Download AOSScheduling

Advanced Operating
Systems
Prof. Muhammad Saeed
The part of the operating
system that makes the choice of
the process is called the scheduler,
and the algorithm it uses is called
the scheduling algorithm.
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The part of the operating
system that makes the choice of
the process is called the scheduler,
and the algorithm it uses is called
the scheduling algorithm.
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The separation of mechanism and policy
is a design principle in computer science. It
states that mechanisms (those parts of a
system implementation that control the
authorization of operations and the
allocation of resources) should not dictate
(or overly restrict) the policies according
to which decisions are made about which
operations to authorize, and which
resources to allocate.
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Scheduling the processor among all ready
processes
The goal is to achieve:
High processor utilization
High throughput
number of processes completed per of
unit time
Low response time
time elapsed from the submission of a
request until the first response is
produced
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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?
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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
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
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Selects from among ready processes in memory
which one is to execute next
The 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
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The selection function determines which
ready process is selected next for execution
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
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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
The dispatch latency must be minimal
Context Switching: Saving and loading registers and memory
maps, updating various tables and lists, flushing and reloading the
memory cache, and so on.
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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 process terminates on a CPU burst
CPU-bound processes have longer CPU
bursts than I/O-bound processes
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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
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First-Come, First-Served Scheduling
Shortest-Job-First Scheduling
Also referred to as Shortest Process
Next
Priority Scheduling
Round-Robin Scheduling
Multilevel Queue Scheduling
Multilevel Feedback Queue
Scheduling
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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”
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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
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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
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Possibility of starvation for longer
processes
Lack of preemption is not suitable in a
time sharing environment
SJF/SPN implicitly incorporates priorities
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
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If the metric is turnaround time (response time),
is SJF or FCFS better?
For FCFS, resp_time=(3+9+13+18+20)/5 = ?
Note that Rfcfs = 3+(3+6)+(3+6+4)+…. = ?
For SJF, resp_time=(3+9+11+15+20)/5 = ?
Note that Rfcfs = 3+(3+6)+(3+6+4)+…. = ?
Which one is smaller? Is this always the case?
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Take each scheduling discipline, they both choose
the same subset of jobs (first k jobs).
At some point, each discipline chooses a different
job (FCFS chooses k1 SJF chooses k2)
Rfcfs=nR1+(n-1)R2+…+(n-k1)Rk1+….+(n-k2)
Rk2+….+Rn
Rsjf=nR1+(n-1)R2+…+(n-k2)Rk2+….+(n-k1)
Rk1+….+Rn
Which one is smaller? Rfcfs or Rsjf?
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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
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Uses preemption based on a clock
An amount of time is determined that
allows each process to use the
processor for that length of time
Clock interrupt is generated at
periodic intervals
When an interrupt occurs, the
currently running process is
placed in the read queue
Next ready job is selected
Known as time slicing
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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
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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
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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
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Preemptive scheduling with dynamic priorities
N ready to execute queues with decreasing priorities:
P(RQ0) > P(RQ1) > ... > P(RQN)
Dispatcher selects a process for execution from RQi
only if RQi-1 to RQ0 are empty
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
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Different RQs may have different quantum values
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Give processes lottery tickets for various system resources,
such as CPU time. Whenever a scheduling decision has to be made,
a lottery ticket is chosen at random, and the process holding that
ticket gets the resource.
When applied to CPU scheduling, the system might hold a lottery
50 times a second, with each winner getting 20 msec of CPU time
as a prize.
More important processes can be given extra tickets, to increase
their odds of winning. If there are 100 tickets outstanding, and one
process holds 20 of them, it will have a 20% chance of winning each
lottery. In the long run, it will get about 20% of the CPU. In contrast
to a priority scheduler, where it is very hard to state what having a
priority of 40 actually means, here the rule is clear: a process
holding a fraction f of the tickets will get about a fraction f of the
resource in question.
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