lecture 8:  interprocess communications ii

-both use busy wait, which is bad, and can be "fatal" with PRIORITY
 INVERSION (high priority proc is busy waiting for resource held by
 low priority proc, which never gets a chance to run)

Sleep and wakeup

PRODUCER-CONSUMER problem (aka BOUNDED BUFFER problem)  what are
examples of this?  disk writes, display writes, in general any kind of
data being prepped for output.

1 buffer
producer puts things in
consumer takes things out

what when buffer is full?
-producer goes to sleep
-consumer wakes producer when it removes item

BUT:  simple solution (pg 109) suffers from race condition problem; 
if count == 0 & consumer is about to sleep, then scheduler switches to
producer, producer puts 1 into buffer and tries to wake consumer, but
consumer's already awake and wake signal is lost.

need WAKEUP WAITING BIT to solve.

this is generalized to the SEMAPHORE, introduced by E. W. Dijkstra
(1965).  semaphore is like a count of waiting wakeup's.

DOWN and UP on a semaphore as generalization of SLEEP and WAKEUP.

Dijkstra's notation:  P == down and V == up.

testing value, changing it, and possibly going to sleep must be a
single, indivisible ATOMIC ACTION.  once a semaphore operation has
started, no other proc can access that semaphore until the operation
has completed or blocked (sleep).

UP increments.  if one or more procs is sleeping on that semaphore
(unable to complete a DOWN), the OS chooses one of the sleeping procs
at random and allows it to complete the down.  

thus, after an UP with a proc sleeping on it, the semaphore will still
be 0, but it will have 1 fewer sleeping proc.

no proc ever blocks using an UP.

producer - consumer solution (pp 112) using semaphores:  1 called
mutex for mutual exclusion, two (empty and full) for SYNCHRONIZATION.