Re: Robert Dewar's great article about the Strengths of Ada over other langauges in multiprocessing!

[Ole-Hjalmar Kristensen <ole-hjalmar.kristensen@substitute_employer_here.com>]

>>>>> "FW" == Florian Weimer <fw@deneb.enyo.de> writes:

    FW> * Ole-Hjalmar Kristensen:
    >> Second, I cannot find anything in the RM which says you can make *any*
    >> assumptions about objects which are outside the protected object so I
    >> cannot see how signaling will help you.

    FW> I think that's what 9.10 (Ada 95 without TC 1) is about.

Yes, you are right. Thanks. I looked in the part describing protected
objects without finding it.

But one part of 9.10 puzzles me:

 9.c    Reason:  The point of this distinction is so that on
        multiprocessors with inconsistent caches, the caches only need
        to be refreshed at the beginning of an entry body, and forced
        out at the end of an entry body or protected procedure that
        leaves an entry open.  Protected function calls, and protected
        subprogram calls for entryless protected objects do not require
        full cache consistency.  Entryless protected objects are
        intended to be treated roughly like atomic objects -- each
        operation is indivisible with respect to other operations
        (unless both are reads), but such operations cannot be used to
        synchronize access to other nonvolatile shared variables.

If you do not refresh the cache at the beginning of a protected
procedure, how do you avoid reading stale data within the protected
object on a multiprocessor? And why the wording "cannot be used to
synchronize access to other *nonvolatile* shared variables"? Is the
implication that it *can* be used to synchronize other *volatile*
shared variables?

Btw., I ran a simple test on a SPRAC multiprocessor with an entryless
protected object containing a single integer versus an integer
declared with pragma atomic, and as expected the pragma atomic
solution was much (40x) faster. 

-- 
   C++: The power, elegance and simplicity of a hand grenade.