Lecture Operating systems: A concept-based approach (2/e): Chapter 4 - Dhananjay M. Dhamdhere
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Chapter 4: Scheduling
Dhamdhere: Operating Systems—
A ConceptBased Approach, 2 ed
Slide No: 1
Copyright © 2008
Scheduling
•
Scheduling is the act of determining the order in which
requests should be taken up for servicing
– A request is a unit of computational work
* It could be a job, a process, or a subrequest made to a process
Chapter 4: Scheduling
Dhamdhere: Operating Systems—
A ConceptBased Approach, 2 ed
Slide No: 2
Copyright © 2008
Scheduling related concepts and terms
•
Request related concepts and terms
– Arrival time
* Request is submitted to the system
– Admission time
* Kernel starts considering the request for scheduling
– Completion time
– Deadline
* Time by which servicing of a request should be completed
– Service time
* Total of CPU time and I/O time required to service a request
– Preemption
– Priority
Chapter 4: Scheduling
Dhamdhere: Operating Systems—
A ConceptBased Approach, 2 ed
Slide No: 3
Copyright © 2008
A schematic of scheduling
• Admission of an arrived request may be delayed for reasons seen later
• An admitted request remains pending until it is scheduled
• A preempted request is put back into the pool of pending requests
Chapter 4: Scheduling
Dhamdhere: Operating Systems—
A ConceptBased Approach, 2 ed
Slide No: 4
Copyright © 2008
Performance related concepts and terms
•
User service related concepts and terms
– Response time
– Turn-around time
– Weighted turn-around = Turn-around time / service time
– Deadline overrun
* Amount of time by which a deadline is missed
– Fair share
* A specified share of CPU time that should be devoted to a process
or a group of processes
– Response ratio
* Response ratio = (time since arrival + service time) / service time
Chapter 4: Scheduling
Dhamdhere: Operating Systems—
A ConceptBased Approach, 2 ed
Slide No: 5
Copyright © 2008
Performance related concepts and terms
•
We shall use the following terms
– Terms related to average service
* Mean response time
* Mean turn-around time
– Terms related to scheduling performance
* Schedule length
The total amount of time required to service a set of requests
* Throughput
Number of requests serviced per unit of time
Chapter 4: Scheduling
Dhamdhere: Operating Systems—
A ConceptBased Approach, 2 ed
Slide No: 6
Copyright © 2008
Fundamental techniques of scheduling
•
Three fundamental techniques are used in scheduling
– Priority-based scheduling
* As seen in the context of multiprogramming
– Reordering of requests: it may be used to
* Enhance system throughput, e.g., as in multiprogramming
* Enhance user service, e.g., as in time sharing
– Variation of time slice
* Small time slice yields better response times
* Large time slice may reduce scheduling overhead
Chapter 4: Scheduling
Dhamdhere: Operating Systems—
A ConceptBased Approach, 2 ed
Slide No: 7
Copyright © 2008
More on priority
•
Features of priority-based scheduling
– Priorities may be static or dynamic
* A static priority is assigned to a request before it is admitted
* A dynamic priority is one that is varied during servicing of a request
– How to handle processes having same priority?
* Round-robin scheduling is performed within a priority level
– Starvation of a low priority request may occur
Q: How to avoid starvation?
Chapter 4: Scheduling
Dhamdhere: Operating Systems—
A ConceptBased Approach, 2 ed
Slide No: 8
Copyright © 2008
Kinds of Scheduling
•
Scheduling may be performed in two ways
– Non-preemptive scheduling
* A process runs to completion when scheduled
– Preemptive scheduling
* Kernel may preempt a process and schedule another one
* A set of processes are serviced in an overlapped manner
Q: What are the benefits of preemptive scheduling?
Chapter 4: Scheduling
Dhamdhere: Operating Systems—
A ConceptBased Approach, 2 ed
Slide No: 9
Copyright © 2008
Processes for scheduling
•
In our discussion, we will consider the following
processes for scheduling
Process
Chapter 4: Scheduling
P1
P2
P3
P4
P5
Arrival time
0
2
3
5
9
Service time
3
3
2
5
3
Deadline
4
14
6
11
12
Dhamdhere: Operating Systems—
A ConceptBased Approach, 2 ed
Slide No: 10
Copyright © 2008
Non-preemptive scheduling
•
Principle
– Reorder the processes to improve performance
* Performance criteria
Throughput
Turn around times of processes
Chapter 4: Scheduling
Dhamdhere: Operating Systems—
A ConceptBased Approach, 2 ed
Slide No: 11
Copyright © 2008
Non-preemptive scheduling policies
•
Three principal non-preemptive scheduling policies are
– First come, first served (FCFS)
– Shortest request next (SRN)
– Highest response ratio next (HRN)
Response ratio = (Time since arrival + service time) / service
time
Q: Which policy will provide better performance under what
conditions? Consider throughput, average turn-around time,
manipulation by users, starvation possibility …
Chapter 4: Scheduling
Dhamdhere: Operating Systems—
A ConceptBased Approach, 2 ed
Slide No: 12
Copyright © 2008
Performance of FCFS and SRN scheduling
Arrives Service
Time
P1 0 3
P2 2 3
P3 3 2
P4 5 5
P5 9 3
• Table shows sequence of decisions made by the scheduling policies
• SRN schedules P3 ahead of P2 because it is shorter
• Mean turnaround time is shorter by SRN than by FCFS scheduling
Qs: Schedule lengths? Should we always use SRN?
Chapter 4: Scheduling
Dhamdhere: Operating Systems—
A ConceptBased Approach, 2 ed
Slide No: 13
Copyright © 2008
Highest response ratio next (HRN) policy
• P14 has a shorter service time than P13
• It has a higher response ratio at 8 seconds, so it is scheduled ahead of P13
Qs: Advantages / disadvantages over FCFS and SRN?
Chapter 4: Scheduling
Dhamdhere: Operating Systems—
A ConceptBased Approach, 2 ed
Slide No: 14
Copyright © 2008
Non-preemptive scheduling policies
•
Consider service provided to different kinds of requests
– Short requests
* SRN provides them a favoured treatment; FCFS does not
Q: Does HRN provide them a favoured treatment?
– Long requests
* May starve if SRN scheduling is used
* They do not starve if HRN scheduling is used
Q: Why?
•
SRN is not a feasible scheduling policy
– Service times of processes are not known
– Service time information provided by users is not reliable
Chapter 4: Scheduling
Dhamdhere: Operating Systems—
A ConceptBased Approach, 2 ed
Slide No: 15
Copyright © 2008
Preemptive scheduling policies
•
Principle
– Preempt a process to permit other processes to operate
* Aimed at improving throughput and response times
•
Scheduling policies
– Round-robin with time slicing (RR)
– Least completed next (LCN)
* Select the process that has received least amount of CPU time
– Shortest time to go (STG)
* Select the process with least remaining service time
Chapter 4: Scheduling
Dhamdhere: Operating Systems—
A ConceptBased Approach, 2 ed
Slide No: 16
Copyright © 2008
Scheduling using preemptive scheduling policies
Arrives Service
time
P1 0 3
P2 2 3
P3 3 2
P4 5 5
P5 9 3
Chapter 4: Scheduling
Dhamdhere: Operating Systems—
A ConceptBased Approach, 2 ed
Slide No: 17
Copyright © 2008
Operation of preemptive scheduling policies
Notes:
• In LCN, n in Pi:n
is CPU time
provided to Pi
• In STG, n in Pi:n
is remaining service
time of Pi
Chapter 4: Scheduling
Dhamdhere: Operating Systems—
A ConceptBased Approach, 2 ed
Slide No: 18
Copyright © 2008
Performance of preemptive scheduling policies
• Column C shows the completion time of a process
• ta is the turnaround time and w is the weighted turnaround
Chapter 4: Scheduling
Dhamdhere: Operating Systems—
A ConceptBased Approach, 2 ed
Slide No: 19
Copyright © 2008
Variation of average response time with time slice
• Response time = n x (time slice + scheduling overhead)
• The response time is larger for time slice of 5 msec than for 10 msec
because of the scheduling overhead
Chapter 4: Scheduling
Dhamdhere: Operating Systems—
A ConceptBased Approach, 2 ed
Slide No: 20
Copyright © 2008
Preemptive scheduling policies
•
Comparison of preemptive scheduling policies
– Performance
* Depends on characteristics of a workload
– Starvation
* The STG policy suffers from starvation
Chapter 4: Scheduling
Dhamdhere: Operating Systems—
A ConceptBased Approach, 2 ed
Slide No: 21
Copyright © 2008
Long, medium, and short-term scheduling
•
A single scheduler cannot provide the desired
combination of performance and user service, so an OS
uses three schedulers
– Long-term scheduler
* Decides when to admit an arrived process
Uses nature of a process, availability of resources to decide
– Medium-term scheduler
* Performs swapping
Maintains a sufficient number of processes in memory
– Short-term scheduler
* Decides which ready process should operate on the CPU
Chapter 4: Scheduling
Dhamdhere: Operating Systems—
A ConceptBased Approach, 2 ed
Slide No: 22
Copyright © 2008
Event handling and scheduling
• An event handler passes control to the long or mediumterm scheduler
• These schedulers pass control to the shortterm scheduler
Chapter 4: Scheduling
Dhamdhere: Operating Systems—
A ConceptBased Approach, 2 ed
Slide No: 23
Copyright © 2008
Scheduling in a time sharing system
The medium term scheduler swaps blocked and ready processes and
changes their states accordingly
Chapter 4: Scheduling
Dhamdhere: Operating Systems—
A ConceptBased Approach, 2 ed
Slide No: 24
Copyright © 2008
Scheduling mechanisms and policy modules
•
The scheduling policy invokes the scheduling
mechanisms when necessary; the mechanisms access
scheduling data
– Scheduling mechanisms
* Process dispatching
* Context save
* Priority computation and reordering
– Data structures
* Scheduling policy uses PCBs; mechanisms update PCBs
Chapter 4: Scheduling
Dhamdhere: Operating Systems—
A ConceptBased Approach, 2 ed
Slide No: 25
Copyright © 2008
