2.2. PRELIMINARIES

2.2. PRELIMINARIES

In dealing with job attributes for the single-machine model, it is useful to distinguish between information that is known in advance and information that is generated as the result of scheduling decisions. Information that is known in advance serves as input to the scheduling process, and we usually use lowercase letters to denote this type of data. Three basic pieces of information that help to describe jobs in the single-machine case are :

Processing time ( pj ) The amount of processing required by job j
Release date (rj) The time at which job j is available for processing
Due date (dj) The time at which the processing of job j is due to be completed

Under condition C3 the processing time pj generally includes both direct processing time and facility setup time. The release date can be thought of as an arrival time the time when job j appears at the processing facility—and in the basic model, the assumption in condition C1 is that rj = 0 for all jobs. Due dates may not be pertinent in certain problems, but meeting them is a common scheduling concern, and the basic model can shed some light on objectives oriented to due dates.
Information that is generated as a result of scheduling decisions represents output from the scheduling function, and we usually use capital letters to denote this type of data. Scheduling decisions determine the most fundamental piece of data to be used in evaluating schedules:
Completion time (Cj) The time at which the processing of job j is finished
Quantitative measures for evaluating schedules are usually functions of job completion times. Two important quantities are: