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

[gpriv@axonx.com]

On Mar 11, 11:41 am, Vadim Godunko <vgodu...@gmail.com> wrote:
> On Mar 11, 4:58 pm, george.p...@gmail.com wrote:
>
> > This code is not multi-core safe.  Are you sure that QString has
> > Vtab?  If not then comparison will be unfair.
>
> What is it "Vtab"?
>
> QString implemented in the same way as Universal_String. Both classes
> internally use shared data - actual string and reference counter.
> Operations on both classes are "reentrant" (see 1) and not "thread-
> safe" (see 2). Both classes will be even not "reentrant" without
> atomic increment/decrement operations.
>
> PS.
>
> (1) A reentrant function can be called simultaneously by multiple
> threads provided that each invocation of the function references
> unique data.
>
> (2) A thread-safe function can be called simultaneously by multiple
> threads when each invocation references shared data. All access to the
> shared data is serialized.

Sorry for double post, did not get it through.

Thanks for some useful education :-)

VTab is virtual call table, created when class has at least one
virtual function. That will make it similar to tagged record.
Otherwise it will be similar to a plain Ada record and destructor may
be inlined.  So to be fair, your QString should have at least virtual
destructor.  That will cause indirect call to actual one.  The
constructors in C++ not virtual by definition, so to be totally equal
should have virtual init and adjust that will be called from
constructor and copy operator.  That may get rid of your 4+ advantage.

With C++ to be totally multicore thread safe, you need to make
"volatile" all the data that has any possibility to be accessed from
different threads. That makes access to this variables much slower.
Otherwise, you may get incorrect readings once in a while. Few years
back I spent months chasing these misreadings when moved heavily
mullti-threded app to multi-core.  These bugs are most nasty, since
they rarely cause big trouble, manifesting themselves only once in a
while (matter of weeks).


Regards,

George Privalov