CS4410/4411: Operating Systems

Fall 2011, Prof. Sirer

Miniproject: Synchronization FAQ

General guideline

Q: Are we allowed to take advantage of the optional argument to the acquire method that causes the semaphore to be non-blocking and return true or false if the acquire does not succeed?

A: No, for MP1, please stick with the semaphore interface we discussed in class (init, P, and V). It's clear and simple to reason about.

Question 10

In q10 all the percentages are with respect to the current number of people in the club.

Monitors in Python

How can I use monitors in Python? There is no object called monitor; only Locks, Semaphores and Condition variables. Should I create a Monitor class?

In Python the behaviors of Monitors are realized by Condition() objects that carry a Lock() inside them. By acquiring and releasing this Lock you enter the monitor and then you can wait, notify and notifyAll on the Condition object(s).

Here is a pseudo example:

class  PrinterRoom():
  def  __init__(self):
          self.printerBusy = False
          self.printerLock = Lock()
          self.printerAvailable = Condition(self.printerLock)

  def  waitforPrinter(self):
          with printerLock:                  #acquire the monitor lock
            while printerBusy:        #check if you need to wait
              printerAvailable.wait()        #wait until somebody notifies you
            printerBusy = True               #set printerBusy to True, to make others wait

  def  donewithPrinter(self):
          with printerLock:                  #acquire the monitor lock
            printerBusy = False              #set printerBusy to False, when you are done
            printerAvailable.notify()     #notify one person waiting for the printer to be available

class  Student():

  def  printdocument(self):
          pr = PrinterRoom()
          while True:
            use printer now
            chat with friends and hang out

For more information on synchronization objects in Python, you can read the Python threading module documentation.

© 2011, Cornell University