1. CSS 430: Operating Systems - Threads, Scheduling, Assignment 2
Lecture 7: Whirlwind tour of Threads, CPU Scheduling ... & Assignment 2
Joe McCarthy
CSS 430: Operating Systems - Threads, Scheduling, Assignment 2

2. CSS 430: Operating Systems - Threads, Scheduling, Assignment 2
Outline
Potentially helpful hints
execlp( “cat”, “cat”, NULL ), testpipe3.cpp
/usr/apps/CSS430/examples
Selected excerpts from
Chapter 4: Threads
Chapter 5: CPU Scheduling
Assignment 2
Next time:
Finish Chapters 4 & 5
CSS 430: Operating Systems - Threads, Scheduling, Assignment 2

3. CSS 430: Operating Systems - Threads, Scheduling, Assignment 2
Hint re: no output
Substitute execlp( “cat”, “cat”, NULL ) for last stage of pipeline
If output comes through, problem is in that last stage
If output does not come through, substitute “cat” for next-to-last stage of pipeline
If output comes through, problem is in that stage
If no output comes through, problem is in first stage (or elsewhere)
CSS 430: Operating Systems - Threads, Scheduling, Assignment 2

4. CSS 430: Operating Systems - Threads, Scheduling, Assignment 2
Hint re: hanging
testpipe3.cpp
uw1-320-lab: /usr/apps/CSS430/examples
Uses
fork(), pipe(), dup2()
echo, cat
Designed to
Echo it’s first argument through a pipe
[…]$ ./testpipe3 hello
hello
… and then hang
CSS 430: Operating Systems - Threads, Scheduling, Assignment 2

5. CSS 430: Operating Systems - Threads, Scheduling, Assignment 2
/* includes omitted from this excerpt */
int main( int argc, char** argv ) {
int fd[2];
int pid; // only need one pid (why?)
assert(argc == 2);
if ( pipe(fd) < 0 ) {
perror( "pipe" ); exit( EXIT_FAILURE );
} else if ( ( pid = fork() ) < 0 ) {
perror( "fork" ); exit( EXIT_FAILURE );
} else if ( pid > 0 ) { // this is the parent
wait( NULL ); exit( EXIT_SUCCESS );
} else if ( pid > 0 ) { // this is [still] the child
dup2( fd[0], 0 ); // copy read-end of pipe to STDIN
close( fd[1] ); // close write-end of pipe (not needed)
execlp( "cat", "cat", NULL );
} else { // this is the grand-child
close( fd[0] ); // close read-end of pipe (not needed)
dup2( fd[1], 1 ); // copy write-end of pipe to STDOUT
execlp( "echo", "echo", argv[1], NULL ); }
CSS 430: Operating Systems - Threads, Scheduling, Assignment 2

6. CSS 430: Operating Systems - Threads, Scheduling, Assignment 2
Chapter 4: Threads
[selected excerpts]
Material derived, in part, from Operating Systems Concepts with Java, 8th Ed. © 2009 Silberschatz, Galvin & Gagne
CSS 430: Operating Systems - Threads, Scheduling, Assignment 2

7. Single and Multithreaded Processes
CSS 430: Operating Systems - Threads, Scheduling, Assignment 2

8. Multithreaded Server Architecture
CSS 430: Operating Systems - Threads, Scheduling, Assignment 2

9. CSS 430: Operating Systems - Threads, Scheduling, Assignment 2
User Threads
Thread management done by user-level threads library
Three primary thread libraries:
POSIX Pthreads
Win32 threads
Java threads
CSS 430: Operating Systems - Threads, Scheduling, Assignment 2

10. CSS 430: Operating Systems - Threads, Scheduling, Assignment 2
Thread Libraries
Thread library provides programmer with API for creating and managing threads
Two primary ways of implementing
Library entirely in user space
Kernel-level library supported by the OS
CSS 430: Operating Systems - Threads, Scheduling, Assignment 2

11. CSS 430: Operating Systems - Threads, Scheduling, Assignment 2
Java Thread States
CSS 430: Operating Systems - Threads, Scheduling, Assignment 2

12. Java Threads Java threads are managed by the JVM
1 process, 1+ threads
Java threads may be created by:
Extending the Thread class
Implementing the Runnable interface
public class MyShell extends Thread { …
void run() { }
/usr/apps/CSS430/examples/threads
CSS 430: Operating Systems - Threads, Scheduling, Assignment 2

13. Java Threads - Producer-Consumer
Main driver: Factory.java
Shared memory via queue variable
Instantiates & starts 2 Threads
import java.util.Date;
public class Factory {
public static void main( String[] args ) {
// create the message queue
Channel<Date> queue = new MessageQueue<Date>();
// create the producer and consumer threads
Thread producer = new Thread( new Producer(queue) );
Thread consumer = new Thread( new Consumer(queue) );
// start the threads
producer.start();
consumer.start(); }
/usr/apps/CSS430/examples/threads
CSS 430: Operating Systems - Threads, Scheduling, Assignment 2

14. Java Threads - Producer-Consumer
Shared memory via abstract data type: Channel.java
public interface Channel<E> {
public void send(E item);
public E receive(); }
/usr/apps/CSS430/examples/threads
CSS 430: Operating Systems - Threads, Scheduling, Assignment 2

15. Java Threads - Producer-Consumer
Implementation of abstract data type: MessageQueue.java
import java.util.Vector;
public class MessageQueue<E> implements Channel<E> {
private Vector<E> queue;
public MessageQueue() {
queue = new Vector<E>(); }
public void send( E item ) {
queue.addElement( item ); // add to tail (end)
public E receive() {
if ( queue.size() == 0 )
return null;
else // remove from head (front)
return queue.remove( 0 );
/usr/apps/CSS430/examples/threads
CSS 430: Operating Systems - Threads, Scheduling, Assignment 2

16. Java Threads - Producer-Consumer
Implementation of producer thread: Producer.java
import java.util.Date;
class Producer implements Runnable {
private Channel<Date> queue;
public Producer( Channel<Date> queue ) {
this.queue = queue; }
@SuppressWarnings("unchecked")
public void run() {
Date message;
while ( true ) {
SleepUtilities.nap();
message = new Date(); // produce an item
System.out.println( "Producer produced " + message );
queue.send(message); // enter it into the buffer
/usr/apps/CSS430/examples/threads
CSS 430: Operating Systems - Threads, Scheduling, Assignment 2

17. Java Threads - Producer-Consumer
Implementation of consumer thread: Consumer.java
import java.util.Date;
class Consumer implements Runnable {
private Channel<Date> queue;
public Consumer( Channel<Date> queue ) {
this.queue = queue; }
@SuppressWarnings("unchecked")
public void run() {
Date message;
while ( true ) {
SleepUtilities.nap();
System.out.println( "Consumer wants to consume." );
message = queue.receive(); // consume an item from buffer
if ( message != null )
System.out.println( "Consumer consumed " + message );
/usr/apps/CSS430/examples/threads
CSS 430: Operating Systems - Threads, Scheduling, Assignment 2

18. Chapter 5: CPU Scheduling
[selected excerpts]
Material derived, in part, from Operating Systems Concepts with Java, 8th Ed. © 2009 Silberschatz, Galvin & Gagne
CSS 430: Operating Systems - Threads, Scheduling, Assignment 2

19. [from Chapter 1: Introduction]
CPU
[from Chapter 1: Introduction]
CSS 430: Operating Systems - Threads, Scheduling, Assignment 2

20. [from Chapter 1: Introduction]
CPU
[from Chapter 1: Introduction]
CSS 430: Operating Systems - Threads, Scheduling, Assignment 2

21. [from Chapter 1: Introduction]
CPU Scheduling
[from Chapter 1: Introduction]
CSS 430: Operating Systems - Threads, Scheduling, Assignment 2

22. [from Chapter 1: Introduction]
CPU Scheduling
Multiprogramming
[from Chapter 1: Introduction]
CSS 430: Operating Systems - Threads, Scheduling, Assignment 2

23. [from Chapter 3: Processes]
CPU Scheduling
[from Chapter 3: Processes]
CSS 430: Operating Systems - Threads, Scheduling, Assignment 2

24. [from Chapter 3: Processes]
CPU Scheduling
Process Control Blocks
[from Chapter 3: Processes]
CSS 430: Operating Systems - Threads, Scheduling, Assignment 2

25. [from Chapter 3: Processes]
CPU Scheduling
Can all PCBs have access to CPU simultaneously?
[from Chapter 3: Processes]
CSS 430: Operating Systems - Threads, Scheduling, Assignment 2

26. [from Chapter 3: Processes]
CPU Scheduling
[from Chapter 3: Processes]
CSS 430: Operating Systems - Threads, Scheduling, Assignment 2

27. [from Chapter 3: Processes]
CPU Queues
[from Chapter 3: Processes]
CSS 430: Operating Systems - Threads, Scheduling, Assignment 2

28. [from Chapter 3: Processes]
Process States
[from Chapter 3: Processes]
CSS 430: Operating Systems - Threads, Scheduling, Assignment 2

29. CSS 430: Operating Systems - Threads, Scheduling, Assignment 2
Basic Concepts
Goal: Maximize CPU utilization via optimized multiprogramming
CPU–I/O Burst Cycle:
Process execution consists of a cycle of
CPU execution
I/O wait
CPU burst durations vary
Within a process, across different processes
CSS 430: Operating Systems - Threads, Scheduling, Assignment 2

30. Histogram of CPU-burst Durations
CSS 430: Operating Systems - Threads, Scheduling, Assignment 2

31. Alternating Sequence of CPU & I/O Bursts
CSS 430: Operating Systems - Threads, Scheduling, Assignment 2

32. CSS 430: Operating Systems - Threads, Scheduling, Assignment 2
CPU Scheduler
Selects next process to run (i.e., to get CPU time)
Dispatches that process (allocates the CPU to it)
CPU scheduler called when a process …
CSS 430: Operating Systems - Threads, Scheduling, Assignment 2

33. CSS 430: Operating Systems - Threads, Scheduling, Assignment 2
CPU Scheduler
Selects next process to run (i.e., to get CPU time)
Dispatches that process (allocates the CPU to it)
CPU scheduler called when a process:
Running  terminated
Running  waiting
Running  ready
Waiting  ready
Preemptive vs. non-preemptive
CSS 430: Operating Systems - Threads, Scheduling, Assignment 2

34. CSS 430: Operating Systems - Threads, Scheduling, Assignment 2
Dispatcher
Activates a process (gives control of the CPU to the process) selected by the short-term scheduler
Three steps:
CSS 430: Operating Systems - Threads, Scheduling, Assignment 2

35. CSS 430: Operating Systems - Threads, Scheduling, Assignment 2
Dispatcher
Activates a process (gives control of the CPU to the process) selected by the short-term scheduler
Three steps:
CSS 430: Operating Systems - Threads, Scheduling, Assignment 2

36. CSS 430: Operating Systems - Threads, Scheduling, Assignment 2
Dispatcher
Activates a process (gives control of the CPU to the process) selected by the short-term scheduler
Three steps:
Switch context
Switch to user mode
Jump to the proper location in user program to start/resume
CSS 430: Operating Systems - Threads, Scheduling, Assignment 2

37. CSS 430: Operating Systems - Threads, Scheduling, Assignment 2
Dispatcher
Activates a process (gives control of the CPU to the process) selected by the short-term scheduler
Three steps:
Switch context
Switch to user mode
Jump to the proper location in user program to start/resume
Dispatch latency: time it takes for the dispatcher to stop one user process and start another one
CSS 430: Operating Systems - Threads, Scheduling, Assignment 2

38. Scheduling Milestones
Submission Time
Time when job is submitted
E.g., when command is entered in shell
Admission Time
Time when job is admitted to Ready queue
PCB creation & other housekeeping has to take place
Activation Time
Time when job is first activated (Ready  Running)
E.g., when first output starts appearing on console
NB: may have several CPU-I/O burst cycles
Completion Time
Time when job finishes executing
CSS 430: Operating Systems - Threads, Scheduling, Assignment 2

39. Job (Process/Thread) Times
Response Time
Time before job is first activated
Activation Time – Submission Time
E.g., time between entering command & first response
Wait Time
Total amount of time a job spends in Ready queue
Execution Time
Total amount of time a job spends Running
NB: may have several CPU-I/O burst cycles
Turnaround Time
Time a job takes to complete after it is submitted
Completion Time – Submission Time
CSS 430: Operating Systems - Threads, Scheduling, Assignment 2

40. CSS 430: Operating Systems - Threads, Scheduling, Assignment 2
Scheduling Criteria
CPU utilization
% of time CPU is executing user processes
Throughput
# of processes that complete their execution per time unit
Average response time
average amount of between when a process is submitted & first response (for time-sharing environment)
Average waiting time
average amount of time a process waits in Ready queue
Average turnaround time
average amount of time to execute a process
CSS 430: Operating Systems - Threads, Scheduling, Assignment 2

41. Scheduling Optimization Criteria
CPU utilization: maximize
% of time CPU is executing user processes
Throughput: maximize
# of processes that complete their execution per time unit
Average response time: minimize
average amount of between when a process is submitted & first response (for time-sharing environment)
Average waiting time: minimize
average amount of time a process waits in Ready queue
Average turnaround time: minimize
average amount of time to execute a process
CSS 430: Operating Systems - Threads, Scheduling, Assignment 2

42. First-Come, First-Served (FCFS)
Example: 3 processes with the following burst times:
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 times?
Average waiting time? P1 P2 P3 24 27 30
CSS 430: Operating Systems - Threads, Scheduling, Assignment 2

43. First-Come, First-Served (FCFS)
Example: 3 processes with the following burst times:
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 times: P1 = 0; P2 = 24; P3 = 27
Average waiting time: ( ) / 3 = 17.0 P1 P2 P3 24 27 30
CSS 430: Operating Systems - Threads, Scheduling, Assignment 2

44. FCFS Scheduling (Cont.)
Suppose that the processes arrive in the order:
P2 , P3 , P1
The Gantt chart for the schedule is:
Waiting times?
Average waiting times? P1 P3 P2 6 3 30
CSS 430: Operating Systems - Threads, Scheduling, Assignment 2

45. FCFS Scheduling (Cont.)
Suppose that the processes arrive in the order:
P2 , P3 , P1
The Gantt chart for the schedule is:
Waiting times: P1 = 6; P2 = 0;; P3 = 3
Average waiting time: ( ) / 3 = 3.0
Much better than previous case
Convoy effect short process behind long process P1 P3 P2 6 3 30
CSS 430: Operating Systems - Threads, Scheduling, Assignment 2

46. CSS 430: Operating Systems - Threads, Scheduling, Assignment 2
Priority Scheduling
A priority number (integer) is associated with each process
The CPU is always allocated to the process with the highest priority
Two basic strategies:
Preemptive can interrupt current process, switch to new one
Nonpreemptive wait until current process finishes, then switch
CSS 430: Operating Systems - Threads, Scheduling, Assignment 2

47. CSS 430: Operating Systems - Threads, Scheduling, Assignment 2
Round Robin (RR)
Each process gets a time quantum (q) of CPU time, usually milliseconds
Selected from front of Ready queue (priority = queue size – process position)
If process does not block (I/O) within q ms: preempted
Added to the end of the Ready queue
General observations:
n processes in the Ready queue
each process gets 1/n of the CPU time (in slices of size q)
no process waits more than (n-1)q time units
CSS 430: Operating Systems - Threads, Scheduling, Assignment 2

48. Example of RR with Time Quantum = 4
Process Burst Time
P1 24
P2 3
P3 3
The Gantt chart is: P1 P2 P3 4 7 10 14 18 22 26 30
CSS 430: Operating Systems - Threads, Scheduling, Assignment 2

49. CSS 430: Operating Systems - Threads, Scheduling, Assignment 2
Multilevel Queue
Ready queue can be partitioned into separate queues
E.g., foreground (interactive) & background (batch)
Each queue can have its own scheduling algorithm
Scheduling must be done between the queues
Simplest: Fixed priority scheduling
One possibility: 100% priority to foreground
Serve foreground if any; serve background only if no foreground
Problem?
CSS 430: Operating Systems - Threads, Scheduling, Assignment 2

50. CSS 430: Operating Systems - Threads, Scheduling, Assignment 2
Multilevel Queue
Ready queue can be partitioned into separate queues
E.g., foreground (interactive) & background (batch)
Each queue can have its own scheduling algorithm
Scheduling must be done between the queues
Simplest: Fixed priority scheduling
One possibility: 100% priority to foreground
Serve foreground if any; serve background only if no foreground
Problem: possibility of starvation
Another possibility?
CSS 430: Operating Systems - Threads, Scheduling, Assignment 2

51. CSS 430: Operating Systems - Threads, Scheduling, Assignment 2
Multilevel Queue
Ready queue can be partitioned into separate queues
E.g., foreground (interactive) & background (batch)
Each queue can have its own scheduling algorithm
Scheduling must be done between the queues
Simplest: Fixed priority scheduling
One possibility: 100% priority to foreground
Serve foreground if any; serve background only if no foreground
Problem: possibility of starvation.
Another possibility: 80/20 priority using time slices
80% to foreground using RR
20% to background using FCFS
CSS 430: Operating Systems - Threads, Scheduling, Assignment 2

52. Multilevel Queue Scheduling
CSS 430: Operating Systems - Threads, Scheduling, Assignment 2

53. Multilevel Feedback Queue
Process can migrate among queues
Queue placement based on past behavior (feedback)
Multilevel feedback queue scheduler
Parameters?
CSS 430: Operating Systems - Threads, Scheduling, Assignment 2

54. Multilevel Feedback Queue
Process can migrate among queues
Queue placement based on past behavior (feedback)
Multilevel feedback queue scheduler:
number of queues
scheduling algorithms for each queue
when to upgrade a process
when to demote a process
when to select a queue for service
CSS 430: Operating Systems - Threads, Scheduling, Assignment 2

55. Multilevel Feedback Queue
CSS 430: Operating Systems - Threads, Scheduling, Assignment 2

56. Example of Multilevel Feedback Queue
Three queues: Q0, Q1, Q2; all use FCFS
If blocked for I/O, job returned to Q0 when I/O done
New jobs enter queue Q0
When activated (given CPU), job receives 8 milliseconds
If not done, job is preempted, demoted to queue Q1
Jobs in Q1
When activated, job receives 16 additional milliseconds
If not done, job is preempted, demoted to Q2
Jobs in Q2
When activated, job runs until terminated or interrupted
If not done, job is preempted, returned to Q2
CSS 430: Operating Systems - Threads, Scheduling, Assignment 2

57. Programming Assignment 2
Scheduler.java
CSS 430: Operating Systems - Threads, Scheduling, Assignment 2

58. CSS 430: Operating Systems - Threads, Scheduling, Assignment 2
For next time
Readings
Chapter 4: Threads
Chapter 5: CPU Scheduling
CSS 430: Operating Systems - Threads, Scheduling, Assignment 2