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Operating Systems
Lecture 4
CPU Scheduling
&
Deadlock
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Process Management
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Concept of a Process
Context-Change
Process Life Cycle
Process Creation
Process Spawning
OS1 - Lecture 2 – Scheduling – Paul Flynn
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Process State Diagrams
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Three State Model
 Ready
 Running
 Blocked
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Five State Model
 Ready
 Running
 Blocked
 Ready
Suspended
 Blocked Suspended
OS1 - Lecture 2 – Scheduling – Paul Flynn
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CPU Scheduling
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Scheduling the processor among all ready
processes
The goal is to achieve:
 High
processor utilization
 High throughput
 number
 Low
of processes completed per of unit time
response time
 time
elapsed from the submission of a request
until the first response is produced
OS1 - Lecture 2 – Scheduling – Paul Flynn
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Classification of Scheduling Activity
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Long-term: which process to admit?
Medium-term: which process to swap in or out?
Short-term: which ready process to execute next?
OS1 - Lecture 2 – Scheduling – Paul Flynn
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Queuing Diagram for Scheduling
OS1 - Lecture 2 – Scheduling – Paul Flynn
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Long-Term Scheduling
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Determines which programs are admitted
to the system for processing
Controls the degree of multiprogramming
Attempts to keep a balanced mix of
processor-bound and I/O-bound processes
 CPU
usage
 System performance
OS1 - Lecture 2 – Scheduling – Paul Flynn
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Medium-Term Scheduling
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Makes swapping decisions based on the
current degree of multiprogramming
 Controls
which remains resident in memory
and which jobs must be swapped out to reduce
degree of multiprogramming
OS1 - Lecture 2 – Scheduling – Paul Flynn
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Short-Term Scheduling
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Selects from among ready processes in
memory which one is to execute next
 The
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selected process is allocated the CPU
It is invoked on events that may lead to
choose another process for execution:
Clock interrupts
I/O interrupts
Operating system calls and traps
Signals
OS1 - Lecture 2 – Scheduling – Paul Flynn
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Characterization of Scheduling Policies
The selection function determines which ready
process is selected next for execution
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The decision mode specifies the instants in time the
selection function is exercised
 Nonpreemptive
 Once a process is in the running state, it will
continue until it terminates or blocks for an I/O
 Preemptive
 Currently running process may be interrupted and
moved to the Ready state by the OS
 Prevents one process from monopolizing the
processor
OS1 - Lecture 2 – Scheduling – Paul Flynn
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Short-Term Scheduler
Dispatcher
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The dispatcher is the module that gives
control of the CPU to the process selected
by the short-term scheduler
The functions of the dispatcher include:
 Switching
context
 Switching to user mode
 Jumping to the location in the user program to
restart execution
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The dispatch latency must be minimal
OS2 - Lecture 2 – Scheduling – Paul Flynn
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The CPU-I/O Cycle
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Processes require alternate use of
processor and I/O in a repetitive fashion
Each cycle consist of a CPU burst followed
by an I/O burst
A
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process terminates on a CPU burst
CPU-bound processes have longer CPU
bursts than I/O-bound processes
OS1 - Lecture 2 – Scheduling – Paul Flynn
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Short-Term Scheduling Criteria
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User-oriented criteria
 Response Time: Elapsed time between the
submission of a request and the receipt of a
response
 Turnaround Time: Elapsed time between the
submission of a process to its completion
System-oriented criteria
 Processor utilization
 Throughput: number of process completed per unit
time
 fairness
OS1 - Lecture 2 – Scheduling – Paul Flynn
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Scheduling Algorithms
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First-Come, First-Served Scheduling
Shortest-Job-First Scheduling
 Also
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referred to as Shortest Job Next
Highest Response Ratio Next (HRN)
Shortest Remaining Time (SRT)
Round-Robin Scheduling
Multilevel Feedback Queue Scheduling
OS1 - Lecture 2 – Scheduling – Paul Flynn
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Process Mix Example
Process
Arrival
Time
Service
Time
1
0
3
2
2
6
3
4
4
4
6
5
5
8
2
Service time = total processor time needed in one (CPU-I/O) cycle
Jobs with long service time are CPU-bound jobs and are referred
to as “long jobs”
OS1 - Lecture 2 – Scheduling – Paul Flynn
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First Come First Served (FCFS)
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Selection function: the process that has
been waiting the longest in the ready
queue (hence, FCFS)
Decision mode: non-preemptive
a
process runs until it blocks for an I/O
OS1 - Lecture 2 – Scheduling – Paul Flynn
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FCFS drawbacks
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Favors CPU-bound processes
 A CPU-bound process monopolizes the processor
 I/O-bound processes have to wait until completion
of CPU-bound process

I/O-bound processes may have to wait even after
their I/Os are completed (poor device utilization)
 Better
I/O device utilization could be achieved if
I/O bound processes had higher priority
OS1 - Lecture 2 – Scheduling – Paul Flynn
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Shortest Job First (Shortest Process Next)
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Selection function: the process with the shortest
expected CPU burst time
 I/O-bound processes will be selected first
Decision mode: non-preemptive
The required processing time, i.e., the CPU burst
time, must be estimated for each process
OS1 - Lecture 2 – Scheduling – Paul Flynn
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SJF / SPN Critique
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Possibility of starvation for longer processes
Lack of preemption is not suitable in a time
sharing environment
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SJF/SPN implicitly incorporates priorities
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Shortest jobs are given preferences
 CPU bound process have lower priority, but a
process doing no I/O could still monopolize the
CPU if it is the first to enter the system

OS1 - Lecture 2 – Scheduling – Paul Flynn
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Highest Response Ratio Next (HRN)
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Based on SJF with formula introduced
Priority Based - P
Time Waiting + Run Time / Run Time = P
The process with the HIGHEST Priority will
be selected for running
Non-Preemptive
Reduces SJF bias against Short Jobs
OS1 - Lecture 2 – Scheduling – Paul Flynn
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Priorities
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Implemented by having multiple ready
queues to represent each level of priority
Scheduler the process of a higher priority
over one of lower priority
Lower-priority may suffer starvation
To alleviate starvation allow dynamic
priorities
 The
priority of a process changes based on its
age or execution history
OS1 - Lecture 2 – Scheduling – Paul Flynn
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Round-Robin
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Selection function: same as FCFS
Decision mode: preemptive
a process is allowed to run until the time slice period
(quantum, typically from 10 to 100 ms) has expired
 a clock interrupt occurs and the running process is put
on the ready queue

OS1 - Lecture 2 – Scheduling – Paul Flynn
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RR Time Quantum
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Quantum must be substantially larger than
the time required to handle the clock
interrupt and dispatching
Quantum should be larger then the typical
interaction
 but
not much larger, to avoid penalizing I/O
bound processes
OS1 - Lecture 2 – Scheduling – Paul Flynn
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Round Robin: critique
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Still favors CPU-bound processes
 An I/O bound process uses the CPU for a time less
than the time quantum before it is blocked waiting for
an I/O
 A CPU-bound process runs for all its time slice and is
put back into the ready queue
 May unfairly get in front of blocked processes
OS1 - Lecture 2 – Scheduling – Paul Flynn
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Multilevel Feedback Scheduling
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Preemptive scheduling with dynamic
priorities
N ready to execute queues with decreasing
priorities:
Dispatcher selects a process for execution
from RQi only if RQi-1 to RQ0 are empty
OS1 - Lecture 2 – Scheduling – Paul Flynn
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Multilevel Feedback Scheduling
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New process are placed in RQ0
After the first quantum, they are moved to
RQ1 after the first quantum, and to RQ2
after the second quantum, … and to RQN
after the Nth quantum
I/O-bound processes remain in higher
priority queues.
 CPU-bound jobs drift downward.
 Hence, long jobs may starve
OS1 - Lecture 2 – Scheduling – Paul Flynn
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Multiple Feedback Queues
Different RQs may have different quantum values
OS1 - Lecture 2 – Scheduling – Paul Flynn
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Algorithm Comparison
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Which one is the best?
The answer depends on many factors:
 the
system workload (extremely variable)
 hardware support for the dispatcher
 relative importance of performance criteria
(response time, CPU utilization, throughput...)
 The evaluation method used (each has its
limitations...)
OS1 - Lecture 2 – Scheduling – Paul Flynn
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Deadlocks
3.1. Resource
3.2. Introduction to deadlocks
3.3. The ostrich algorithm
3.4. Deadlock detection and recovery
3.5. Deadlock avoidance
3.6. Deadlock prevention
3.7. Other issues
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Deadlock Recap
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Process is deadlocked if it is waiting for an
event that will never occur
Most common situation is where two
processes are involved on is holding
resource required by the other and also
looking for resource held by the other
process.
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Deadlock Example
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Airline booking example. Alan wants to
books seat on flight AB123 so locks this
file. Same time Brian wants to book flight
on AB456 and locks this file. Alan wants to
book return flight on AB456 and Brian
wants to book return flight on AB123. No
each user as a file locked and is requesting
a file locked by the other.
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Deadlock Example cont
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Alan locks AB123
Brian lock AB456
Alan requests AB456
Brian requests AB123
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Conditions for deadlock
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There are 4 conditions necessary for
deadlock
 Mutual
exclusion
 Resource holding
 No premption
 Circular wait
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Conditions explained
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Mutual Exclusion only one process can
use a resource at a time
Resource holding process can hold a
resource while requesting another
No premption resources cannot be forcibly
removed from a process
Circular wait closed circle exists where
each process is holding a resource
required by another.
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Dealing with deadlock
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Three ways of dealing with deadlocks
 Deadlock
prevention
 Deadlock avoidance
 Deadlock detection
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Deadlock prevention
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Prevent any one of the 4 conditions from
occuring will prevent deadlock
Mutual exclusion – cannot prevent
Resource holding – to prevent this a
process must be allocated all it resources
at once called one shot allocation very
inefficient. Process may have to wait for
resources it might not need. Process may
hold resources for long time without using.
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Deadlock prevention cont
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No premption to deny this condition two
ways
 If
a process is holding a resource requests
another it could be forces to give up the
resource it is holding.
 A resources required by a process and held by
a second could be forcibly removed from the
second. Not possible with serially reusable
resources.
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Deadlock prevention
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Circular wait – this condition can be
prevented if resources are organsied in a
particual order and require that resources
follow an order.
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Deadlock avoidance
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Deadlock prevention means that deadlock
will not occur due to fact that we deny one
of the 4 conditions necessary. Innefficient
Deadlock avoidance attempts to predict
the possibility of deadlock as each
resources request is made.
Example if process A requests a resource
held by process B then make sure that
process B is not waiting for resource held
by A
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Bankers Algorithm
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The most common method of deadlock
avoidance is to use the bankers algorithm.
Uses banking anology, banker will only
grant loan if he can meet the needs of
customers based on their projected future
loan requirements.
Example three processes P1,P2 & P3 and
10 resources available. Table shows
requirements
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Example cont.
Process Max need Current usage
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P1
P2
P3
8
5
8
3
1
2
Total maximum needs =21 means that allocation cannot be met at one time. We need a sequence
of allocations which will allow all processes to finish. If we start with P2 when it is finished it
will release 5 resources. Next if we allow P1 to run it will release 8 resources and so P3 will be
able to finish.
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Problems with deadlock avoidance
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Each process has to pre-declare its
maximum rersource requirements. This is
not realistic for interactive systems.
The avoidance algorithm must be executed
every time a resource request is made. For
a multi-user system with a large number of
user processes, the processing overhead
would be severe.
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Deadlock detection
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A deadlock detection strategy accepts the
risk of deadlock occuring and periodically
executes a procedure to detect any in
place.
Breaking a deadlock implies that a process
must be aborted or resources prempted
from processes, either could result in loss
of work.
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