dispatching rules

Rules used to decide the priorities for fulfilling orders. Examples include:

• first-come-first-served (FCFS);

• first-in-first-out (FIFO) - often seen as a fair rule, especially by the customer, but in practice it leads to overall inefficiency;

• earliest-due-date (EDD) first - the job due out first is processed first: this ignores arrival times and processing times;

• shortest processing time (SPT) first - an ideal method for getting small jobs out of the way and minimizing work in progress, but it can make large jobs late and it ignores due dates and arrival times;

• longest processing time (LPT) first - this tends to increase work in progress and to make short jobs late;

• truncated shortest processing time (TSPT) - jobs that have been waiting longer than a predetermined time are given priority (if no work has been waiting long, SPT rules apply);

• critical ratio (CR) - this is the ratio of the time until the due date to the processing time: jobs with the lowest CR are processed first.