[Bug driver/36081] gcc optimizations and threads (pthread)

["snes2002 at freenet dot de" <gcc-bugzilla at gcc dot gnu dot org>]

------- Comment #9 from snes2002 at freenet dot de  2008-04-30 12:26 -------
Subject: Re:  gcc optimizations and threads (pthread)

Hello,

I read about mutexes, cond_wait and cond_signal.
When I use these things instead of "busy loop" there is no performance gain at
all for my problem (=generating the first 50000 prime numbers).
I tested the execution-times of all variants:

Single-thread (compiled with -O2) : 17,5 s
Double-thread with busy-loop (compiled with -O0) : 11 s
Double-thread with mutexes (compiled with -O2) : 47,1 s !!!

But all this is not the problem or the reason why I'm reporting this.

The problem is, that the same code (whatever it does) compiled with "gcc -O0"
or "bcc32 -O0" (with wine) or "bcc32 -O2" works.
But if compiled with "gcc -O2" it doesn't work.
In this case the second thread doesn't read or dereference the actual pointer.
It seems to be cached somehow in a register and not read realtime.

If there are additional exit-conditions in loops (because of
speed-optimization)
they won't be recognized.



----- original Nachricht --------

Betreff: [Bug driver/36081] gcc optimizations and threads (pthread)
Gesendet: Di 29 Apr 2008 21:11:58 CEST
Von: "pinskia at gcc dot gnu dot org"<gcc-bugzilla@gcc.gnu.org>

> 
> 
> ------- Comment #8 from pinskia at gcc dot gnu dot org  2008-04-29 19:11
> -------
> >And for syncing the threads, that's the easiest way.
> 
> Use mutexes then, they are designed exactly for this.
> 
> >I don't want so serialize the threads, I want to use all 2 processors of
> my
> dual-core.
> 
> You are serializing them by doing a busy loop.  In fact using a busy loop
> will
> just make the CPUs spin without doing anything except for checking a
> condition.
>  Using a mutex allows for other stuff to happen while that thread is
> waiting
> for more work to do.
> 
> I guess you need to read up on threading programming more because right now
> you
> are causing extra work to happen on thread for no reason.
> 
> -- Pinski
> 
> 
> -- 
> 
> 
> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36081
> 
> ------- You are receiving this mail because: -------
> You reported the bug, or are watching the reporter.
> 

--- original Nachricht Ende ----


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36081