CPS 356 Lecture notes: Scheduling

Coverage: [OSCJ8] Chapter 5 (pp. 193-239); [OSC8] Chapter 5 (pp. 183-223)

Scheduling management

  • processes are managed through the use of multiple queues (or lists) of PCB's; the word queue (in an OS context) has a loose interpretation
  • the job queue contains all jobs submitted to the system, but not yet in main memory
  • the ready queue contains all jobs in main memory ready to execute
  • each I/O device has a queue of jobs waiting for various I/O operations
  • a process is dispatched from the ready queue to the CPU; its processing may cause it to be put on a device queue
  • all of these events are signaled by interrupts
  • job scheduling versus process scheduling (or CPU scheduling)
  • here we are primarily discussing process scheduling

Process scheduling

  • allocating the CPU to a different process to reduce idle time
  • each process change requires a context switch
  • a context switch is pure overhead (i.e., involves no useful work)

CPU and I/O Bursts

  • a process cycles between CPU processing and I/O activity
  • a process generally has many short CPU bursts or a few long CPU bursts

  • I/O bound processes have many short CPU bursts
  • CPU bound processes have few long CPU bursts

  • this can affect the choice of CPU scheduling algorithm used in an OS

Preemptive scheduling

  • CPU scheduling decisions may take place when a process
    1. switches from the running to waiting state
    2. switches from the running to ready state
    3. switches from the waiting to ready state
    4. terminates
  • scheduling under conditions 1 and 4 is called non-preemptive (context switch is caused by the running program, e.g., I/O or termination)
  • scheduling under conditions 2 and 3 is preemptive (context switch caused by external reasons, e.g., time slice)

Scheduling Criteria

Each scheduling algorithm favors particular criteria:
  • CPU utilization (maximize)
  • throughput: number of processes which complete execution per time unit (maximize)
  • turnaround time (TA): total amount of time to execute a particular process (minimize)
  • waiting time: amount of time a process has been waiting in the ready queue (minimize)
  • response time: amount of time it takes from when a request is submitted to when the response is produced (minimize); does not include the time for a response to be output
Some work is being done to minimize response time variance, to promote predictability.

CPU Scheduling Algorithms

  • First-Come, First-Served (FCFS or FIFO) (non-preemptive)
  • Priority (e.g., Shortest Job First (SJF; non-preemptive)
  • or Shortest Remaining Time First (SRTF; preemptive))
  • Round Robin (preemptive)
  • Multi-level Queue
  • Multi-level Feedback Queue

First-Come, First-Served

  • non-preemptive scheduling management
  • ready queue is managed as a FIFO queue
  • example: 3 jobs arrive at time 0 in the following order (batch processing):

    ProcessBurst TimeArrivalStartWaitFinishTA

    Gantt chart:

    (regenerated from [OSC8] p. 189)
    (regenerated from [OSCJ8] p. 199)

    average waiting time: (0+24+27)/3 = 17

    average turnaround time: (24+27+30) = 27

  • consider arrival order: 2, 3, 1

    ProcessBurst TimeArrivalStartWaitFinishTA

    Gantt chart:

    (regenerated from [OSC9] p. 189)
    (regenerated from [OSCJ8] p. 199)

    average waiting time: (0+3+6)/3 = 3

    average turnaround time: (3+6+30) = 13

  • another example:

    ProcessBurst TimeArrivalStartWaitFinishTA

    Gantt chart:

    average waiting time: (0+11+14)/3 = 8.33

    average turnaround time: (12+17+23) = 52/3 = 17.33

  • another example:

    ProcessBurst TimeArrivalStartWaitFinishTA

    Gantt chart:

    (regenerated from [OSC8] p. 214)
    (regenerated from [OSCJ8] p. 229)

    average waiting time: (0+10+39+42+49)/5 = 28

    average turnaround time: (10+39+42+49+61)/5 = 40.2

Priority Scheduling

  • associate a priority with each process, allocate the CPU to the process with the highest priority
  • any 2 processes with the same priority are handled FCFS
  • SJF is a version of priority scheduling where the priority is defined using the predicted CPU burst length
  • priorities are usually numeric over a range
  • high numbers may indicate low priority (system dependent)
  • internal (process-based) priorities: time limits, memory requirements, resources needed, burst ratio
  • external (often political) priorities: importance, source (e.g., faculty, student)
  • priority scheduling can be non-preemptive or preemptive
  • problem: starvation --- low priority processes may never execute because they are waiting indefinitely for the CPU
  • a solution: aging --- increase the priority of a process as time progresses
  • nice in UNIX executes a utility with an altered scheduling priority
  • renice in UNIX alters the priority of running processes

Shortest Job First (SJF)

  • associate with each process the length of its next CPU burst
  • schedule the process with the shortest time
  • two schemes
    • non-preemptive: once scheduled, a process continues until the end of its CPU burst
    • preemptive: preempt if a new process arrives with a CPU burst of less length than the remaining time of the currently executing process; known as the Shortest Remaining Time First (SRTF) algorithm
  • SJF is provably optimal; it yields a minimum average waiting time for any set of processes
  • however, we cannot always predict the future (i.e., we do not know the next burst length)
  • we can only estimate its length
  • an estimate can be formed by using the length of its previous CPU bursts:

      Tn = actual length of the nth CPU burst

      ψn = predicted value of nth CPU burst

      0 <= w <= 1

      ψn+1 = w * Tn + (1-w) * ψn

SJF (non-preemptive) examples

  • example 1:

    ProcessBurst TimeArrivalStartWaitFinishTA

    Gantt chart:

    (regenerated from [OSC8] p. 190)
    (regenerated from [OSCJ8] p. 200)

    average waiting time: (3+16+9+0)/4 = 7

    average turnaround time: (9+24+16+3)/4 = 13

  • example 2:

    ProcessBurst TimeArrivalStartWaitFinishTA

    Gantt chart:

    average waiting time: (0+6+3+7)/4 = 4

    average turnaround time: (7+4+10+11)/4 = 8

  • example 3:

    ProcessBurst TimeArrivalStartWaitFinishTA

    Gantt chart:

    (regenerated from [OSC8] p. 214)
    (regenerated from [OSCJ8] p. 229)

    average waiting time: (10+32+0+3+20)/5 = 13

    average turnaround time: (10+39+42+49+61)/5 = 25.2

SRTF (preemptive) examples

  • example 1:

    ProcessBurst TimeArrivalStartWaitFinishTA

    Gantt chart:

    (regenerated from [OSC8] p. 192)
    (regenerated from [OSCJ8] p. 202)

    average waiting time: (9+0+15+2)/4 = 6.5

    average turnaround time: (17+4+24+7)/4 = 13

  • example 2:

    ProcessBurst TimeArrivalStartWaitFinishTA

    Gantt chart:

    average waiting time: (9+1+0+2)/4 = 3

    average turnaround time: (16+5+1+6)/4 = 7

Priority Scheduling example

    ProcessBurst TimePriorityArrivalStartWaitFinishTA

    Gantt chart:

    (regenerated from [OSC8] p. 193)
    (regenerated from [OSCJ8] p. 203)

    average waiting time: (6+0+16+18+1)/5 = 8.2

    average turnaround time: (1+6+16+18+19)/5 = 12

Round Robin

  • time sharing (preemptive) scheduler where each process is given access to the CPU for 1 time quantum (slice) (e.g., 20 milliseconds)
  • a process may block itself before its time slice expires
  • if it uses its entire time slice, it is then preempted and put at the end of the ready queue
  • the ready queue is managed as a FIFO queue and treated as a circular
  • if there are n processes on the ready queue and the time quantum is q, then each process gets 1/n time on the CPU in chunks of at most q time units
  • no process waits for more than (n-1)q time units
  • the choice of how big to make the time slice (q) is extremely important
    • if q is very large, Round Robin degenerates into FCFS
    • if q is very small, the context switch overhead defeats the benefits

  • example 1 (q = 20):

    ProcessBurst TimeArrivalStartWaitFinishTA

    Gantt chart:

    waiting times:

      p1: (77-20) + (121-97) = 81

      p2: (20-0) = 20

      p3: (37-0) + (97-57) + (134-117) = 94

      p4: (57-0) + (117-77) = 97

    average waiting time: (81+20+94+97)/4 = 73

  • example 2 (q = 4):

    ProcessBurst TimeArrivalStartWaitFinishTA

    Gantt chart:

    (regenerated from [OSC8] p. 194) (regenerated from [OSCJ8] p. 204)

    average waiting time: (6+4+7)/3 = 5.67

    average turnaround time: (30+7+10)/3 = 15.67

  • example 3 (q = 10):

    ProcessBurst TimeArrivalStartWaitFinishTA

    Gantt chart:

    (regenerated from [OSC8] p. 214)
    (regenerated from [OSCJ8] p. 229)

    average waiting time: (0+32+20+23+40)/5 = 23

    average turnaround time: (10+39+42+49+61)/5 = 35.2

Multilevel Queue

  • the ready queue is managed as multiple queues based on various characteristics. For instance,
    • foreground (interactive)
    • background (batch)
  • each queue uses a particular scheduling algorithm. For instance,
    • foreground (round robin)
    • background (FCFS)
  • scheduling must be done between queues:
    • fixed priority (may lead to starvation) (e.g., foreground jobs have absolute priority over background jobs)
    • time slice per queue

Multilevel Feedback Queue

  • processes move between the various queues
  • a multilevel feedback queue is characterized by
    • number of queues
    • scheduling algorithm 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 on which queue a process begins (each time it returns to the ready state)
  • example:
    • 3 queues
    • fixed priority based on length of CPU burst
    • RR for 1st queue, FCFS for last queue
    • each process begins on top queue (quantum = 8)

    (regenerated from [OSC8] Fig. 5.7 on p. 198)
    (regenerated from [OSCJ8] Fig. 5.7 on p. 208)

Algorithm Evaluation

  • which algorithm should be used in a particular system?
  • how should the parameters (e.g., q, number of levels) be defined?
  • on which criteria do we base our decisions?

Four approaches to evaluation

  • deterministic modeling
  • queue models
  • simulation
  • implementation

Deterministic modeling

  • define a workload and compare it across algorithms
  • simple to execute and results in distinct values to compare
  • however, the results apply only to that case and cannot be generalized
  • a set of workload scenarios with varying characteristics can be defined and analyzed
  • must be careful about any conclusion drawn

Queuing models

  • n = average queue length
  • W = average waiting time in the queue
  • λ = average arrival rate
  • Little's Formula: n = λ * W
  • Little's formula can be applied to the CPU and ready queue, or the wait queue for any device
  • values can be obtained by measuring a real system over time and mathematically estimating
  • the estimates are not always accurate due to:
    • complicated algorithms
    • assumptions
  • therefore, the queuing model may not reflect reality to the level needed


    [OSC8] A. Silberschatz, P.B. Galvin, and G. Gagne. Operating Systems Concepts. John Wiley and Sons, Inc., Eighth edition, 2009.
    [OSCJ8] A. Silberschatz, P.B. Galvin, and G. Gagne. Operating Systems Concepts with Java. John Wiley and Sons, Inc., Eighth edition, 2010.

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