Re: Tasking Techniques - Opinions?

["Samuel A. Mize" <smize@magellan.bgm.link.com>]

Jon S Anthony wrote:
> In article <3395A448.41C6@magellan.bgm.link.com> "Samuel A. Mize" <smize@magellan.bgm.link.com> writes:
> 
> > Absolutely true.  However, do be aware that protected types have
> > significant overhead -- not as much as task rendezvous, but on the
> > order of ten times a procedure call.
> 
> I find this surprising. On what do you base this 10X figure?

It surprised me too, and forced me to redesign data-sharing
in some real-time code.  (Measure everything, early.  Thank
heaven we DID.)

As Tom Quiggle of SGI pointed out to me in email, Ada semantics
surround a protected call with a lot of required processing:
a) deferral of abortion/asynchronous transfer of control (ATC)
   for the current task
b) acquisition of of a mutex
c) execution of the protected actions
d) release the mutex
e) re-enable abortion/ATC
f) check for pending abort/ATC

Some of this will be system calls, which take longer than
simple procedure calls on most systems I know of.  In a
multi-processor environment, those system calls will have to
coordinate between processors, which takes even more time.

Sam Mize

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