[M3devel] condition variables/win32

[Jay K]

condition variables/win32

 


So..one way I think about condition variables
is that you want to be woken when someone else
leaves the mutex that guards the data that you are dealing with.
You want to know when another thread modifies the data.
(If you have a reader/writer lock, you only want to be
woken when someone exits a write.)

 


Now, if you consider a producer/consumer queue.
There are two interesting occurences.
Transitions from empty to non-empty
and transitions from full to non-full (optionally,
if it is fixed size).

 


Consumers wait for empty to non-empty.
Consumers signal full to non-full.
Producers wait for full to non-full.
Producers signal non-empty to empty.

 


So, in this case, one mutex is likely used with with two condition variables.

 


But, what if we take a simplifying deoptimization and assume that a condition
variable is only ever associated with one mutex?
Anyone existing that mutex wakes up anyone waiting on any condition associated with it?
Like, a condition variable I think becomes stateless and everything is
about the mutex?

 

 

What is the downside?

 


Condition variables are allowed to have spurious wakeups.
This would "just" increase them. Too much?

 


So, therefore, what would be wrong with the following design?
 a mutex contains an event 
 and a number of waiters, zero or non-zero 
 if a mutex is exiting with a non-zero number of waiters, signal the event

 


To handle Signal vs. Broadcast

method 1:
 the number of waiters might be interlocked
 the woken would decrement it
 if it isn't zero, signal the event again

 


method 2:
 the number of waiters is both an integer and a semaphore
 and the lock exiter raises the semaphore by the the integer


 

method 3:
 it is not an auto-reset event and there is a count
  and when the count goes to 0, reset the event
 I think in this case you have to maintain a "wait generation" 
 so that new waiters don't prevent the count from ever hitting 0.

 I think this #3 is what Java might be doing, and is described here:
http://www.cs.wustl.edu/~schmidt/win32-cv-1.html
 "3.3. The Generation Count Solution"


 

also:
http://www.cs.wustl.edu/~schmidt/win32-cv-1.html
3.2. The SetEvent Solution
Evaluating the SetEvent Solution
Incorrectness -- 
 


Is that incorrect case really necessarily incorrect?
It seems unfair, since first waiter should be first woken, but..?


 

Am I missing something? A lot?

 


 - Jay
 		 	   		  
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