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Mi-Jung [email protected]
Dept. of Computer and Science
Silberschatz, Galvin and Gagne ©2006Operating System Principles
Chapter 5: Process Scheduling
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Ch05 - Process Scheduling
Chapter 5: Process Scheduling Basic Concepts Scheduling Criteria Scheduling Algorithms
Multiple-Processor Scheduling Real-Time Scheduling Thread Scheduling
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Basic Concepts Process Scheduling is
the basis of multi-programmed operating system
Terminology CPU scheduling, Process scheduling, Kernel Thread scheduling used interchangeably, we use process scheduling
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Basic Concepts In a single-processor system
Only one process can run at a time Any others must wait until the CPU is free and can be rescheduled. When the running process goes to the waiting state,
the OS may select another process to assign CPU to improve CPU utilization.
Every time one process has to wait, another process can take over use of the CPU
Process scheduling is a fundamental function of operating-system. to select a process from the ready queue and assign the CPU
Maximum CPU utilization obtained with multiprogramming by process scheduling
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Diagram of Process State from ch.3
It is important to realize that only one process can be running on any processor at any instant.
Many processes may be ready and waiting states.
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Process Scheduling from ch.3
Two types of queues: one ready queue, a set of device queues Two types of resources: CPU, I/O
Arrow indicates the flow of processes in the system
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CPU - I/O burst Cycle Process execution consists of
a cycle of CPU execution (CPU burst) and I/O wait (I/O burst)
Process alternate between these two states Process execution begins with a CPU burst, which is followed by
an I/O burst, and so on. Eventually, the final CPU burst ends with an system call to
terminate execution.
CPU burst distribution of a process varies greatly from process to process and from computer to
computer
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Alternating Sequence of CPU & I/O Bursts
CPU burst time
I/O burst time
CPU burst time
CPU burst time
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Histogram of CPU-burst Times
CPU burst distribution is generally characterized as exponential or hyper-exponential with large number of short CPU burst and small number of long CPU burst
I/O bound process has many short CPU bursts CPU bound process might have a few long CPU bursts.
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Process Scheduler is one of OS modules. selects one of the processes in memory that are ready to
execute, and allocates the CPU to the selected process.
CPU scheduling decisions may take place when a process:1. switches from running to waiting state: I/O request, invocation of
wait() for the termination of other process2. switches from running to ready state: when interrupt occurs3. switches from waiting to ready: at completion of I/O4. terminates
Scheduling under 1 and 4 is non-preemptive Scheduling under 2 and 3 is preemptive
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Non-preemptive vs. Preemptive Non-preemptive scheduling
Once the CPU has been allocated to a process, the process keeps the CPU until it releases the CPU
either by terminating or by switching to the waiting state. used by Windows 3.x
Preemptive scheduling Current running process can be switched with another at any time
because interrupt can occur at any time Most of modern OS provides this scheme. (Windows XP, Max OS, UNIX) incurs a cost associated with access to shared data among processes affects the design of the OS kernel
Certain OS (UNIX) waits either for a system call to complete or for an I/O block to take place before doing a context switch.
protects critical kernel code by disabling and enabling the interrupt at the entry and exit of the code.
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Dispatcher Dispatcher module is a part of a Process Scheduler
gives control of the CPU to the process selected by the short-term scheduler; this involves: switching context switching to user mode jumping to the proper location in the user program to restart that
program
Dispatch latency – time it takes for the dispatcher to stop one process and start another running
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Context Switch from ch. 3
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Scheduling Criteria Based on the scheduling criteria, the performance of various
scheduling algorithm could be evaluated. Different scheduling algorithms have different properties.
CPU utilization – ratio (%) of the amount of time while the CPU was busy per time unit.
Throughput – # of processes that complete their execution per time unit.
Turnaround time – the interval from the time of submission of a process to the time of completion. Sum of the periods spent waiting to get into memory, waiting in the ready queue, executing on the CPU, and doing I/O
Waiting time – Amount of time a process has been waiting in the ready queue, which is affected by scheduling algorithm
Response time – In an interactive system, amount of time it takes from when a request was submitted until the first response is produced, not output (for time-sharing environment)
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Optimization Criteria It is desirable to maximize:
The CPU utilization The throughput
It is desirable to minimize: The turnaround time The waiting time The response time
However in some circumstances, it is desirable to optimize the minimum or maximum values rather than the average. Interactive systems, it is more important to minimize the variance
in the response time than minimize the average response time.
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Process Scheduling Algorithms First-Come, First-Served Scheduling (FCFS) Shortest-Job-First Scheduling (SJF) Priority Scheduling Round-Robin Scheduling
Our measure of comparison is the average waiting time. CPU utilization, Throughput, Turn arround time,
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First-Come, First-Served (FCFS) SchedulingProcess Burst Time
P1 24
P2 3
P3 3
Suppose that the processes arrive in the order: P1 , P2 , P3
The Gantt Chart for the schedule is:
Waiting time for P1 = 0; P2 = 24; P3 = 27 Average waiting time: (0 + 24 + 27)/3 = 17
P1 P2 P3
24 27 300
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FCFS SchedulingSuppose that the processes arrive in the order
P2 , P3 , P1
The Gantt chart for the schedule is:
Waiting time for P1 = 6; P2 = 0; P3 = 3
Average waiting time: (6 + 0 + 3)/3 = 3
Much better than previous case
P1P3P2
63 300
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FCFS Scheduling
P0
P1
P2
CPU (10) I/O (20) CPU(11)
CPU(6) I/O(17) CPU(9)
CPU(4) I/O(4) CPU (4)
0 10 20 30 40 50
I/O(24) CPU (4)
P0 CPU(10)P2
CPU(4)P1
CPU(6)P2
CPU(4)P0
I/O(6) P0 CPU(11) P1 CPU(9) P2 CPU(4)
10 14 20 24 30 41 50 54
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FCFS Scheduling FCFS scheduling algorithm is non-preemptive
Once the CPU has been allocated to a process, that process keeps the CPU until it releases the CPU, either by terminating or by requesting I/O.
is particularly troublesome for time-sharing systems.
Convoy effect occurs. When one CPU-bound process with long CPU burst and many
I/O-bound process which short CPU burst. All I/O bound process waits for the CPU-bound process to get off
the CPU while I/O is idle All I/O- and CPU- bound processes executes I/O operation while
CPU is idle. results in low CPU and device utilization
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Shortest-Job-First (SJF) Scheduling associate with each process the length of its next CPU
burst. use these lengths to schedule the process with the
shortest time
Two schemes: non-preemptive – once CPU given to the process it cannot be
preempted until completes its CPU burst preemptive – if a new process arrives with CPU burst length less
than remaining time of current executing process, preempt. This scheme is known as the Shortest-Remaining-Time-First (SRTF)
SJF is optimal – gives minimum average waiting time for a given set of processes
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Process Arrival Time Burst Time
P1 0.0 7
P2 2.0 4
P3 4.0 1
P4 5.0 4
SJF (non-preemptive)
Average waiting time = (0 + 6 + 3 + 7)/4 = 4
Example of Non-Preemptive SJF
P1 P3 P2
73 160
P4
8 12
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Example of Preemptive SJFProcess Arrival Time Burst Time
P1 0.0 7
P2 2.0 4
P3 4.0 1
P4 5.0 4
SJF (preemptive)
Average waiting time = (9 + 1 + 0 +2)/4 = 3
P1 P3P2
42 110
P4
5 7
P2 P1
16
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Preemptive SJF Scheduling
P0
P1
P2
CPU (10) I/O (20) CPU(11)
CPU(6) I/O(17) CPU(9)
CPU(4) I/O(4) CPU (4)
0 10 20 30 40 50
I/O(24) CPU (4)
P0 CPU(1)
P2 CPU(4)
P1 CPU(6)
P2 CPU(4)
P0 CPU(9)
P1 I/O(4) P1 CPU(9) P2
I/O(2)P2
CPU(4)P0
I/O(1)P0
CPU(11)
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Non-preemptive SJF Scheduling
P0
P1
P2
CPU (10) I/O (20) CPU(11)
CPU(6) I/O(17) CPU(9)
CPU(4) I/O(4) CPU (4)
0 10 20 30 40 50
I/O(24) CPU (4)
P0 CPU(10)P2
CPU(4)P1
CPU(6)P2
CPU(4) Idle (6) P0 CPU(11) P1 CPU(9) P2 CPU(4)
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Determining Length of Next CPU Burst can only estimate the length can be done by using the length of previous CPU bursts,
using exponential averaging
The value of tn contains our most recent information. n+1 stores the past history The parameter controls the relative weight of recent and
past history in our prediction.
.1 :Define 4.
10 number, real a is 3.
burst CPUnext for the valuepredicted 2.
burst CPU oflenght actual 1.
1
1
nnn
n
thn
t
nt
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Prediction of the Length of the Next CPU Burst
In this example, 0 = 10, = ½ 1 = x t0 + (1- ) x 0 = ½ x 6 + ½ x 10 = 8 2 = x t2 + (1- ) x 2 = ½ x 4 + ½ x 8 = 6
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Examples of Exponential Averaging = 0
n+1 = n = n-1 = n-2 . … = 0
Recent history does not count = 1
n+1 = tn
Only the actual last CPU burst counts
If we expand the formula, we get:n+1 = tn + (1 - ) tn - 1 + …
+ (1 - )j tn -j + …
+ (1 - )n +1 0
Since both and (1 - ) are less than or equal to 1, each successive term has less weight than its predecessor
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Priority Scheduling A priority number (integer) is associated with each process The CPU is allocated to the process with the highest
priority (smallest integer highest priority) Preemptive Non-preemptive
SJF is a priority scheduling where priority is the predicted next CPU burst time
Problem Starvation – low priority processes may never execute
Solution Aging – as time progresses, increase the priority of the process
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Process Burst Time Priority
P1 10 3
P2 1 1
P3 2 4
P4 1 5
P5 5 2
Priority Scheduling (non-preemptive)
Average waiting time = (0 + 1 + 6 + 16 + 18)/5 = 8.2
Example of Non-Preemptive Priority
P2 P1 P3
1610
P4
18
P5
6 19
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Preemptive Priority Scheduling
P0(1)
P1(2)
P2(3)
CPU (10) I/O (20) CPU(11)
CPU(6) I/O(17) CPU(9)
CPU(4) I/O(4) CPU (4)
0 10 20 30 40 50
I/O(24) CPU (4)
P0 CPU(10)
P2 CPU(4)
P1 CPU(6)
P2 CPU(4)
Idle: P2(I/O4)
P2 I/O(2)P1 CPU(9)P0
I/O(2)P2
CPU(4)
I/O is same to idle ( 원래 I/O 에는 idle 이 들어가야 합니다 .)
P0 CPU(11)
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Non-preemptive Priority Scheduling
CPU (10) I/O (20) CPU(11)
CPU(6) I/O(17) CPU(9)
CPU(4) I/O(4) CPU (4)
0 10 20 30 40 50
I/O(24) CPU (4)
P0 (1)
P1 (2)
P2 (3)
이 경우는 특수 case 로 preemptive 나 non-preemptive 나 답이 같습니다 . ( 즉 앞장과 답이 같습니다 .)
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Round Robin (RR) Scheduling Each process gets a small unit of CPU time (time quantum),
usually 10-100 milliseconds. After this time has elapsed, the process is preempted and
added to the end of the ready queue.
If there are n processes in the ready queue and the time quantum is q, then each process gets 1/n of the CPU time (= q time units) in chunks of at most n x q time units at once.
No process waits more than (n-1) x q time units.
Performance depends on the size of the time quantum. q large RR is same as FIFO q small provides high concurrency: each of n processes has its
own processor running at 1/n the speed of the real processor
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Example of RR with Time Quantum = 20Process Burst Time
P1 53
P2 17
P3 68
P4 24
The Gantt chart is:
Typically, higher average turnaround than SJF, but better response
P1 P2 P3 P4 P1 P3 P4 P1 P3 P3
0 20 37 57 77 97 117 121 134 154 162
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RR Scheduling
Time Quantum = 2 (9 번째 P1 과 10 번째 P2 는 바뀌어도 상관 없음 ) (20 번째 P0 와 21 번째 P2 도 바뀌어도 상관 없음 )
P0(1)
P1(2)
P2(3)
CPU (10) I/O (20) CPU(11)
CPU(6) I/O(17) CPU(9)
CPU(4) I/O(4) CPU (4)
0 10 20 30 40 50
I/O(24) CPU (4)
P0 CPU(2)
Idle P1 I/O (11)
P2 CPU(2)
P0 CPU(2)
P1 CPU(2)
P2 CPU(2)
P0 CPU(2)
P1 CPU(2)
P0 CPU(2)
P1 CPU(2)
P2 CPU(2)
P0CPU(2)
P2 CPU(2)
P1 CPU(2)
P1 CPU(2)
P1 CPU(2)
P1 CPU(2)
P0CPU(2)
P1 CPU1)
P0CPU(2)
P0CPU(2)
P2CPU(2)
P0CPU(2)
P2CPU(2)
P0CPU(2)
P0CPU(1)
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RR Scheduling
Time Quantum = 5
P0(1)
P1(2)
P2(3)
CPU (10) I/O (20) CPU(11)
CPU(6) I/O(17) CPU(9)
CPU(4) I/O(4) CPU (4)
0 10 20 30 40 50
I/O(24) CPU (4)
P0 CPU(5)
P2 CPU(4)
P1CPU(5)
P0 CPU(5)
P2 CPU(4)
IdleP1 I/O(15)
P0CPU(5)
P0CPU(5)
P2CPU(4)
P1 CPU(1)
P1CPU(5)
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Time Quantum and Context Switch Time
The effect of context switching on the performance of RR scheduling, for example one process of 10 time quantum. quantum = 12 time units, finished in less than 1 time quantum quantum = 6 time units, requires 2 quanta, 1 context switch quantum = 1 time units, requires 10 quanta, 9 context switch
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Round Robin (RR) Scheduling The time quantum q must be large with respect to context
switch, otherwise overhead is too high
If the context switching time is 10% of the time quantum, then about 10% of the CPU time will be spent in context switching
Most modern OS have time quanta ranging from 10 to 100 milliseconds,
The time required for a context switch is typically less than 10 microseconds; thus the context-switch time is a small fraction of the time quantum.
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Turnaround Time varies with the Time Quantum The turnaround time
depends on the size ofthe time quantum
The average turnaroundtime can be improvedif most processes finish their next CPU burst in a single time quantum.
When SJF and RR used If quantum = 6 and 7, average turnaround time = 10.5 If quantum = 1, average turnaround time = 11
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Scheduling Algorithm with multi-Queues Multi-level Queue Scheduling Multi-level Feedback Queue Scheduling
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Multilevel Queue Ready queue is partitioned into separate queues:
foreground (interactive)background (batch)
The processes are permanently assigned to one queue, generally based on some property, or process type.
Each queue has its own scheduling algorithm foreground – RR background – FCFS
Scheduling must be done between the queues Fixed priority scheduling - serve all from foreground then from
background, Possibility of starvation. Time slice scheduling – each queue gets a certain amount of CPU
time which it can schedule amongst its processes; i.e., 80% to foreground in RR, 20% to background in FCFS
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Multilevel Queue Scheduling
No process in the batch queue could run unless the queues with high priority were all empty.
If an interactive editing process entered the ready queue while a batch process was running, the batch process would be preempted.
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Multi-level Queue Scheduling
CPU (10) I/O (20) CPU(11)
CPU(6) I/O(17) CPU(9)
CPU(4) I/O(4) CPU (4)
0 10 20 30 40 50
I/O(24) CPU (4)
P0 (1)
P1 (0)
P2 (0)
Two-level Queues: SJF with priority 0, FCFS with priority 1 Fixed Priority Queue Preemptive
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Multilevel Feedback Queue A process can move between the various queues; aging
can be implemented in this way
Multilevel-feedback-queue scheduler defined by the following parameters: number of queues scheduling algorithms for each queue method used to determine when to upgrade a process method used to determine when to demote a process method used to determine which queue a process will enter
when that process needs service
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Example of Multilevel Feedback Queue Three queues:
Q0 – RR with time quantum 8 milliseconds
Q1 – RR time quantum 16 milliseconds
Q2 – FCFS
Scheduling A new job enters queue Q0 which is served RR. When it gains
CPU, job receives 8 milliseconds. If it does not finish in 8 milliseconds, job is moved to queue Q1.
At Q1 job is again served RR and receives 16 additional milliseconds. If it still does not complete, it is preempted and moved to queue Q2.
The job is serverd based on FCFS in queue Q2
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Multilevel Feedback Queues
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Ch05 - Process Scheduling
Multi-level Feedback Queue Scheduling
CPU (10) I/O (20) CPU(11)
CPU(6) I/O(17) CPU(9)
CPU(4) I/O(4) CPU (4)
0 10 20 30 40 50
I/O(24) CPU (4)
P0 (1)
P1 (0)
P2 (0)
Three-level Queues: RR with quantum 3, RR with quantum 6, FCFS with priority 1
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Multiple-Processor Scheduling CPU scheduling more complex when multiple CPUs are available
Homogeneous processors within a system or heterogeneous processors within a system
Asymmetric multiprocessing vs. Symmetric multiprocessing (SMP) Symmetric Multiprocessing (SMP) – each processor makes its own
scheduling decisions. Asymmetric multiprocessing – only one processor accesses the
system data structures, alleviating the need for data sharing.
Load sharing on SMP system keeps the workload evenly distributed across all processors in an SMP
system. Push migration vs. Pull migration
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Real-Time Scheduling Hard real-time systems – required to complete a critical
task within a guaranteed amount of time
Soft real-time computing – requires that critical processes receive priority over less fortunate ones
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Thread Scheduling On operating systems that support kernel-level thread, it
is kernel-level threads, not processes, that are being scheduled by the operating system.
Local Scheduling – How the threads library decides which thread to put onto an available LWP
Global Scheduling – How the kernel decides which kernel thread to run next
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Summary CPU scheduling is the task of selecting a waiting process from the
ready queue and allocating the CPU to it. The CPU is allocated to the selected process by the dispatcher. FCFS scheduling is simple, cause short processes to wait for long time SJF scheduling is provably optimal, providing the shortest averaging
waiting time. But predicting the length of the next CPU bursts is difficult. Priority scheduling allocates the CPU to the heights priority process. Both priority and SJF may suffer from starvation. Aging is a technique
to prevent starvation. RR scheduling is more appropriate for a time-shared system. Major problem of RR scheduling is the selection of the time quantum. FCFS is non-preemptive, RR is preemptive, SJF and Priority may be
preemptive and non-preemptive. Multilevel queue allows different scheduling algorithm for each queue. Multilevel feedback queue allow process to move from one queue to
another.
Mi-Jung [email protected]
Dept. of Computer and Science
Silberschatz, Galvin and Gagne ©2006Operating System Principles
End of Chapter 5