Re: Threads: monitors vs. primitives

From: scleary_at_[hidden]
Date: 2000-08-11 14:25:54

> > I am fairly sure we can design a monitor/condition interface
> > that is no less efficient than the primitives.
> 
> . . .  For instance, to implement a CV on the Win32 
> platform correctly (i.e. it's both fair and correct) requires the use 
> of a mutex (or critical section), a semaphore and an event.

How would you need anything more than just an event?  Or is your "fair and
correct" definition guaranteeing first-come-first-serve?

> . . . if all you're doing is preventing 
> simultaneous access to a resource that's way too high of a cost.  A 
> monitor, which combines the functionality of a mutex with a CV, is a 
> pretty expensive synchronization object.

I agree with this.  The fact is, monitors are not the Holy Grail of MT
programming.  They are just another sync prim, another tool, that we can
use.  Keep a good collection of tools around; then you can use the right
tool for the right job.  It's a good idea to keep the kids (i.e.,
programmers without MT experience) away from power saws ;), but that doesn't
mean we shouldn't ever use power saws.  (Actually, in this case it's the
"simpler" sync prim's that are more dangerous).

> . . .  So I see the choice as being whether or not 
> to eliminate the CV in favor of only a monitor or should we include 
> both.  Either way, the mutex must stay.

I say keep both; who knows -- an OS may yet surface which provides only a
monitor.  In which case I would have no problem implementing the other tools
in terms of this one.

> . . .  What happens in two years when we find 
> some new synchronization technique that's safer and easier to use 
> than the monitor and all we have is the monitor type in our library?  
> It's going to be harder and less efficient to model the new technique 
> off of only this one synchronization object.

I think it would only be harder because you & I are used to mutexes, CVs,
and semaphores.  If we were already used to monitors, I think it would come
fairly naturally.  OTOH, I would prefer to add a new technique to my
toolbox, and keep all my old tools too.  :)

        -Steve