EECE.4810/EECE.5730 Operating Systems

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

2. 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 Operating Systems: Lecture 8

3. 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 Operating Systems: Lecture 6

4. Example Thread AThread 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? 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 Operating Systems: Lecture 8

6. 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 Operating Systems: Lecture 8

7. 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 Operating Systems: Lecture 6

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

9. 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! Operating Systems: Lecture 8

10. 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 Operating Systems: Lecture 8