Scheduling

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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 (batch processing)
• The decision to add to the pool of processes to
  be executed.
• Medium Term Scheduling (swapping)
• The decision to add to the process in main
  memory.
• Short Term Scheduling(CPU 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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CPU 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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Basic Concepts (CPU Scheduling)

• Maximum CPU utilization obtained with
  multiprogramming
• CPUI/O Burst Cycle Process execution consists
  of a cycle of CPU execution and I/O wait.
• CPU burst distribution

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Alternating Sequence of CPU And I/O Bursts
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Histogram of CPU-burst Times
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Types of CPU Scheduling

• A scheduling algorithm is NON-PREEMPTIVE (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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CPU Scheduler

• Selects from among the processes in memory that
  are ready to execute, and allocates the CPU to
  one of them.
• CPU scheduling decisions may take place when a
  process
• 1. Switches from running to waiting state.
• 2. Switches from running to ready state.
• 3. Switches from waiting to ready.
• 4. Terminates.
• Scheduling under 1 and 4 is nonpreemptive.
• All other scheduling is preemptive.

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Dispatcher

• Dispatcher module gives control of the CPU to the
  process selected by the short-term scheduler
  this involves
• switching context
• switching to user mode
• jumping to the proper location in the user
  program to restart that program
• Dispatch latency time it takes for the
  dispatcher to stop one process and start another
  running. (context switch overhead)

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The Interrupting Clock- Timer

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

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Scheduling Criteria

• CPU utilization keep the CPU as busy as
  possible
• Throughput of processes that complete their
  execution per time unit
• Turnaround time amount of time to execute a
  particular process
• Waiting time amount of time a process has been
  waiting in the ready queue
• Response time amount of time it takes from when
  a request was submitted until the first response
  is produced, not output (for time-sharing
  environment)

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Optimization 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 maximize deadlines
met
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System-oriented, Performance Criteria

• Criteria Aim
• Throughput allow maximum number of jobs to
  complete
• Processor maximize percentage of time processor
  is busy
• utilization
• Overhead minimize 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 / CPU boundedness of a process
• Is the process interactive or batch?
• Process priority
• Page fault frequency (Virtual Memory )
• Preemption frequency (time slice)
• Execution time received
• Execution time required to complete

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Popular research area in 1970s..

• Assumptions
• One program per user
• One thread per program
• Programs are independent

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Scheduling Algorithms

• FCFS -----------------------FCFS
• Shortest Job First ------------------ SJF
• Shortest Remaining Time
• Highest Response Ratio Next
• Round Robin -------------------------RR
• Virtual Round Robin
• Priority -------------------------------Priority
• Priority Classes(Multilevel Queues)
• 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
• Non-premptive
• Penalizes short and I/O-bound processes

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First-Come, First-Served (FCFS) Scheduling

• Process Burst Time
• P1 24
• P2 3
• P3 3
• Suppose that the processes arrive in the order
  P1 , P2 , P3 The Gantt Chart for the schedule
  is
• Waiting time for P1 0 P2 24 P3 27
• Average waiting time (0 24 27)/3 17

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FCFS Scheduling (Cont.)

• Suppose that the processes arrive in the order
• P2 , P3 , P1 .
• The Gantt chart for the schedule is
• Waiting time for P1 6 P2 0 P3 3
• Average waiting time (6 0 3)/3 3
• Much better than previous case.
• Convoy effect short process behind long process

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Approximating Load

• Let ? mean arrival rate
• So 1/ ? mean time between arrivals
• And µ mean service rate
• So 1/ µ mean service time (avg t(pi))
• CPU busy ? ? 1/ µ ? / µ
• Notice must have ? / µ (i.e., ? lt 1)
• What if ? approaches 1?

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Predicting Wait Time in FCFS

• In FCFS, when a process arrives, all in ready
  list will be processed before this job
• Let µ be the service rate
• Let L be the ready list length
• Wavg(p) L1/ µ 0.5 1/ µ L/ µ 1/(2 µ )
• Compare predicted wait with actual in earlier
  examples

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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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Example of Non-Preemptive SJF

• Process Arrival Time Burst Time
• P1 0.0 7
• P2 2.0 4
• P3 4.0 1
• P4 5.0 4
• SJF (non-preemptive)
• Average waiting time (0 6 3 7)/4 4

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SJF-continued

• Characteristics
• Non-premptive
• 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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Example of Preemptive SJF

• Process Arrival Time Burst Time
• P1 0.0 7
• P2 2.0 4
• P3 4.0 1
• P4 5.0 4
• SJF (preemptive)
• Average waiting time (9 1 0 2)/4 3

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SRT-continued

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

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Determining Length of Next CPU Burst

• Can only estimate the length.
• Can be done by using the length of previous CPU
  bursts, using exponential averaging.

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Prediction of the Length of the Next CPU Burst
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Examples of Exponential Averaging

• ? 0
• ?n1 ?n
• Recent history does not count.
• ? 1
• ?n1 tn
• Only the actual last CPU burst counts.
• If we expand the formula, we get
• ?n1 ? tn(1 - ?) ? tn-1
• (1 - ? )j ? tn-1
• (1 - ? )n1 tn ?0
• Since both ? and (1 - ?) are less than or equal
  to 1, each successive term has less weight than
  its predecessor.

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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 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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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
• Penalizes 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 (overhead 10)
• For 100ms ts - context switch time lt 10ms

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Example of RR with Time Quantum 20

• Process Burst Time
• P1 53
• P2 17
• P3 68
• P4 24
• The Gantt chart is
• Typically, higher average turnaround than SJF,
  but better response.

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Time Quantum and Context Switch Time
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Turnaround Time Varies With The Time Quantum
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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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Multilevel Queue

• Ready queue is partitioned into separate
  queuesforeground (interactive)background
  (batch)
• Each queue has its own scheduling algorithm,
  foreground RRbackground FCFS
• Scheduling must be done between the queues.
• Fixed priority scheduling (i.e., serve all from
  foreground then from background). Possibility of
  starvation.
• Time slice each queue gets a certain amount of
  CPU time which it can schedule amongst its
  processes i.e., 80 to foreground in RR 20 to
  background in FCFS

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Multilevel Queue Scheduling
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Multilevel Feedback Queue

• A process can move between the various queues
  aging can be implemented this way.
• Multilevel-feedback-queue scheduler defined by
  the following parameters
• number of queues
• scheduling algorithms for each queue
• method used to determine when to upgrade a
  process
• method used to determine when to demote a process
• method used to determine which queue a process
  will enter when that process needs service

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Example of Multilevel Feedback Queue

• Three queues
• Q0 time quantum 8 milliseconds
• Q1 time quantum 16 milliseconds
• Q2 FCFS
• Scheduling
• A new job enters queue Q0 which is served FCFS.
  When it gains CPU, job receives 8 milliseconds.
  If it does not finish in 8 milliseconds, job is
  moved to queue Q1.
• At Q1 job is again served FCFS and receives 16
  additional milliseconds. If it still does not
  complete, it is preempted and moved to queue Q2.

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Multilevel Feedback Queues
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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

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Summary

• A CPU Scheduling Mechanism Should
• Favour short jobs
• Favour I/O bound jobs to get good I/O device
  utilization
• Determine the nature of a job and schedule
  accordingly