Chapter 9: Processor Scheduling

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Chapter 9 Processor Scheduling

• CS 472 Operating Systems
• Indiana University Purdue University Fort Wayne

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Time frame for scheduling

• Long-term scheduling
• decide to admit a new process
• often a processes is admitted in the suspended
  state
• Medium-term scheduling
• decide to suspend / resume
• involves allocating memory to the admitted
  process
• Short-term scheduling
• choose process to dispatch

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Dispatcher

• This is the short-term scheduler
• Invoked whenever appropriate
• I/O request
• clock interrupt
• system service call
• etc.
• Attempts to meet established criteria

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Use of priorities

• Scheduler will always choose a process of higher
  priority over one of lower priority
• Have multiple ready queues to represent each
  level of priority
• Lower-priority processes may suffer starvation
• To overcome starvation, allow a process to change
  its priority based on its age or execution
  history

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Decision Mode

• Nonpreemptive
• Once a process is in the running state, it will
  continue until it terminates or blocks itself for
  I/O
• Preemptive
• The currently running process may be interrupted
  and moved to the Ready state by the operating
  system
• Allows for better service since no one process
  may monopolize the processor for very long

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Dispatcher policies

• FCFS
• Round-robin
• SPN
• SRT
• HRRN
• Feedback
• Fair-share

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Running example
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FCFS

• First Come First Served or FIFO
• Ready queue ordered by waiting time
• Batch oriented
• Nonpreemptive
• Poor turnaround time
• Normalized turnaround time poor also
• Ratio of turnaround time to service time

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FCFS

• A short process may have to wait a very long time
  before it can execute
• Favors CPU-bound processes
• A process waiting for I/O must wait until
  CPU-bound process completes
• However, very low overhead

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FCFS example
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Round-robin

• Preemptive version of FCFS
• Time slicing is used with high overhead
• A clock interrupt is generated periodically
• Quantum time slice size
• should slightly exceed the length of a typical
  user interaction
• Time sharing / transaction processing oriented

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• Round-robin refinement
• Processes in the auxiliary queue take priority
  over those in the main ready queue
• In this case, the quantum is the basic quantum
  minus time used before I/O block

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SPN

• Shortest Process Next
• Ready queue ordered by estimated service time
• until next block (interactive environment)
• until termination (batch environment)
• Nonpreemptive
• Improves average wait over FCFS
• A long process may starve and short processes may
  have long waits due to no preemption
• Difficult to estimate service time

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SPN estimated service time

• Batch environment service time
• Estimated by programmer
• Available from previous runs in a production
  environment
• Interactive environment
• OS may keep a running average of each burst for
  each process
• Sn1 1/n ? nTi where Ti historical
  time
• i1

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SPN interactive environment

• Exponential moving average estimate is good
• Choose a such that 0 lt a lt 1
• Then define
• S0 0
• Sn1 a Tn (1- a)Sn
• a Tn (1- a) a Tn-1 (1- a)2 a Tn-2
• This gives higher weight to more recent times

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SPN example
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SRT

• Shortest Remaining Time
• Preemptive version of SPN
• When a job enters the Ready queue with a small
  Sn, it may preempt the current job
• A long process may starve here
• Good turnaround time for short jobs

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HRRN

• Highest Response Ratio Next
• RR Response Ratio (w s) / s
• w time waiting for processor
• s expected service time
• The Ready queue is ordered by RR values
• Prevents starvation
• Short jobs favored
• Service time still needs to be estimated

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HRRN example
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Feedback
Q1

• Involves
• multiple
• round-robin
• queues

Q2
Q3
Qn
running
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Feedback

• Queues have decreasing priority
• A process is demoted to the next lower queue
  based on cumulative execution time or each
  time-out
• A process is promoted to a higher queue based on
  events like I/O request
• Lower queues may get a larger time-slice
• Short and I/O-bound jobs keep higher priority
• Adapts to changing process behavior

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Feedback example
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Fair-share

• Assign each user a fair share of processor time
• A user may have many processes
• Monitor usage and adjust priorities so each user
  gets a fair share

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