[Beowulf] Threaded code

[Robert G. Brown]

On Tue, 17 Aug 2004, Art Edwards wrote:

> I mean that when we run the threaded code and look at top, don't see two
> processors running. The load is always less than or equal to one. When
> we run two separate jobs, we see a load of 2. 

If you see a load of 2 and they complete in the time of 1, nothing is
wrong with SMP or the two processors, but your code, threaded or not,
isn't running in parallel.  As Mark suggested earlier, that makes me
suspect that your copy of ATLAS is either not threaded or not built for
your (SMP) system or both.

I'd suggest doing a proper build of ATLAS >>just<< for that system,
exploring the build options enough to be sure that the options you are
shooting for are turned on.  I also wouldn't be terribly surprised if
ATLAS doesn't speed up a whole lot when multithreaded.  It depends a bit
on where it is bottlenecked -- if it is bottlenecked at a shared
resource point, it could even slow down.  You're right back to the
fundamental problem of parallel program design -- it is very difficult
to automate because there are so many ways things can interact and
because work cannot always be done faster in parallel.

   rgb

> 
> Art Edwards
> 
> On Tue, Aug 17, 2004 at 07:45:32AM -0700, Alvin Oga wrote:
> > 
> > hi ya
> > 
> > yup.. that is dual cpu
> > 
> > On Tue, 17 Aug 2004, Art Edwards wrote:
> > 
> > > We are running SuSE 9.1 on a system with 8 GB of ram and two scsi disks.
> > > Here is /proc/cpuinfo
> > > processor       : 0
> > 
> > good
> > 
> > > processor       : 1
> > 
> > good
> > 
> > > Linux bladerunner 2.6.4-52-smp #1 SMP Wed Apr 7 01:58:54 UTC 2004 x86_64
> > > x86_64 x86_64 GNU/Linux
> > 
> > good
> > 
> > 
> > > > Newbie question:
> > > > 
> > > > We just purchased a dual opteron box that is running an SMP kernel. We
> > > > have built a fortran code using g77 and a pthreaded atlas. Currently, we
> > > > see only one processor. What really simple thing have we missed?
> > 
> > so what do you mean by "see only one processor" ??
> > 
> > c ya
> > alvin
> 
> 

-- 
Robert G. Brown	                       http://www.phy.duke.edu/~rgb/
Duke University Dept. of Physics, Box 90305
Durham, N.C. 27708-0305
Phone: 1-919-660-2567  Fax: 919-660-2525     email:rgb at phy.duke.edu