1. Monitors and Inter-Process Communication
© 2004, D. J. Foreman

2. Basic Concept
Consists of a lock (a mutex) and zero or more condition variables
A queue
© 2004, D. J. Foreman

3. Monitors Look like C++ "classes"
encapsulation
private mutex (semaphore) variable
public interfaces
public interfaces use P, V for protection
acts like a "critical section"
© 2004, D. J. Foreman

4. Condition Variables (cv's)
Private to monitor
Access via:
wait() – suspends process
signal() – lets ONE process resume
queue() – TRUE if #waiters on cv > 0
2 approaches
Hoare – p1 waiting, p0 signals, p1 starts now
Mesa (Hansen) – p1 waiting, p0 signals and continues to run, p1 re-checks when p0 ends
fewer context switches
© 2004, D. J. Foreman

5. Comparison Hoare semantics if (R is held) R.wait(); //proceed
Mesa semantics
while (R held) R.wait(); // forces re-try
© 2004, D. J. Foreman

6. Mesa vs. Hoare
Hoare: signaler releases lock, waking thread acquires and runs
Mesa: signaler keeps lock, waking thread must wait on acquire
© 2004, D. J. Foreman

7. IPC Pipes Message passing anonymous named send() & receive()
limited to parent-child due to file reference
child inherits pipe-end as an open file
named
pipe is opened with a name
names are system-wide
managed like files
Message passing
send() & receive()
synchronous & asynchronous
© 2004, D. J. Foreman

8. Notes on Java The JVM uses monitors for mutual exclusion
provides wait and notify for cooperation
© 2004, D. J. Foreman

9. Condition Variables vs Semaphores
CV's
Semaphores
Only in monitors
Anywhere BUT monitors
Wait always blocks
Wait blocks if count>0
Signal releases a blocked thread or does nothing
Signal releases a blocked thread or increments count
Either caller or released thread continues (Hoare vs Mesa)
Both continue
© 2004, D. J. Foreman