Operating System 9 UNIPROCESSOR SCHEDULING

TYPES OF PROCESSOR SCHEDULING

• The aim of processor scheduling is to assign processes to be executed by the processor or processors over time, in a way that meets system objectives, such as response time, throughput, and processor efficiency. In many systems, this scheduling activity is broken down into three separate functions: long-, medium-, and shortterm scheduling.The names suggest the relative time scales with which these functions are performed.

• Long-term scheduling is performed when a new process is created.

• Medium-term scheduling is a part of the swapping function.

• Short-term scheduling is the actual decision of which ready process to execute next.

Long-Term Scheduling

The long-term scheduler determines which programs are admitted to the system for processing. Thus, it controls the degree of multiprogramming.

The decision as to when to create a new process is generally driven by the desired degree of multiprogramming.The more processes that are created, the smaller is the percentage of time that each process can be executed (i.e., more processes are competing for the same amount of processor time).

Medium-Term Scheduling

Medium-term scheduling is part of the swapping function. The issues involved are discussed in Chapters 3, 7, and 8.Typically, the swapping-in decision is based on the need to manage the degree of multiprogramming.

Short-Term Scheduling

The short-term scheduler, also known as the dispatcher, executes most frequently and makes the fine-grained decision of which process to execute next.

The short-term scheduler is invoked whenever an event occurs that may lead to the blocking of the current process

SCHEDULING ALGORITHMS

Short-Term Scheduling Criteria

The main objective of short-term scheduling is to allocate processor time in such a way as to optimize one or more aspects of system behavior.

The commonly used criteria can be categorized along two dimensions. First, we can make a distinction between user-oriented and system-oriented criteria. User oriented criteria relate to the behavior of the system as perceived by the individual user or process. An example is response time in an interactive system.

Other criteria are system oriented. That is, the focus is on effective and efficient utilization of the processor. An example is throughput.

Another dimension along which criteria can be classified is those that are performance related and those that are not directly performance related.

•> Table 9.2 summarizes key scheduling criteria. These are interdependent, and it is impossible to optimize all of them simultaneously.

The Use of Priorities

In many systems, each process is assigned a priority and the scheduler will always choose a process of higher priority over one of lower priority.

Alternative Scheduling Policies

Table 9.3 presents some summary information about the various scheduling policies that are examined in this subsection. The selection function determines which process, among ready processes, is selected next for execution.

•> Preemptive policies incur greater overhead than nonpreemptive ones but may provide better service to the total population of processes,

First-Come-First-Served

The simplest scheduling policy is first-come-firstserved (FCFS), also known as first-in-first-out (FIFO)

•> Another difficulty with FCFS is that it tends to favor processor-bound processes over I/O-bound processes.

Round Robin

A straightforward way to reduce the penalty that short jobs suffer with FCFS is to use preemption based on a clock.The simplest such policy is round robin. A clock interrupt is generated at periodic intervals.

Round robin is particularly effective in a general-purpose time-sharing system or transaction processing system.

Generally, an I/O-bound process has a shorter processor burst (amount of time spent executing between I/O operations) than a processor-bound process.

[HALD91] suggests a refinement to round robin that he refers to as a virtual round robin (VRR) and that avoids this unfairness. Figure 9.7 illustrates the scheme. New processes arrive and join the ready queue, which is managed on an FCFS basis. When a running process times out, it is returned to the ready queue.

When a process is dispatched from the auxiliary queue, it runs no longer than a time equal to the basic time quantum minus the total time spent running since it was last selected from the main ready queue.

Shortest Process Next•> Another approach to reducing the bias in favor of long processes inherent in FCFS is the Shortest Process Next (SPN) policy. This is a non preemptive policy in which the process with the shortest expected processing time is selected next. Thus a short process will jump to the head of the queue past longer jobs.

•> One difficulty with the SPN policy is the need to know or at least estimate the required processing time of each process.

A risk with SPN is the possibility of starvation for longer processes, as long as there is a steady supply of shorter processes.

Shortest Remaining Time The shortest remaining time (SRT) policy is a preemptive version of SPN. In

this case, the scheduler always chooses the process that has the shortest expected remaining processing time.

In Table 9.5, we have used the normalized turnaround time, which is the ratio of turnaround time to actual service time, as a figure of merit. For each individual process, we would like to minimize this ratio, and we would like to minimize the average value over all processes. In general, we cannot know ahead of time what the service time is going to be, but we can approximate it, either based on past history or some input from the user or a configuration manager.Consider the following ratio:

Thus, our scheduling rule becomes the following:When the current process completes or is blocked, choose the ready process with the greatest value of R.

Highest Response Ratio Next

Feedback If we have no indication of the relative length of various processes, then none

of SPN, SRT, and HRRN can be used.Another way of establishing a preference for shorter jobs is to penalize jobs that have been running longer. In other words, if we cannot focus on the time remaining to execute, let us focus on the time spent in execution so far.

•> Figure 9.10 illustrates the feedback scheduling mechanism by showing thepath that a process will follow through the various queues.5 This approach is known as multilevel feedback, meaning that the operating system allocates the processor to a process and, when the process blocks or is preempted, feeds it back into one of several priority queues.

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