Re: ||ism

[manor]

> Is this thread-safety something that completely surrounds the object's
> methods; meaning if one thread calls a method on the object, that object
> is locked and no other thread can call any method on that object?

Almost...

> I can see how that might be used as a large-grain mechanism;
> it's expensive though because it requires maintaining a recursive
> mutex for every instance in the system, plus checking operations
> with each method call.

Which is why the programmer specifies thread-saftey on a per_method basis:

class thing {

  private int height_ ;

  int getHeight(){
    return height_ ;
  }

  synchronized void changeHeight(int newHeight){
    height_ = newHeight ;
  }
}

The mutex is there per instance (only one per instance, btw, which seems to
address the common case), but is only tested per call to changeHeight().

Obviously, no mutex is generated by the compiler if the synchronized keyword
is missing from all the methods.

Oh, if you want a per-class lock, you can specify 'synchronized' on a 'static'
(class) method.

Also, there is language support for a monitor-queue per lock (so, the locks
are potentially full-blown monitors which support a wait() and notify()
method):

class HoldEmAll {

  synchronized void willBlock(){

    // All Threads Do This (In Turn)

    wait() ;

    // After Waiting, All Threads Do This, (In Turn - Mutex is
    // still maintained)!!!

  }

  synchronized void releaseThemAll(){
    notifyAll() ;
  }
}

[All the threads which call HoldEmAll will end up on an internal wait queue
and will lose the lock they got when calling willBlock() (they don't lose
any other locks), then if some other thread calls releaseThemAll(), there
is a race to re-capture the lock and continue the method right after the
wait()].

One of the reasons I am optimistic about threads/SMP for ||ism is that many
programmers are now being introduced to this approach at the language level
when they learn Java. I guess the same effect is achieved by wrapping up
the platform dependent thread-crud into ObjC (pthread vs. lwp etc.).
What is the API? Is this a port of a similar mechanism from Next?

Manor.


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