Operating System 9 UNIPROCESSOR SCHEDULING

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Operating System 9 UNIPROCESSOR SCHEDULING
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TYPES OF PROCESSOR SCHEDULING
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• 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.

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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).

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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.

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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

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SCHEDULING ALGORITHMS
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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.

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

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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.

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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.

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

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First-Come-First-Served

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

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

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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.

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Shortest Process Next

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

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A risk with SPN is the possibility of starvation
for longer processes, as long as there is a
steady supply of shorter processes.
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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
  followingWhen the current process completes or
  is blocked, choose the ready process with the
  greatest value of R.

Highest Response Ratio Next
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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.

• gt Figure 9.10 illustrates the feedback scheduling
  mechanism by showing the
• path 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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