Until now, we have only considered scheduling policies that do not have any knowledge of the job sizes. In this chapter and the next two chapters, we will look at size-based scheduling policies, starting with non-preemptive size-based policies (this chapter) and followed by preemptive size-based policies (next two chapters). The size-based policies that we will be studying include the following:

SJF – (non-preemptive) Shortest-Job-First (Chapter 31)

PSJF – Preemptive-Shortest-Job-First (Chapter 32)

SRPT – (preemptive) Shortest-Remaining-Processing-Time (Chapter 33)

It will be convenient to evaluate these size-based policies as special cases of priority queueing, so we start by analyzing priority queues, which are important in their own right.

Size-based scheduling is a very important topic, which is why we devote three chapters to it. The proper size-based scheduling policy can greatly improve the performance of a system. It costs nothing to alter your scheduling policy (no money, no new hardware), so the performance gain comes for free. The above size-based policies are implemented in real systems. For web servers serving static content, SRPT scheduling has been implemented in the Linux kernel to schedule HTTP requests [92]. It has also been used to combat transient overload in web servers [162]. Priority queues are likewise prevalent in computer systems. Prioritization of jobs is used in databases to provide differentiated levels of service, whereby high-priority transactions (those that bring in lots of money) are given priority over low-priority transactions (those that are less lucrative).