[Omp] Overhead of #pragma omp for static nowait

[Greg Bronevetsky]

What kind of false sharing issues? Are you referring to my code below or
to typical OpenMP implementations?

                             Greg Bronevetsky

On Mon, 11 Dec 2006, Dieter an Mey wrote:

> Greg,
> if you really use static(1) you may hit false sharing issues ...
> 
> Dieter
> 
> Greg Bronevetsky schrieb:
> > I mean the following compiler transformation:
> >   #pragma omp for static(1) nowait
> >   for(int i=0; i<n; i++){}
> > should become:
> >   for(int i=omp_get_thread_num(); i<n; i+=omp_get_num_threads())
> >   {}
> > 
> > and
> >   #pragma omp for static nowait
> >   for(int i=0; i<n; i++){}
> > should become:
> >   // the id of the last thread that gets 1 more iteration than others
> >   int midPoint=n%omp_get_num_threads();
> >   // number of iterations assigned to threads with smaller ids
> >   int itersBeforeMe;
> >   if(omp_get_thread_num()<=midPoint)
> >      itersBeforeMe = omp_get_thread_num()*(n/omp_get_num_threads()+1);
> >   else
> >      itersBeforeMe = midPoint*(n/omp_get_num_threads()+1)+
> >                      (omp_get_thread_num()-midPoint)*(n/omp_get_num_threads());
> >   // number of iterations assigned to this thread
> >   int numIter;
> >   if(omp_get_thread_num()<=midPoint)
> >      numIter = n/omp_get_num_threads()+1;
> >   else
> >      numIter = n/omp_get_num_threads();
> > 
> >   for(int i=itersBeforeMe; i<itersBeforeMe+numIter; i++)
> >   {}
> > 
> > Other chunk sizes or loop bounds would involve more complex arithmetic to
> > set up loop bounds but the basic idea is pretty much the same. The overall
> > cost of the above implementation of "#pragma omp for static(1) nowait"
> > should be several ns per iteration. However, I am seeing much higher
> > overheads in my experiments.
> > 
> >                              Greg Bronevetsky
> > 
> > On Fri, 8 Dec 2006, Meadows, Lawrence F wrote:
> > 
> >> What do you mean by "converting to a set of serial loops" 
> >>
> >> -----Original Message-----
> >> From: omp-bounces at openmp.org [mailto:omp-bounces at openmp.org] On Behalf
> >> Of Greg Bronevetsky
> >> Sent: Friday, December 08, 2006 12:48 PM
> >> To: omp at openmp.org
> >> Subject: [Omp] Overhead of #pragma omp for static nowait
> >>
> >> I have recently executed the EPCC microbenchmarks on several machines
> >> and
> >> noticed that there is a consistent overhead of ~1us (~several thousand
> >> cycles) for #pragma omp for static nowait and its variants on the
> >> platforms I've tried. Given the simplicity of this scheduling policy, it
> >> seems to me that it should be possible to convert the parallel loop into
> >> a
> >> set of serial loops at compile-time. This would result in a loop that
> >> requires no inter-thread communication and costs only a few tens of
> >> cycles. 
> >>
> >> What is the reason for this much-higher than expected overhead? Is it
> >> just
> >> that the above compiler analysis is not typically performed or is there
> >> a
> >> more fundamental reason. Here at LLNL, we have applications that would
> >> like to use OpenMP to parallelize loops with ~50 iterations and ~.25us
> >> of
> >> work per iteration. ~1us overheads for the #pragma omp for static nowait
> >> make OpenMP too expensive for this task.
> >>
> >>                              Greg Bronevetsky
> >>
> >> _______________________________________________
> >> Omp mailing list
> >> Omp at openmp.org
> >> http://openmp.org/mailman/listinfo/omp
> >>
> >>
> > 
> > _______________________________________________
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> > http://openmp.org/mailman/listinfo/omp
> > 
> 
> -- 
> --------------------------------------------------------------------
> Dieter an Mey
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>