Global rules use the information pertaining to other machine also. For example, the processing time of current job on next machine on its route or current queue length at the next machine on its route may be considered.

STANDARD SEQUENCING RULES

There are basic sequencing rules, which are well known and considered to be standard ones. These are namely:

• First In First Out [FIFO] or First Come First Serve [FCFS]
• Shortest Processing Time [SPT]
• Longest Processing Time [LPT]
• Earliest Due Date [EDD]
• Operation Slack [OPS]
• Critical Ratio [CRR]

(I) FIRST IN FIRST OUT [FIFO] OR FCFS
This sequencing rule gives priority to the operation, which comes first to that particular resource. This means, the operation which arrives first in the queue is loaded / dispatched first. The functionality of this rule is to try to minimize waiting time as it tries to move the tasks as soon as it arrives.


(II) SHORTEST PROCESSING TIME [SPT]

This sequencing rule orders the tasks in the increasing value of processing time. This means, the task with the smallest processing time is scheduled first and subsequent tasks in ascending order of processing time. This rule maximizes throughput by minimizing the total completion times.


(III) LONGEST PROCESSING TIME [LPT]
This sequencing rule is opposite to the SPT rule. LPT creates a sequence based on descending order of processing time. This means, the task with the longest processing time is dispatched first and later other tasks are scheduled sin the decreasing order of processing time values. LPT rule creates an environment for work balancing, especially for parallel machine case.


(IV) EARLIEST DUE DATE [EDD]
As name suggests, this rule works on the due date of the tasks. The task with the earliest due date is scheduled first and other task in the ascending order of the due dates. The core objective of applying this rule is to minimize the delay / lateness.

(V) OPERATION SLACK [OPS]

This rule gives priority to the operation with minimum slack. The operation slack is computed as the difference between the total remaining time and the remaining processing time (i.e. process time plus setup time).

OPS = (Total remaining time) – (scheduled setup and running time remaining)
OPS = (Due date) – (scheduled setup and running time remaining) – (Today’s date)
OPS = (Time remaining) – (work remaining)

This rule could be applied at the end of production, where the client due date becomes relatively important. Jobs are sequenced in ascending order of CR (i.e. Critical Ratio).



Jobs with CR of less than one, exactly one and more than one are respectively behind schedule, on schedule and ahead of schedule.

The following table illustrates the comparison between these sequencing rule, which gives us a good overview data.