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Chapter 5: CPU Scheduling. Outline. Why scheduling? Scheduling Criteria Scheduling Algorithms Real-Time Scheduling Thread Scheduling Example: Solaris Algorithm Evaluation. Process Execution. Process execution consists of alternating cycles of CPU execution and I/O wait

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outline
Outline
  • Why scheduling?
  • Scheduling Criteria
  • Scheduling Algorithms
  • Real-Time Scheduling
  • Thread Scheduling
  • Example: Solaris
  • Algorithm Evaluation
process execution
Process Execution
  • Process execution consists of alternating cycles of CPU execution and I/O wait
  • Long-term scheduler gets a good mix of CPU-bound & I/O-bound jobs
  • Given that, how to scheudle ready processes? – Shor-term scheduling: focus of this chapter
why scheduling is necessary
Why scheduling is necessary?
  • Maximize CPU utilization via multiprogramming, while minimizing response time and/or maximizing throughput → Efficient CPU scheduling required
cpu scheduler
CPU Scheduler
  • Decide which process to execute next among the ready processes in memory
  • Order ready processes according to the specified policy such as FCFS (First Come First Serve)
preemptive vs nonpreemptive scheduling
Preemptive vs. nonpreemptive scheduling
  • Preemptive scheduling:
    • A hihger priority process can preempt a currently running low priority process to avoid priority inversion
    • Most real-time scheduling algorithms, e.g., EDF & RM
    • More details about RT scheduling will be discussed later
  • Nonpreemptive scheduling:
    • A higher priority process waits a currently running process to finish regardless of the priority
    • e.g., FCFS
    • Others such as I/O & networking devices are not preemptive
dispatcher
Dispatcher
  • Gives control of the CPU to the process selected by the short-term scheduler
    • Context switch between the two user processes
    • Context switch to user mode
    • Jump to the proper location in the user program to restart it

→ Dispatch latency : Time for dispatcher to stop one process and start another

scheduling criteria
Scheduling Criteria
  • CPU utilization: #CPU cycles used for computation/#total CPU cycles per unit time
  • Throughput: # of processes that complete the execution per unit time
  • Response time: Amount of time it takes from when a request was submitted until the first response is produced
scheduling criteria9
Scheduling Criteria
  • Waiting time
    • Amount of time a process has been waiting in the ready queue
    • Response time = waiting time + execution time
optimization criteria
Optimization Criteria
  • Maximize CPU utilization
  • Maximize throughput
  • Minimize response time
  • Minimize waiting time
  • Sometimes they may conflict with each other
  • e.g., system can be overloaded while trying to maximize CPU utilization; response time may significantly increase as a result
first come first served fcfs scheduling

P1

P2

P3

0

24

27

30

First-Come, First-Served (FCFS) Scheduling

ProcessBurst Time (Execution 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
fcfs scheduling cont

P2

P3

P1

0

3

6

30

FCFS Scheduling (Cont.)
  • Suppose that processes arrive in the order P2 , P3 , P1
  • Gantt chart:
  • Average waiting time: (6 + 0 + 3)/3 = 3
  • Much better than previous case
  • Convoy effect: A short process behind a long process suffer long wating/response time
shortest job first sjf
Shortest-Job-First (SJF)
  • Schedule the process with the shortest execution time
  • Two schemes:
    • Nonpreemptive
      • Once the CPU is given to a process it cannot be preempted until completes its execution
    • Preemptive
      • Shortest-Remaining-Time-First (SRTF)
      • If a new process arrives with exec time shorter than the remaining time of the currently running process executing process, preempt
shortest job first sjf14
Shortest-Job-First (SJF)
  • SJF is optimal: It minimizes average waiting time for a given set of processes
example of non preemptive sjf

P1

P3

P2

P4

0

3

7

8

12

16

Example of Non-Preemptive SJF

ProcessArrival TimeExec Time

P1 0.0 7

P2 2.0 4

P3 4.0 1

P4 5.0 4

  • Average waiting time = (0 + 6 + 3 + 7)/4 = 4
example of preemptive sjf

P1

P2

P3

P2

P4

P1

11

16

0

2

4

5

7

Example of Preemptive SJF

ProcessArrival TimeBurst Time

P1 0.0 7

P2 2.0 4

P3 4.0 1

P4 5.0 4

  • Average waiting time = (9 + 1 + 0 +2)/4 = 3
hard to implement sjf
Hard to implement SJF
  • Don’t know the exact exec time when a process arrives
  • Can only estimate it by using the length of previous exec time (Take moving average)
    • Doesn’t always work
priority scheduling
Priority Scheduling
  • A priority number (integer) is associated with each process
  • The CPU is allocated to the process with the highest priority (smallest integer  highest priority)
    • Preemptive
    • nonpreemptive
priority scheduling20
Priority Scheduling
  • SJF is a priority scheduling where priority is determined based on the exec time
  • Problem:Starvation
    • Low priority processes may never execute or suffer very long waiting time
  • Solution: Aging
    • As time progresses increase the priority of the process
  • How to support fairness while preferring high priority processes?
    • Hard problem
round robin rr
Round Robin (RR)
  • Each process gets a small unit of CPU time (time quantum), usually 10-100 milliseconds
  • After the time quantum, preempt the process and add to the end of the ready queue
round robin
Round Robin
  • If there are n processes in the ready queue and the time quantum is q, then each process gets 1/n of the CPU time in chunks of at most q time units at once. No process waits more than (n-1)q time units.
  • Performance
    • q large  FIFO
    • q small  q must be large with respect to context switch, otherwise overhead is too high
example of rr with time quantum 20

P1

P2

P3

P4

P1

P3

P4

P1

P3

P3

0

20

37

57

77

97

117

121

134

154

162

Example of RR with Time Quantum = 20

ProcessBurst Time

P1 53

P2 17

P3 68

P4 24

  • Gantt chart:
multilevel queue
Multilevel Queue
  • Ready queue is partitioned into separate queues, e.g., foreground (interactive) &background (batch)
  • Each queue can have its own scheduling algorithm
    • foreground – RR
    • background – FCFS
multilevel queue26
Multilevel Queue
  • Scheduling must be done between the queues
    • Fixed priority scheduling
    • Serve all foreground process first
    • Serve background process only if the foreground queue is empty
    • Possibility of starvation
  • Time slice
    • Each queue gets a certain amount of CPU time which it can schedule amongst its processes, e.g.,
      • 80% to foreground in RR
      • 20% to background in FCFS
multilevel feedback queue
Multilevel Feedback Queue
  • A process can move between queues
  • Aging can be implemented this way
  • Defined by the following parameters:
    • number of queues
    • scheduling algorithms for each queue
    • method used to determine when to demote/upgrade a process
    • method used to determine which queue a process will enter when that process needs service
example of multilevel feedback queue
Example of Multilevel Feedback Queue
  • Example:
    • Q0– RR with time quantum 8 milliseconds
    • Q1– RR time quantum 16 milliseconds
    • Q2– FCFS
    • A new job enters queue Q0 . When it gets CPU, it receives 8 ms. If it does not finish in 8 milliseconds, it is moved to Q1.
    • At Q1 job receives 16 ms. If it does not complete within 16ms, it is preempted and moved to queue Q2.
real time scheduling
Real-Time Scheduling
  • Hard real-time systems
    • A deadline miss may cause catastrophic results, e.g., air traffic control, flight control, engine control, etc.
  • Soft real-time systems
    • A deadline miss does not cause catastrophic results
    • Meet as many deadlines as possible, e.g., video streaming
earliest deadline first
Earliest Deadline First
  • Optimal Dynamic scheduling algorithm
  • Schedule the task with the earliest absolute deadline
  • Advantage: 100% utilization bound
  • Disadvantage: Domino effect under overload
rate monotonic scheduling
Rate Monotonic Scheduling
  • Optimal fixed priority scheduling algorithm
  • Assign the highest priority to the task with the shortest period
  • Advantage: Easy to implement
  • Disadvantage
    • Low utilization bound, i.e., 69%
    • Blind to deadlines

RMS schedules T1 first

T1

T2

multiple processor scheduling
Multiple Processor Scheduling
  • Asymmetric multiprocessing
    • One processor, called master, makes all scheduling decisions
  • Symmetric multiprocessing (SMP)
    • Each processor is self-scheduling
    • Need to avoid for two processors to schedule one process at the same time
    • Supported by most OS – Linux, Solaris, Mac OS X, Windows XP, Windows 2000 …
    • Load balancing: Push vs. pull migration
thread scheduling
Thread Scheduling
  • Local Scheduling (process contention scope)
    • How the threads library decides which thread to put onto an available LWP
    • Threads belonging to one process compete for a LWP
  • Global Scheduling (system contention scope)
    • How the kernel decides which kernel thread to run next
    • Assign priority
  • POSIX Thread API
    • Select scheduling algorithm
    • Assign priority to threads
example solaris 2 scheduling
Example: Solaris 2 Scheduling
  • Assign the highest priority to RT threads
  • Apply fixed priority among thread groups
algorithm evaluation
Algorithm Evaluation
  • Deterministic model
  • Queuing model
    • Little’s Formula: Queue length n = average arrival rate * average waiting time
  • Simulation
  • Implementation