1. EECE.4810/EECE.5730 Operating Systems
Instructor:
Dr. Michael Geiger
Spring 2019
Lecture 9:
Threads

2. Operating Systems: Lecture 8
Lecture outline
Announcements/reminders
Program 1 due today
Write one program that does everything
Objective list is an outline that could be used to guide development, but feel free to skip steps
If wait() returns -1, called from process without child!
Please respond to the exam scheduling poll
Today’s lecture
Review: multithreading basics
Continue threading discussion
Pthread example programs
8/26/2019
Operating Systems: Lecture 8

3. Operating Systems: Lecture 6
Review: Threads
Thread: active sequence of instructions
Basic unit of CPU utilization
Thread creation is lightweight
Multiple threads in same process can share address space
Each thread needs own PC, register copies, stack + SP
Threads provide concurrency within application
HW support necessary for parallelism
Major issue: non-deterministic ordering
Solutions require atomic operations
Avoid race condition: solution depends on timing/ordering of earlier events
8/26/2019
Operating Systems: Lecture 6

4. Operating Systems: Lecture 8
Example
Thread A Thread B
i = 0 i = 0
while (i < 10) while (i > -10)
i++ i--
print “A done” print “B done”
Which thread finishes first?
Is winner guaranteed to print first?
Is it guaranteed that one thread will win?
What’s required to guarantee one thread will win?
8/26/2019
Operating Systems: Lecture 8

5. Multithreaded debugging
Non-deterministic ordering makes debugging difficult
“Heisenbug”: bug that occurs non-deterministically
All possible interleavings must be correct
Race condition: output/result dependent on timing or ordering of earlier events
Becomes bug when unanticipated ordering occurs
Potentially disastrous consequences
Over-radiation in Therac-25
2 modes of operation: direct low-power radiation, high-powered radiation + safeguards
Race condition activated high-powered beam w/o safeguards
Northeast blackout of 2003
Race condition in control software
8/26/2019
Operating Systems: Lecture 8

6. Operating Systems: Lecture 8
Synchronization
Constrain interleavings between threads
Goal: force all possible interleavings to produce correct result
Correct concurrent program should work regardless of processor speed
Try to constrain as little as possible
Some events are independent—order irrelevant
Order only matters in dependent events
Synchronization: Controlling execution and order of threads
8/26/2019
Operating Systems: Lecture 8

7. Operating Systems: Lecture 6
Pthreads
POSIX thread API (Pthreads) supported on most systems
Necessary functions included in <pthread.h>
Files may need to be compiled with –pthread or –lpthread option
Library contains functions for:
Thread creation and termination
Thread joining (forced waiting  simple synch)
Synchronization (locks, condition variables, barriers)
Other thread management
8/26/2019
Operating Systems: Lecture 6

8. Operating Systems: Lecture 6
Pthread examples
Three example programs
Functions covered
pthread_create(thread, attr, start_routine, arg)
pthread_exit(status)
pthread_join(thread, status)
8/26/2019
Operating Systems: Lecture 6

9. Operating Systems: Lecture 8
Final notes
Next time
Detailed synchronization discussion
Reminders:
Program 1 due today
Write one program that does everything
Objective list is an outline that could be used to guide development, but feel free to skip steps
If wait() returns -1, called from process without child!
8/26/2019
Operating Systems: Lecture 8

10. Operating Systems: Lecture 8
Acknowledgements
These slides are adapted from the following sources:
Silberschatz, Galvin, & Gagne, Operating Systems Concepts, 9th edition
Chen & Madhyastha, EECS 482 lecture notes, University of Michigan, Fall 2016
8/26/2019
Operating Systems: Lecture 8