CS533 Concepts of Operating Systems Class 3

1
CS533 Concepts of Operating SystemsClass 3

• Monitors

2
Questions

• Which multi-threaded 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?
• How does encapsulation help with synchronization?
• How can mutual exclusion in a monitor be
  enforced?
• on uniprocessors and multiprocessors?

3
Questions

• Do monitors solve the deadlock problem?
• Why restrict signal to affect only waiting
  processes in the same monitor?
• Why hand-off the resource directly from the
  signaling process to the signaled waiting
  process?
• What is the advantage of restricting signal calls
  to be the last statement of a monitor procedure?
• What implications does this have for assumptions
  that can be made by processes waking up from a
  wait?

4
Questions

• 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)?
• How might the queuing discipline for waiting
  processes avoid starvation?

5
Questions

• In Hoares bounded buffer example, he assumes
  that the time taken to produce or consume is very
  large in comparison with the time taken to append
  or remove an item from a list
• What are the implications of this?
• Are there similar assumptions in the
  readers/writers example?

6
Questions

• What are the implications of allowing monitor
  calls to make calls to different monitors?
• Which kinds of monitor procedures do not need to
  acquire/release the monitor lock?

7
Questions

• How does non-preemption solve concurrency
  problems?
• What problem does it have on multiprocessors?
• What problem does I/O present?
• What problem does page faults present?

8
Questions

• Mesa operates in an environment that doesnt use
  hardware protection
• What are the implications for 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 Detachp?

9
Questions

• How is handling exceptions in root procedures of
  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?

10
Questions

• What is the difference between Hoares semantics
  for wait/signal and Mesas 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 Hoares semantics for
  wait/signal vs Mesas wait/notify?
• Why do Hoare semantics require more context
  switches?
• Why do they require the signaling mechanism to be
  reliable?

11
Questions

• What are the advantages of Mesas wait/notify
  semantics?
• Why does it help to enable timeouts for waiting
  processes?
• Why does it help to enable broadcast?
• Why is Mesas use of broadcast encouraged by
  their use of compiler-supported protection in a
  single user environment?

12
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 cant 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?

13
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?

14
Reminder

• Please mail me your slides to put on the web page
• The next paper is work in progress, presented by
  Paul McKenney of IBM.