Scheduling

1
Scheduling

• Operating Systems
• Spring 2004

2
Multiprogramming

• Multiprogramming having multiple jobs
  (processes) in the system
• Interleaved (time sliced) on a single CPU
• Concurrently executed on multiple CPUs
• Both of the above
• Why multiprogramming?
• Responsiveness, utilization, concurrency
• Why not?
• Overhead, complexity

3
Responsiveness
Job 1 arrives
Job 2 arrives
Job 3 arrives
Job1
Job2
Job3
4
Utilization
1st I/O operation
I/O ends
2nd I/O operation
I/O ends
3rd I/O operation
idle
CPU
idle
idle
Disk
idle
idle
idle
CPU
idle
Job1
Job2
Job1
Job2
Disk
idle
Job1
Job2
idle
idle
Job1
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Workload matters!

• Does it really matter?
• Yes, of course
• If all jobs are CPU bound (I/O bound),
  multiprogramming does not help to improve
  utilization
• A suitable job mix is created by a long-term
  scheduling
• Jobs are classified on-line to be CPU (I/O) bound
  according to the jobs history

6
Concurrency

• Concurrent programming
• Several process interact to work on the same
  problem
• ls l more
• Simultaneous execution of related applications
• Word Excel PowerPoint
• Background execution
• Polling/receiving Email while working on smth else

7
The cost of multiprogramming

• Switching overhead
• Saving/restoring context wastes CPU cycles
• Degrades performance
• Resource contention
• Cache misses
• Complexity
• Synchronization, concurrency control, deadlock
  avoidance/prevention

8
Short-Term Scheduling
terminated
running
schedule
wait for event
preempt
created
ready
blocked
event done
9
Short-Term scheduling

• Process execution pattern consists of alternating
  CPU cycle and I/O wait
• CPU burst I/O burst CPU burst I/O burst...
• Processes ready for execution are hold in a ready
  (run) queue
• STS schedules process from the ready queue once
  CPU becomes idle

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Preemptive/non-preemptive

• Four events in which the scheduler may decide to
  switch the CPU to a new process
• Running process terminates
• Running process blocks (e.g., for I/O)
• Non-ready process becomes ready (or starts)
• Clock interrupt
• The latter two cause preemption
• Advantages
• Better time sharing, responsiveness
• Guard against endless loops
• Disadvantages
• Higher programming complexity
• Example Unix disables interrupts within kernel
  mode
• Disrupts real-time

11
Metrics Response time

• Response time (turnaround time) is the average
  over the jobsTresp

Job terminates/ blocks waiting for I/O
Job arrives/ becomes ready to run
Starts running
Trun
Twait
Tresp
Tresp Twait Trun
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Metrics

• Wait time average of Twait
• This parameter is under the system control
• Throughput
• Number of complete process terminations within a
  time unit
• Utilization
• Fraction of time CPU is utilized

13
Off-line vs. On-line scheduling

• Off-line algorithms
• Get all the information about all the jobs to
  schedule as their input
• Outputs the scheduling sequence
• Preemption is never needed
• On-line algorithms
• Jobs arrive at unpredictable times
• Very little info is available in advance
• Preemption compensates for lack of knowledge

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First-Come-First-Serve (FCFS)

• Schedules the jobs in the order in which they
  arrive
• Off-line FCFS schedules in the order the jobs
  appear in the input
• Runs each job to completion
• Both on-line and off-line
• Simple, a base case for analysis
• Poor response time

15
Shortest Job First (SJF)

• Best response time

Long job
Short
Short
Long job

• Inherently off-line
• All the jobs and their run-times must be
  available in advance

16
Preemption

• Preemption is the action of stopping a running
  job and scheduling another in its place
• Context switch Switching from one job to another

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Using preemption

• On-line short-term scheduling algorithms
• Adapting to changing conditions
• e.g., new jobs arrive
• Compensating for lack of knowledge
• e.g., job run-time
• Periodic preemption keeps system in control
• Improves fairness
• Gives I/O bound processes chance to run

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Shortest Remaining Time first (SRT)

• Job run-times are known
• Job arrival times are not known
• When a new job arrives
• if its run-time is shorter than the remaining
  time of the currently executing job
• preempt the currently executing job and schedule
  the newly arrived job
• else, continue the current job and insert the new
  job into a sorted queue
• When a job terminates, select the job at the
  queue head for execution

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Round Robin (RR)

• Both job arrival times and job run-times are not
  known
• Run each job cyclically for a short time quantum
• Approximates CPU sharing

Job 1 arrives
Job 2 arrives
Job 3 arrives
Job 1 terminates
Job 2 terminates
Job 3 terminates
Job1
3
2
1
2
3
1
2
3
1
2
1
Job2
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Responsiveness
Job 1 arrives
Job 2 arrives
Job 3 arrives
Job1
Job2
Job3
21
Priority Scheduling

• Example prioritize processes according to their
  past CPU usage
• Equivalent to SJF with predicted next CPU burst
  time

• Tn is the duration of the n-th CPU burst
• En1 is the estimate of the next CPU burst

22
Multilevel feedback queues
terminated
new jobs
quantum10
quantum20
quantum40
FCFS
23
Multilevel feedback queues

• Priorities are implicit in this scheme
• Very flexible
• Starvation is possible
• Short jobs keep arriving gt long jobs get starved
• Solutions
• Let it be
• Aging

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Real-life examples

• Solaris 2
• Window 2000
• Linux