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Scheduling
Operating Systems
Spring 2004
OS Spring’04
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
OS Spring’04
Responsiveness
Job 1
arrives
Job 2
arrives
Job 3
arrives
Job1
Job 1
terminates
Job1
Job3
Job2
Job 2
Job 3
terminates terminates
Job3
Job 1 terminates
OS Spring’04
Job2
Job 3
terminates
Job 2 terminates
Utilization
1st I/O
operation
CPU
I/O 2nd I/O
ends operation
idle
CPU
Job1
Disk
idle
idle
Job2
Job1
3rd I/O
operation
idle
idle
Disk
I/O
ends
idle
idle
Job1 idle
idle
Job2
OS Spring’04
Job2
Job1
idle
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 longterm scheduling
Jobs are classified on-line to be CPU (I/O)
bound according to the job’s history
OS Spring’04
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
OS Spring’04
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
OS Spring’04
Short-Term Scheduling
terminated
running
schedule
wait for event
preempt
created
ready
blocked
event done
OS Spring’04
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
OS Spring’04
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
OS Spring’04
Metrics: Response time
 Response time (turnaround time) is the
average over the jobs’Tresp
Job arrives/
becomes ready to run
Starts running
Job terminates/
blocks waiting for I/O
Trun
Twait
Tresp
Tresp= Twait + Trun
OS Spring’04
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
OS Spring’04
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
OS Spring’04
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
OS Spring’04
Shortest Job First (SJF)
 Best response time
Short
Long job
Long job
Short
 Inherently off-line
All the jobs and their run-times must be
available in advance
OS Spring’04
Preemption
 Preemption is the action of stopping a
running job and scheduling another in its
place
 Context switch: Switching from one job to
another
OS Spring’04
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
OS Spring’04
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
OS Spring’04
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 3
terminates
Job1 2 1 2 3 1 2 3 1 2 3 1
OS Spring’04
Job 1
terminates
Job2
Job 2
terminates
Responsiveness
Job 1
arrives
Job 2
arrives
Job 3
arrives
Job1
Job 1
terminates
Job1
Job3
Job2
Job 2
Job 3
terminates terminates
Job3
Job 1 terminates
OS Spring’04
Job2
Job 3
terminates
Job 2 terminates
Priority Scheduling
 Example: prioritize processes according to their
past CPU usage
Equivalent to SJF with predicted next CPU burst
time
En1  Tn  (1   ) En , 0    1
1
1
1
1
1
  : En1  Tn  Tn1  Tn2  Tn3  
2
2
4
8
16
 Tn is the duration of the n-th CPU burst
 En+1 is the estimate of the next CPU burst
OS Spring’04
Multilevel feedback queues
new jobs
quantum=10
quantum=20
quantum=40
FCFS
OS Spring’04
terminated
Multilevel feedback queues
 Priorities are implicit in this scheme
 Very flexible
 Starvation is possible
Short jobs keep arriving => long jobs get
starved
 Solutions:
Let it be
Aging
OS Spring’04
Real-life examples
 Solaris 2
 Window 2000
 Linux
OS Spring’04