Chapter 2.2 : Process Scheduling

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Chapter 2.2 Process Scheduling

• Process concept ?
• Process scheduling
• Interprocess communication
• Deadlocks
• Threads

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Scheduling

• Select process(es) to run on processor(s)
• Process state is changed from ready to
  running
• The component of the OS which does the scheduling
  is called the scheduler

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Types of Scheduling

• Scheduling is divided into various levels.
• These levels are defined by the location of the
  processes
• A process can be
• available to be executed by the processor
• partially or fully in main memory
• in secondary memory
• is not started yet

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Types of Scheduling

• Long Term Scheduling.
• The decision to add to the pool of processes to
  be executed.
• Medium Term Scheduling.
• The decision to add to the process in main
  memory.
• Short Term Scheduling.
• The decision as to which process will gain the
  processor.
• I/O Scheduling.
• The decision as to which process's I/O request
  shall be handled by a device.

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Scheduling Criteria

• Fairness each process should get a fair share
  of the CPU
• Efficiency keep CPU 100 utilized
• Response time should be minimized for
  interactive users
• Turnaround minimize batch turnaround times
• Throughput maximize number of jobs processed
  per hour

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User-Oriented, Performance Criteria
Criteria Aim Response Time low response time,
maximum number of interactive users
Turnaround Time time between submission
and completion Deadlines maximise deadlines
met
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System-oriented, Performance Criteria

• Criteria Aim
• Throughput allow maximum number of jobs to
  complete
• Processor maximise percentage of time processor
  is busy
• utilisation
• Overhead minimise time processor busy executing
  OS

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System oriented, other criteria
Criteria Aim Fairness treat processes the
same avoid starvation Enforcing Priorities give
preference to higher priority processes Balancing
Resources keep the system resources busy
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Important Factors

• I/O boundedness of a process
• CPU boundedness of a process
• Is the process interactive or batch?
• Process priority
• Page fault frequency
• Preemption frequency
• Execution time received
• Execution time required to complete

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Types of Scheduling

• A scheduling algorithm is non-premptive (run to
  completion) if the CPU cannot be taken away by
  the OS.
• A scheduling algorithm is preemptive if the CPU
  can be taken away by the OS.

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The Interrupting Clock

• The OS sets the interrupting clock to generate an
  interrupt at some specified future time.
• This interrupt time is the process quantum.
• Provides reasonable response times and prevents
  the system being held up by processes in infinite
  loops.

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Scheduling Algorithms

• FCFS
• Round Robin
• Virtual Round Robin
• Priority
• Priority Classes
• Shortest Job First
• Shortest Remaining Time
• Highest Response Ratio Next
• Feedback Queues

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FCFS (First Come First Serve)

• Implementation
• As each process becomes ready, it joins the ready
  queue.
• When the current process finishes the oldest
  process is selected next.
• Characteristics
• Simple to implement
• Nonpremptive
• Penalises short and I/O-bound processes

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Round Robin (RR)

• Implementation
• Processes are dispatched FIFO. But are given a
  fixed time on the CPU (quantum - time slice).
• Characteristics
• Preemptive
• Effective in time sharing environments
• Penalises I/O bound processes

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Quantum Size

• Some Options
• Large or small quantum
• Fixed or variable quantum
• Same for everyone or different
• If quantum is to large RR degenerates into FCFS
• If quantum is to small context switching becomes
  the primary job being executed
• A good guide is quantum should be slightly larger
  than the time required for a typical interaction

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Virtual Round Robin (VRR)

• A modification to the RR algorithm to remove the
  bias towards CPU bound processes.
• Implementation
• Two ready queues, one called an AUX queue for
  storing completed IO processes
• AUX queue has priority over READY queue
• IO processes only runs for remaining time
• Characteristics
• Performance studies indicate fairer than RR

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Priority

• Implementation
• Each process is assigned a priority and the
  scheduler always selects the highest priority
  process first
• Characteristics
• High priority processes may run indefinitely, so
  decrease the priority of these processes at
  regular intervals
• Assign high priority to system processes with
  known characteristics such as being I/O bound

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Priority Classes
Priority Class 4
Highest
Priority Class 3
Priority Class 2
Priority Class 1
Lowest
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• Implementation
• Processes are grouped into priority classes
• Round Robin is used within a class
• When selecting process start with the highest
  class. If the class is empty, use a lower class
• Characteristics
• If priorities are not adjusted from time to time,
  lower classes may starve to death

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Shortest-Job-First (SJF)

• Sometimes known as Shortest Process Next (SPN)
• Implementation
• The process with the shortest expected execution
  time is given priority on the processor

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• Characteristics
• Nonpremptive
• Reduces average waiting time over FIFO
• Always produces the minimum average turnaround
  time
• Must know how long a process will run
• Possible user abuse
• Suitable for batch environments. Not useful in a
  timesharing environment

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Shortest Remaining Time (SRT)

• Preemptive counterpart of SPN
• Implementation
• Process with the smallest estimated run-time to
  completion is run next
• A running process may be preempted by a new
  process with a shorter estimate run-time

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• Characteristics
• Still requires estimates of the future
• Higher overhead than SJF
• No additional interrupts are generated as in RR
• Elapsed service times must be recorded

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Highest Response Ratio Next (HRRN)

• How do you get around the problem of Indefinite
  postponement?
• Implementation
• Once a job gets the CPU it runs it to completion
• The priority of a job is a function of the job's
  service time and the time it has been waiting for
  service
• priority (time waiting service time) /
  service time

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• Characteristics
• Nonpremptive
• Shorter jobs still get preference over longer
  jobs
• However aging ensures long jobs will eventually
  gain the processor
• Estimation still involved

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Feedback Queues

• Sometimes called multi-level feedback queues
• Implementation
• There is a network of ready queues
• A new process enters at the top queue
• Moves through the queue FIFO

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• I/O processes
• If the job requires I/O before quantum expiration
  it leaves the network and comes back at the same
  level queue
• CPU bound processes
• If the quantum expires first, the process is
  placed on the next lower queue
• This continues until it reaches the bottom queue

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• Dispatching
• A process is only placed on the CPU if all higher
  level queues are empty
• A running process is preempted by a process
  arriving in a higher queue
• Processes from lower level queues receive a
  larger quantum
• Modifications
• In some systems processes can proceed back up the
  network by becoming I/O bound

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A Scheduling Mechanism Should

• Favour short jobs
• Favour I/O bound jobs to get good I/O device
  utilisation
• Determine the nature of a job and schedule
  accordingly