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Operating Systems
Dr. Jerry Shiao, Silicon Valley University
Fall 2015
SILICON VALLEY UNIVERSITY
CONFIDENTIAL
1
CPU Scheduling
 Overview

CPU Scheduling

Basic Concepts




Scheduling Criteria






User-Level Threads and Kernel-Level Threads
Multiple-Processor Scheduling





First-Come-First-Serve
Shortest-Job-First
Priority
Round Robin
Multilevel Queue Scheduling
Thread Scheduling


Comparing CPU Scheduling Algorithms
Scheduling Algorithms


Scheduler in a Multi-Programming Environment.
Maximize CPU Utilization.
Concept of CPU Bursts and I/O Bursts.
Processor Afinitity
Load Balancing
Multicore Processors
Virtualization
Computer System Schedulers
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CPU Scheduling

Objective of Multiprogramming Maximize CPU Utilization:
To have some process running at all times.


Several processes kept in memory.
Alternating sequence of CPU execution – I/O Wait …
. . . Load Store . . . Wait for I/O . . . Store Inc . . . Wait for I/O . . .
Add Store
Index
Read From File
Write to File
CPU Burst 1-2 MSec


CPU Burst 1-2 MSec
I/O Bound Process has many short CPU bursts.
CPU Bound Process has few long CPU bursts.
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CPU Scheduling

CPU-Burst Duration



I/O Bound Process has many short CPU bursts.
CPU Bound Process has few long CPU bursts.
Burst distribution important for CPU-Scheduling Algorithm.
Curve showing large
number of short burst
(Time-Sharing
processes).
A smaller number of
long CPU bursts (Batch
processes).
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CPU Scheduling
 CPU Scheduler
 Scheduler manages queues to minimize queuing delay
and to optimize performance in a queuing environment.
 Types of Scheduling:

Long-Term Scheduling



Medium-Term Scheduling


Decision to Swap in a process, manage degree of Multiprogramming.
Short-Term Scheduling



Decision on which programs are admitted to the system for processing,
controlling the degree of Multiprogramming.
Long-Term Scheduler may limit the degree of Multiprogramming to
provide satisfactory service to current set of processes.
Dispatcher, makes decision on which process to execute next.
Invoked whenever event can lead to blocking of current process or
preempt the current running process: Clock Interrupts, I/O Interrupts,
System Calls, Signals (Semphores).
I/O Scheduling

Decision as to which process pending I/O request shall be handled by
the available I/O Device.
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CPU Scheduling
 CPU Scheduler
 Scheduling and Process State Transition
Scheduling types and relation to Process
State Transition.
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CPU Scheduling


CPU Scheduler
Queuing Diagram for Scheduling:
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CPU Scheduling

CPU Scheduler



Short Term Scheduler: Selects from Ready Queue the Process
Control Block ( PCB ).
Ready Queue can be: FIFO Queue, Priority Queue, a Binary
Tree, or Unordered Link List.
Preemptive and NonPreemptive Scheduling

NonPreemptive or Cooperative: Process keeps the CPU until it
releases the CPU or . . .






Switch Running State to Waiting State (i.e. I/O Request or Wait for
Child Process Termination).
Process Terminates.
Used by hardware platforms without special timer hardware for realtime clock.
Process can monopolize CPU, hang the system.
Windows 3.x, Old version of Macintosh
Real-Time Operating System
 Cannot allow real-time event processing to be interrupted.
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CPU Scheduling

Preemptive and NonPreemptive Scheduling (Cont)

Preemptive:




Switch Running State to Ready State (i.e. on Interrupt).
Switch Waiting State to Ready State (i.e. I/O Completion).
Uses special timer hardware for real-time clock.
Problems:




Process Shared Data: Using semaphores, mutexes, and spinlocks.
Kernel critical data guarded from simultaneous use.
 Process updating critical data is interrupted and second process
reads the critical data, which the interrupted process has not
completed updating.
Interrupt Handlers disable/enable interrupts.
Windows 95, Mac OS X, Linux
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CPU Scheduling
 Scheduling Criteria
 Criteria for comparing CPU-Scheduling Algorithm:


Usually favors one class over another.
CPU Utilization



Throughput




Minimize.
Process execution time: Sum of periods waiting to swap into memory,
waiting in the Ready Queue, waiting for I/O, and executing on CPU.
Waiting Time



Maximize.
Schedule 10 processes per second ( Interactive Load ) to 1 process per
second ( Batch Load ).
Turnaround Time


Maximize.
CPU 40% (lightly loaded) to 90% (heavily loaded).
Minimize.
Sum of the periods the process spends waiting in the Ready Queue.
Response Time


Minimize.
Measure of time from the submission of a request until the time it takes to
start responding. SILICON VALLEY UNIVERSITY
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CPU Scheduling
 Scheduling Criteria
 Criteria for comparing CPU-Scheduling Algorithm (Cont)
 Short-Term Scheduling Criterias: Optimized towards System
Behavior (Real-Time, Time-Sharing, Batch System).

User-Oriented:





System-Oriented:



Behavior of the system as perceived by the user or process.
Interactive System: Response Time is elapsed time between the submission of a
request until the response appearing as output.
Scheduling Policy minimize Response Time: Round-Robin.
Switching processes frequently increases overhead, reducing throughput.
Effective and efficent utilization of the processor or System Performance.
Scheduling Policy minimize Turnaround Time or maximize Throughput: Shortest
Job First ( SJF ).
Criteria not readily measureable:

Predictability:


Service provided to users to exhibit same characteristics over time.
Variation in Response Time or Turnaround Time mean wide swing in system
workloads, needing system tuning to stabilize.
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CPU Scheduling


Scheduling Algorithms
First-Come, First-Served Scheduling



FCFS Scheduling is nonpreemptive.
Problem for Time-Sharing Systems (needs CPU at regular
intervals).
Simple algorithm, but long wait time.
P1
P2
P1
P2
P3
P3
Average Wait Time = ((P1 = 0) + P2 + P3) / 3
P2
P2
P3
P3
P1
P1
Average Wait Time = (( P2 = 0) + P3 + P1) / 3

Wait Time.
Average Wait Time varies on order and CPU bursts.
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CPU Scheduling


Scheduling Algorithms
Shortest-Job-First Scheduling






CPU is assigned to the process that has the smallest next CPU
burst.
Minimum average waiting time for a set of processes.
In Short term scheduling, predict the next CPU burst value.
Preemptive SJF takes CPU when arrived process burst time is
less than the remaining time of current process.
NonPreemptive SJF allows current process to complete burst
time before scheduling next process based on burst time.
Length of CPU burst? Predict value of next CPU burst.
P4
P4
P2
P2
P3
P3
P1
P1
Average Wait Time = ((P4 = 0) + P2 + P3 + P1 )/ 4
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Wait Time.
13


CPU Scheduling
Scheduling Algorithms
FirstCome, First-Served Scheduling
P1
P2
0
20


40
P5
56
60
Wait Time.
P1: 20, P2: 12, P3: 8, P4: 16, P5: 4
Wait Time ( Order of Arrival ).


32
P4
Burst Time:


P3
P1: 0, P2: 20, P3: 32, P4: 40, P5: 56
Average Wait Time: 148 / 5 = 29.6
Shortest-Job-First Scheduling
P5
0
P3
4

12
P4
24
P1
40
60
Wait Time ( In Ready Queue at Same Time ).


P2
P1: 40, P2: 12, P3: 4, P4: 24, P5: 0
Wait Time.
Average Wait Time: 80 / 5 = 16
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CONFIDENTIAL
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CPU Scheduling


Scheduling Algorithms
Priority Scheduling




SJF Algorithm is Priority Scheduling where priority is inverse of
predicted next CPU burst ( Lower burst, higher priority ).
Preemptive: Compare arriving process priority with current
running process priority.
Nonpreemptive: Arriving process priority higher than current
running process priority placed in front of the Ready Queue.
Problem:

Indefinite blocking of low priority processes until system lightly
loaded.

Solution: Increase priority of processes in the Ready Queue ( Aging ).
Process/Burst Time (msec)/Priority: P1/10/3, P2/1/1, P3/2/4, P4/1/5, P5/5/2 = 8.2 msec Average
P2
0
1
P5
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6
P1
16
P3
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P4
15
CPU Scheduling



Scheduling Algorithms
Priority Scheduling
Numbering Scheme


Priority 0 can be highest or lowest
Priorities

Internal


External


Importance, Process cost, Process sponsorship.
Static


Set according to Operating System (i.e. memory requirements, number of
open files, number of I/O bursts).
Fixed priority when process enter Operating System.
Dynamic

Changing during processing (i.e. Amount of CPU usage, or aging (Length of
time waiting (a solution to starvation))
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CPU Scheduling
 Scheduling Algorithms
 Compare FCFS, SJF, Priority Scheduling



FCFS (First-come, First-Served)
 Non-preemptive.
SJF (Shortest Job First)
 Non-preemptive or Preemptive.
 When a new (shorter burst) job arrives, can preempt current
job or not.
 Possible process starvation.
Priority
 Non-preemptive or Preemptive.
 When a new (higher priority job arrives).
 When a processes priority changes.
 Possible process starvation.
 Linux 2.4 Non-realtime nonpreemptive.
 Linux 2.6 “Real-time” preemptive.
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CONFIDENTIAL
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CPU Scheduling


Scheduling Algorithms
Round-Robin Scheduling







Designed for Time-Sharing or Multi-Tasking Operating Systems.
Based on First-Come-First-Serve Algorithm, but use Preemption
to switch between processes.
Preemption: Process will be preempted when quantum expires.
Quantum: Time slice (unit of time) in which a process executes.
Circular Queue: Process alternates quantum execution time.
Average waiting time often long (process has to wait for other
processes to complete their quantum).
Timer Interrupt when Quantum Expires.


Current process placed at end of Circular Queue.
Current process completes before Quantum Expires, next process
selected, no Interrupt.
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CPU Scheduling


Scheduling Algorithms
Round-Robin Scheduling
Order of Queue and CPU Burst
P1: 20 P2: 12 P3: 8 P4: 16 P5: 4
Quantum = 4 Process runs until:
a) Remaining CPU Burst < Quantum
b) Remaining CPU Burst > Quantum
Interrupt Context Switch to Next
Process in Ready Queue.
P4 completes
P5 completes P3 completes
P2 completes
P1 completes
P1 P2 P3 P4 P5 P1 P2 P3 P4 P1 P2 P4 P1 P4 P1

0
4
8
12 16 20 24 28 32 36 40 44
Waiting times:
Average wait time: 30.4
P1: 60 - 20 = 40
P2: 44 - 12 = 32
P3: 32 - 8 = 24
P4: 56 - 16 = 40
P5: 20 - 4 = 16
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56
60
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CPU Scheduling



Scheduling Algorithms
Round-Robin Scheduling
Choosing a Quantum:


Time Quantum too large, RR Policy becomes FCFS Policy.
Time Quantum too small, RR Policy process time affected by
Context Switch time.




Context Switch slows the execution of the process.
Context Switch approximately less than 10 percent of the CPU time.
Typically Context switch < 10 microseconds.
But, more Context Switch, longer process has to wait for the CPU
again.
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CPU Scheduling



Scheduling Algorithms
Round-Robin Scheduling
Context Switch
Process Time = 10
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Quantum
Context Switches
12
0
6
1
1
9
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CPU Scheduling

Scheduling Algorithms

Sample distribution based on Nonpreemptive FCS and SFJ. Preemptive RR.
Response time
 Measure of time from the submission of a request until the time it takes to start responding.
 Assume all jobs submitted at same time, in order given.
Turnaround time
 Process execution time. Time interval from submission of process until completion of
process.
 Assume all jobs submitted at same time


SJF
P5 P3
4
RR
P2
P4
12
24
P1
40
60
P1 P2 P3 P4 P5 P1 P2 P3 P4 P1 P2 P4 P1 P4 P1
4
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8
12
16
20
24
28
32
36
40
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48
52
56
60
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CPU Scheduling

Scheduling Algorithms

Sample Response Time Calculation (Measure of time from the submission of a
request until the time it takes to start responding).
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CPU Scheduling


SJF
P5 P3
4
Scheduling Algorithms
P2
12
P4
24
P1
40
60
Sample Response Time Calculation (Measure of time from the submission of a
request until the time it takes to start responding).
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RR
CPU Scheduling
P1 P2 P3 P4 P5 P1 P2 P3 P4 P1 P2 P4 P1 P4 P1
4
8
12
16
20
24
28
32
36
40
44
48

Scheduling Algorithms

Sample Response Time Calculation (Measure of time from the submission of a
request until the time it takes to start responding).
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56
25
60
CPU Scheduling

Scheduling Algorithms

Sample Turnaround Time Calculation (Process burst execution time and sum of wait
periods).
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SJF
CPU Scheduling
P5 P3
4
P2
12
P4
24
P1
40
60

Scheduling Algorithms

Sample Turnaround Time Calculation (Process burst execution time and sum of wait
periods).
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RR
CPU Scheduling
P1 P2 P3 P4 P5 P1 P2 P3 P4 P1 P2 P4 P1 P4 P1
4
8
12
16
20
24
28
32
36
40
44
48
52
56

Scheduling Algorithms

Sample Turnaround Time Calculation (Process burst execution time and sum of wait
periods).
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60
CPU Scheduling


Scheduling Algorithms
Performance Characteristics of Scheduling Algorithms

Round Robin ( RR )

Average waiting time, often high, causing above average
Turnaround Time.


Quantum provides good response time.





Turnaround Time good when CPU bursts shorter than Quantum.
Quantum too long, approach FCFS Policy behavior.
Quantum too short, causes additional Context Switch overhead,
Quantum 10 to 100 milliseconds.
Important for Interactive or Timesharing.
Shortest-Job-First ( SJF ).


Best average waiting time, causing best Turnaround Time.
CPU burst length or estimates difficult to predict.
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CPU Scheduling
 Scheduling Algorithms


Processes are classified into different process types.
Multilevel Queue Scheduling



Partition Ready Queue into several separate queues.
Queues are defined based on process property: interactive process type,
background process type, process priority.
Each queue has its own Scheduling Algorithm.


Time-Sharing Queue scheduled by RR Algorithm.
Batch Queue scheduled by FCFS Algorithm.

Each queue has priority:

No process in the lower priority queues will run until the higher priority
queues has completed.
 When process in higher priority queue is received, the lower priority queue
processing is preempted.
 Prevent Starvation, Time-Slice between queues, each queue has % of CPU.
Inflexible: Process assigned to queue, process “type” does not change.

System Processes
Interactive Processes
Batch Processes
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CPU Scheduling



Scheduling Algorithms
Multilevel Feedback Queue Scheduling
Multilevel Queues, but allow process to move between
queues.



Process exceeds quantum, placed in lower priority queue.
Process not getting CPU time, placed in higher priority queue.
Defined with:





The number of queue.
Scheduling algorithm for each queue.
Policy to upgrade a process to a higher-priority queue.
Policy to downgrade a process to a lower-priority queue.
Policy to determine which queue to place a process.
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CPU Scheduling


Scheduling Algorithms
Multilevel Feedback Queue Scheduling
Multilevel Queue (Fixed)
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Multilevel Feedback Queue (Movement)
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CPU Scheduling


Thread Scheduling
User-level and Kernel-level Threads

User-level thread mapped to kernel-level thread.



Operating System schedules kernel-level threads.
Process-Contention Scope ( PCS ) Scheduling




Lightweight Process ( LWP )
User-level threads scheduled by the User-level Thread Library.
Kernel scheduler is NOT involved.
The User-level thread mapped to Kernel-level thread for scheduling
by Operating System ( One-to-One, Many-to-One, Many-to-Many ).
PTHREAD SCOPE PROCESS schedules threads using PCS.
System-Contention Scope ( SCS ) Scheduling



Kernel-level threads scheduled by the Operating System.
PTHREAD SCOPE SYSTEM schedules threads using SCS.
LWP creates/bind User-level thread to LWP (One-to-One policy).
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CPU Scheduling

Thread Scheduling
#include <pthread.h>
#include <stdio.h>
int main(int argc, char *argv[])
...
/* set the scheduling algorithm to PCS or SCS */
pthread attr setscope(&attr, PTHREAD SCOPE SYSTEM);
/* create the threads */
for (i = 0; i < 5; i++)
pthread create(&tid[i],&attr,runner,NULL);
/* now join on each thread */
for (i = 0; i < 5; i++)
pthread join(tid[i], NULL);
}
/* Each thread will begin control in this function */
void *runner(void *param) {
/* do some work ... */
pthread exit(0);
}
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Sets the attribute set for the thread
as Process or System Contention
scope. This attribute set to System
Contention.
Create the thread based on the
attributes.
Wait for the threads to complete.
NOTE: Contention Scopes are Operating
System Dependent.
Linux and MAC OS X Operating Systems
allow only PTHREAD_SCOPE_SYSTEM
(One-to-One Mapping).
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CPU Scheduling


Multiple-Processor Scheduling
Processor Affinity

Cache Memory: Process prefers running on a specific CPU.



Keep Cache Memory Validated.
Soft Affinity: Operating System has policy to keep process on same
CPU, but not guaranteed.
Hard Affinity: Process cannot migrate to other CPU.
Linux, Solaris.


Main Memory Architecture: Non-Uniform Memory Access
(NUMA) when memory access speed non-uniform.
CPU and Memory
Affinity.
CPU
Slow Access
( NUMA )
CPU
Fast Access
Memory
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Fast Access
Memory
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CPU Scheduling
 Multiple-Processor Scheduling
 Load Balancing

Keep CPU processing balanced.



Distributing processes between CPUs can counteracts
Processor Affinity ( Imbalance determined by queue threshold ).
Push Migration: Linux/FreeBSD


Only when processors have separate CPU Ready Queues.
Special process checks CPU Ready Queues and redistributes processes by
pushing process from one CPU Ready Queue to another.
Pull Migration: Linux/FreeBSD

Idle processor pulls a waiting process from a CPU’s Ready Queue.
Ready Queue
CPU
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Migration
Thread
Ready Queue
Every 200
Milliseconds
CPU
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CPU Scheduling


Multicore Processors Scheduling
Multiple Processor Cores on Same Physical Chip



Each core separate set of registers.
Multicore SMPs are faster and consume less power than single
core multiprocessor SMPs.
Multithreading processor cores: Two or more hardware threads
assigned to each core.


Memory Stall: A hardware thread will block when waiting for I/O to arrive or
write, cache memory miss.
Computer Cycle
Allows another hardware thread to use the CPU.
Memory Stall
Cycle
Thread 0
Thread 1
C
C
M
C
M
C
M
M
C
M
C
M
C
C
Time
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CPU Scheduling


Multicore Processors Scheduling
Each hardware thread is a logical processor.


Dual-core dual-threaded CPU has four logical processors.
Multithread Processor Implementation

Coarse-grained: Hardware thread executes until memory stall ( or long-latency
event ) occurs. Processor now switch to another hardware thread. Cost of thread
switching is high ( instruction pipeline has to be flushed ).
 Fine-grained ( Interleaved ): Hardware thread switches at boundary of instruction
cycle. All interleaved multithreading processor has hardware support for thread
switching and is fast.

Two Levels of Scheduling:

Scheduling Algorithm by Operating System to select software thread to run.
 Each core decides which hardware thread to run.
 Round Robin (UltraSPARC T1).
 Events to trigger thread switch given priority (Intel Itanium).
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CPU Scheduling



Virtualization and Scheduling
Virtualization Software Schedules Physical CPU (Host
OS) for Each Virtual Machine (Guest OS).
Multiple Scheduling Layers All Sharing CPU Cycle



Host Operating System Scheduling.
Virtualization Software Scheduling Virtual Machine.
Virtual Machine Operating System Running Own Scheduler.




100 Millisecond Virtual CPU time might take 1 second.
Poor response times to uses logged into Virtual Machine.
Time-of-day clocks are incorrect, timers take longer to trigger.
Make a Virtual Machine scheduling algorithm appear poor.
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CPU Scheduling


Solaris Operating System
Priority-Based Thread Scheduling


Thread belongs to one of six classes: each Class maintains
different priorities and Scheduling Algorithm.
Time Sharing Class





Default Class.
Dynamic alters priorities using multilevel feedback queue.
Inverse Relationship between priorities and time slices (quantum).
Higher priority, lower time slice.
Good Response time for Interactive processes and good throughput
for CPU-bound processes.
Interactive Class


Similar Scheduling Policy as Time Sharing Class.
Used by KDE or GNOME Window Managers for higher priority.
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CPU Scheduling


Solaris Operating System
Priority-Based Thread Scheduling

Real Time Class



System Class



Executes kernel threads only (Scheduler and Paging Daemon).
Priority fixed.
Fair Share



Highest Priority.
Guaranteed Response within bounded time period.
Use CPU shares (not priorities), indicate entitlement to CPU.
Solaris 9 (One-to-One Model).
Fixed Priority


Same priority range as Time-Sharing Class, but not dynamically adjusted.
Solaris 9 (One-to-One Model).
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Lowest Priority has highest
Time Quantum
CPU Scheduling

Solaris Operating System
 Time-Sharing and Interactive
Priority
Time Quantum
New Priority when used
entire Quantum.
Threads
Time Quantum
Expired
Return From Sleep
0
200
0
50
5
200
0
50
10
160
0
15
160
5
51
New Priority when NOT used
entire Quantum51
(waiting for I/O).
20
120
10
52
25
120
15
52
30
80
20
53
35
80
25
54
40
40
30
55
45
40
35
56
50
40
40
58
55
40
45
58
20
49
59
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CPU Scheduling
 Solaris Operating System
Highest
169
Scheduling Classes
Interrupt Threads
First
159
RealTime (RT) Threads
Global Priority
Scheduling Order
99
System (SYS) Threads
59
Lowest
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Fair Share (FSS) Threads
Fixed Priority (FX) Threads
TimeShare (TS) Threads
Interactive (IA) Threads
0
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43
CPU Scheduling




Windows XP Operating System
Priority-Based, Preemptive Scheduling
Scheduling on the basis of threads
Highest priority thread always be dispatched, and runs
until:





Preempted by a higher priority thread
Terminates
Time quantum ends
Makes a blocking system call (e.g., I/O)
32 priorities, each with own queue:

Mem. Mgmt.: 0; Variable: 1 to 15; Real-Time: 16 to 31
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CPU Scheduling

Windows XP Operating System

Within Priority Class contains Relative Priority
All Classes Except Real-Time are Variable


Thread Quantum Runs Out: Lower Priority.
 Thread Released From Wait State: Raise Priority.
Priority Classes
Relative Priority
Real-Time
High
Above
Normal
Normal
Below
Normal
Idle
Priority
Time-Critcal
31
15
15
15
15
15
Highest
26
15
12
10
8
6
Above Normal
25
14
11
9
7
5
Normal
24
13
10
8
6
4
Below Normal
23
12
9
7
5
3
Lowest
22
11
8
6
4
2
Idle
16
1
1
1
1
1
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CPU Scheduling


Linux Operating System
Preemptive, Priority-Based Scheduler


Real-Time Priority Range: 0 – 99
Nice Range: 100 - 140
Relative Priority
Highest
Numeric Priority
0
Time Quantum
200
MSec
Higher Priority
Tasks are
assigned longer
Time Quantums.
Real-Time Tasks
99
100
Other Tasks
Lowest
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CPU Scheduling

Linux Operating System
Real-Time Tasks Assigned Static Priorities
Other Tasks Has Dynamic Priorities

Nice Value and how long task is sleeping.
 Longer sleep time has higher priority adjustments ( typically I/O
bound tasks ).
 Shorter sleep time has lower priority adjustments ( typically
CPU-bound tasks ).
Runqueue Data Structure per CPU




Active Priority Array: All tasks with time remaining in Quantum.
 Expired Priority Array: All tasks with expired Quantum.
 Scheduler selects highest priority task from the Active Priority Array.
 When Active Priority Array is empty (transitioned to Expired Priority
Array or in Wait Queues), arrays are swapped.
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CPU Scheduling

Linux 2.6 Scheduler Algorithm
runqueue
Active [0]
[1]
[2]
...
[140]
Expired [0]
[1]
[2]
...
[140]
Priority 140
Bitmap
1 0
Task 3
Task 2
Task 1
Task 4
Task 5
Task 6
Task 7
Task 68
Migration Thread
Fall 2015
0
...
3
2
1
...
0
0
1
...
Task N
Task M
...
Priority Arrays
exchanged when all
tasks have
executed.
Task X
Task Z
Used to Load Balance when CPU heavily
utilized. Tries to distribute some processes.
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CPU Scheduling


Algorithm Evaluation Methods
Criteria: CPU Utilization, Response Time, Throughput

Analytic Evaluation: Uses given algorithm and system workload,
produce a formula that evaluates performance of algorithm.




Deterministic Modeling: Predetermined workload ( i.e. measure wait time for
FCFS, SJF, RR algorithms).
 Problem: Requires exact numbers and ONLY applies to specific cases.
Queueing Modeling: Estimate arrival rate and service rate, compute
utilization, average queue length, average wait time.
 Problem: Approximations of real systems and may not be accurate.
Simulations: Programming model of computer system, software data
structures represents Operating System components.
 Data ( i.e. CPU burst times, arrivals, departures) randomly generated
(i.e. uniform, Poisson) or empirical.
 Problem: Designing simulator is expensive.
Only completely accurate way to evaluate a scheduling algorithm is to
implement it and observe the behavior in the Operating System.
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CPU Scheduling



Summary
CPU Scheduling selects a waiting process from Ready Queue.
Scheduling Algorithms

FCFS: Simple, but Short Processes Waits for Long Processes.
 SJF: Shortest Average Wait Time, but Predicting Next CPU Burst is Difficult, and
if Preemptive, can cause Starvation of Long Jobs.
 RR: Time-Shared (Interactive) System, but Selecting Quantum is Problematic.




Multilevel Queue Algorithms: Uses different algorithms for classes.
Multilevel Feedback Queues: Moves process between queues.
Multi-Processor Scheduling: Processor Affinity, Load Balancing.
Operating Systems:

Solaris: Priority-based Thread Scheduling with 6 Priority Classes.
 Windows XP: Priority-based Thread Scheduling with 32 Priority Queues.
 Linux: Priority-based with Real-Time and Other Priority Ranges.

Algorithm Evaluation:

Deterministic Modeling, Queueing Models, Simulations.
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