What is online scheduling and offline scheduling?

Answer 1

Online scheduling is the useage of competitive analysis (or online algorithms) on scheduling problems. Online algorithms is characterized by making decision "online", which means a point in the time axe. In this point of time, we can not see the future jobs or tasks, whereas we only know the jobs or tasks before or at this point of time. In contrast, in offline scheduling problems, there is no the conception of "point of time". We are lords of the world. We stay outside the real world and can see the past and future (jobs or tasks). TThis aspect is called "offline". Since we can see the past and the future, we know the total knowledge of the problem before we make decision (not depending the time). Even use the simpliest method, such as enumeration, then we can obtain the optimal solution. Nontheless, by not knowing the future knowledge of problem, we must make decision. Then we use a critierion to meaure the performance of online algorithm, called competitive ratio. This is a conception like approximation ratio, compering the objective value obtained by online algorithm and that of offline (optimal) algorithm. Offline scheduling is concerned of the classical scheduling problems. Not introducing the conception of "online". Offline scheduling problem is scheduling problem.