CS533 Concepts of Operating Systems Class 3

1. CS533 Concepts of Operating SystemsClass 3 Monitors

2. Questions • Which concurrent programming problems do monitors address? • Are monitors useful as a programming convention, i.e., without compiler support? • What is the convention? • With compiler support for monitors, what are the important tasks of the compiler? • Why does encapsulation help with synchronization? CS533 - Concepts of Operating Systems

3. Questions • Do monitors solve the deadlock problem? • How might the queuing discipline for waiting processes avoid starvation? • Why do we need condition variables instead of a single waiting queue per monitor? • Does a condition variable have a value (such as true/false)? CS533 - Concepts of Operating Systems

4. Questions • What are the implications of allowing monitor calls to make calls to different monitors? • Should the monitor lock be released before such a call? • Which kinds of monitor procedures do not need to acquire/release the monitor lock, and why? CS533 - Concepts of Operating Systems

5. Questions • How does non-preemptive scheduling solve concurrency problems? • What problem does it have on multiprocessors? • What problem does I/O present? • Why are page faults a problem for this approach? • How can mutual exclusion in a monitor be enforced? • … on uniprocessors and multiprocessors? CS533 - Concepts of Operating Systems

6. Questions • Mesa operates in an environment that doesn’t use hardware protection • How does this affect the cost of monitor operations? • How is processes creation in Mesa similar to thread creation in Pthreads? • Why join? • Why detatch? • Why might you still use p after calling Detach[p]? • What dangling reference problems can arise with process pointers? CS533 - Concepts of Operating Systems

7. Questions • How is handling exceptions in root procedures of Mesa processes more complicated than in normal Mesa procedures? • What issues arise when an exception is generated within a monitor procedure? • What does UNWIND do in Mesa? CS533 - Concepts of Operating Systems

8. Questions • What is the difference between Hoare’s semantics for wait/signal and Mesa’s wait/notify? • What invariants can be assumed in each case? • What implications does this have for code surrounding a wait call? • What is the advantage of Hoare’s semantics for wait/signal vs Mesa’s wait/notify? • Why do Hoare semantics require more context switches? CS533 - Concepts of Operating Systems

9. Questions • What are the advantages of Mesa’s wait/notify semantics? • In what way does it enable the use of timeouts? • In what way does it encourage the use of broadcast? CS533 - Concepts of Operating Systems

10. Questions • How might you use monitors for I/O • Can interrupts be treated as signals? • What if no process was waiting? • How can devices make monitor calls? • If a device can’t wait on the monitor lock, how can it update monitor state? • What is a naked notify, and who is waiting on it? • Why is a wakeup-waiting switch needed with naked notifies? CS533 - Concepts of Operating Systems

11. Questions • Why implement process creation as a monitor? • How is fork implemented? • How are join and end implemented? • Why is fork/join so expensive compared to other operations? CS533 - Concepts of Operating Systems

12. Reminder • Please mail me your slides to put on the web page • The next paper is work in progress, presented by me CS533 - Concepts of Operating Systems