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Operating Systems
Part of E1.9 - “Principles of Computers and Software
Engineering”
Lecture 4: Process scheduling
Objectives
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To introduce process scheduling algorithms
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Non-priority based
Priority based
Examine their advantages and disadvantages
E1.9 - Operating systems (Lect 4)
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Concepts
(1)
Bursts
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Processes consist of CPU and I/O bursts.
Majority of processes alternate between the two.
CPU-bound processes spend most of their time
computing, while I/O-bound spend most of their time
waiting for I/O.
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Concepts
(2)
Scheduler and interrupts
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Short-term scheduling is invoked by interrupts
“ Timer interrupts: they are set by the OS (to regain
control regularly)
“ Hardware interrupts
“ System call interrupts
Short-term scheduling is invoked to decide which
process (from the ready queue) will be dispatched to
the processor
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Concepts
(3)
Preemptive and non-preemptive scheduling
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A non-preemptive scheduling algorithm selects a
process and allows it to run until:
“ It blocks (e.g. asks for I/O)
“ It voluntarily releases the CPU
“ It terminates
Requires cooperative processes and users
A preemptive scheduling algorithm selects a process
and allows it to run for a maximum of some fixed
time. Uses the timer interrupts to regain control
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Concepts
(4)
Short-term Scheduling criteria
Vary according to particular needs, but usually
“ Maximize:
“ CPU utilization
“ Throughput, i.e. number of processes completed per unit of
time
“ Fairness
“ Minimize:
“ Response time (time from submission of request till first
response)
“ Waiting time (time a process is waiting for the CPU)
“ Turnaround time (time it takes a process to complete)
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Scheduling algorithms
(1)
Overview (1)
We will examine the following algorithms:
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First-come, first served (FCFS)
Shortest job first (SJF)
Shortest remaining job first (SRJF)
Round-robin (RR)
Priority scheduling (PS)
Multilevel queue scheduling (MQS)
Multilevel feedback queue scheduling (MFQS)
E1.9 - Operating systems (Lect 4)
Non
priority
Priority
based
7
Scheduling algorithms
(1)
Overview (2)
We will use the following example in order to compare the algorithms:
Process
A
B
C
D
E
Processing
Time
3s
6s
4s
5s
2s
Arrival Time
0s
2s
4s
6s
8s
Example and Gantt charts courtesy of Daniel Ruckert
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Scheduling algorithms
(2)
First-come, first-served (1)
Algorithm description
“ CPU is allocated to the process that requests it first
“ Advantages
“ Simple implementation (FIFO queue)
“ Disadvantages
“ Can result in long waits for short processes, and for I/Obound processes
“
E1.9 - Operating systems (Lect 4)
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Scheduling algorithms
(2)
First-come, first-served (2)
Process A
(3s)
Process B
(6s)
Process C
(4s)
Process D
(5s)
Process E
(2s)
5s
10 s
15 s
20 s
Average Waiting time = (0 + 1 + 5 + 7 + 10) / 5 = 4.6s
Average Turnaround time = (3 + 7 + 9 + 12 + 12) / 5 = 8.6s
E1.9 - Operating systems (Lect 4)
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Scheduling algorithms
(3)
Shortest Job First (1)
Algorithm description
“ CPU is allocated to the process that has the shortest next
CPU burst
“ Advantages
“ Provably optimal in that it gives the minimum average
waiting time for a given set of processes
“ Disadvantages
“ Knowing the length of next CPU burst is difficult; frequently
this is predicted utilising previous lengths as estimates, or is
user-specified.
“ Can result in long waits for long processes
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E1.9 - Operating systems (Lect 4)
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Scheduling algorithms
(3)
Shortest Job First (2)
Process A
(3s)
Process B
(6s)
Process C
(4s)
Process D
(5s)
Process E
(2s)
5s
10 s
Average Waiting time = (0 + 1 + 7 + 9 + 1) / 5 = 3.6s
Average Turnaround time = (3 + 7 + 11 + 14 + 3) / 5 = 7.6s
E1.9 - Operating systems (Lect 4)
15 s
20 s
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Scheduling algorithms
(4)
Shortest Remaining Job First (1)
Algorithm description
“ Variation of SJF, adding preemption; CPU again is allocated
to the process that has the shortest next CPU burst; if a new
process comes along that has a shorter CPU burst than the
remaining one in the running process, the running process is
removed and the new process is allocated the CPU.
“ Advantages
“ Allows new short jobs to get a good service
“ Good handling of interactive processes since it results in a
short response time
“ Disadvantages
“ Same as SJF
“ Plus, this algorithm can be particularly bad for long
processes (starvation).
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E1.9 - Operating systems (Lect 4)
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Scheduling algorithms
(4)
Shortest Remaining Job First (2)
Process A
(3s)
Process B
(6s)
Process C
(4s)
Process D
(5s)
Process E
(2s)
5s
10 s
Average Waiting time = (0 + 7 + 0 + 9 + 0) / 5 = 3.2s
Average Turnaround time = (3 + 13 + 4 + 14 + 2) / 5 = 7.2s
E1.9 - Operating systems (Lect 4)
15 s
20 s
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Scheduling algorithms
(5)
Round Robin (1)
Algorithm description
“ The CPU is allocated to a process for a time quantum (or
time-slice); if process is still running at the end of the
quantum, CPU is preempted and given to the next process.
Preempted process is put at the end of the queue. New
processes are added at the end of the queue.
“ Advantages
“ Simple to implement and fair
“ Disadvantages
“ Difficult to determine appropriate time quantum:
“ Too small: good response time, but large overheads
(scheduler is called too often)
“ Too large: bad response time
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E1.9 - Operating systems (Lect 4)
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Scheduling algorithms
(5)
Round Robin (2)
Time-slice of 1s
Process A
(3s)
Process B
(6s)
Process C
(4s)
Process D
(5s)
Process E
(2s)
5s
10 s
15 s
20 s
Average Waiting time = (1 + 10 + 9 + 9 + 5) / 5 = 6.8s
Average Turnaround time = (4 + 16 + 13 + 14 + 7) / 5 = 10.8s
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Scheduling algorithms
(5)
Round Robin (3)
Time-slice of 4s
Process A
(3s)
Process B
(6s)
Process C
(4s)
Process D
(5s)
Process E
(2s)
0
2s
5s
8s
10 s
Average Waiting time = (0 + 9 + 3 + 9 + 9) / 5 = 6 s
Average Turnaround time = (3 + 15 + 7 + 14 + 11) / 5 = 10 s
E1.9 - Operating systems (Lect 4)
15 s
20 s
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Scheduling algorithms
(5)
Comparison
Algorithm
Average waiting
time
Average
turnaround time
Type
FCFS
4.6 s
8.6 s
Nonpreemptive
SJF
3.6 s
7.6 s
Nonpreemptive
SRJF
3.2 s
7.2 s
Preemptive
RR
6s
10 s
Preemptive
6.8 s
10.8 s
Preemptive
(Time slice of 4s)
RR
(Time slice of 1s)
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Scheduling algorithms
(6)
Priority-based algorithms
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Algorithms shown so far assume all processes are of
equal importance.
Frequently, we need to incorporate levels of priority
to the different processes that request the CPU to
indicate their relative importance.
We will examine:
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Priority scheduling
Multilevel queue scheduling
Multilevel feedback queue scheduling
E1.9 - Operating systems (Lect 4)
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Scheduling algorithms
(7)
Priority scheduling (1)
Algorithm description
“ A priority is associated with each process; CPU is allocated
to the process with the highest priority. FCFS is used to
resolve situations involving processes with equal priority.
“ There are two versions, preemptive and non-preemptive.
“ Priority can be static, or dynamic.
“ Advantages
“ Takes into account external factors regarding the importance
of the various processes
“ Disadvantages
“ Might result in starvation of low-priority processes – this can
be avoided using an aging procedure; processes that wait
for too long have their priorities gradually increased.
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E1.9 - Operating systems (Lect 4)
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Scheduling algorithms
(7)
Priority scheduling (2)
The following priorities are added to demonstrate this algorithm
(smaller numbers indicate higher priority):
Process
A
B
C
D
E
Processing
Time
3s
6s
4s
5s
2s
Arrival
Time
0s
2s
4s
6s
8s
E1.9 - Operating systems (Lect 4)
Priority
3
5
4
2
0
21
Scheduling algorithms
(7)
Priority scheduling (3)
Non-preemptive version
Process A
(3s) – Priority 3
Process B
(6s) – Priority 5
Process C
(4s) – Priority 4
Process D
(5s) – Priority 2
Process E
(2s) – Priority 0
5s
10 s
Average Waiting time = (0 + 1 + 13 + 5 + 1) / 5 = 4 s
Average Turnaround time = (3 + 7 + 16 + 10 + 3) / 5 = 7.8 s
E1.9 - Operating systems (Lect 4)
15 s
20 s
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Scheduling algorithms
(7)
Priority scheduling (4)
Preemptive version
Process A
(3s) – Priority 3
Process B
(6s) – Priority 5
Process C
(4s) – Priority 4
Process D
(5s) – Priority 2
Process E
(2s) – Priority 0
5s
10 s
Average Waiting time = (0 + 12+ 7 + 2 + 0) / 5 = 4.2 s
Average Turnaround time = (3 + 18 + 11 + 7 + 2) / 5 = 8.4 s
E1.9 - Operating systems (Lect 4)
15 s
20 s
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Scheduling algorithms
(8)
Multilevel queue scheduling (MQS)
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Algorithm description
“ Ready processes are divided into several separate queues; each
queue has a priority associated with it. Queues with higher priority
can be given more time-slices for each process. Different
scheduling algorithms can be used for scheduling within queues.
“ E.g. separate ready-queue in foreground (for interactive processes)
and background (for batch processes) queues. Assign low priority
to background queue and high to foreground queue. Implement RR
on the foreground queue and FCFS to background queue.
Advantages
“ Flexible; allows more fine control of scheduling
Disadvantages
“ Does not allow for possibility of processes that change
requirements throughout their execution (e.g. a process that started
with a long CPU-burst, but requires interaction later – such process
will be penalised indefinitely)
E1.9 - Operating systems (Lect 4)
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Scheduling algorithms
(9)
Multilevel feedback queue scheduling (MFQS)
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Algorithm description
“ Same with MQS, but processes are allowed to move between queues
“ For example, if a process uses too much CPU it will be moved to a
lower-priority queue; if a process is waiting for too long in a lower
priority queue, it can moved to a higher priority queue.
Advantages
“ Same with MQS, with the added adaptability that feedback brings
“ This is the most general scheme you can have
Disadvantages
“ It is also the most complex – have to decide on several issues:
“ Number of queues and the scheduling algorithm for each queue
“ Method used to determine how to promote or demote a process
“ Method used to determine which queue a process will enter
“ Tuning of several parameters will be required to define the best
scheduler
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Summary
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Several process scheduling algorithms were
presented
Can be divided in non-priority and priority-based
algorithms
Their characteristics (incl. average waiting time and
average turnaround time) were examined
Next lecture:
Process synchronization
E1.9 - Operating systems (Lect 4)
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Recommended Reading
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Read chapter 6 of Silberschatz’s book
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Read chapter 2, section 2.5 of Tanenbaum’s book
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Read chapter 9 of Stallings’ book
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