Re: OpenMP problem

[Tim Prince <n8tm at aol dot com>]

Jerry DeLisle wrote:
> Carlos Alberto Bulant wrote:
> > Hello,
> >
> > I'm using cygwin, gfortran-4 compiler, in NetBeans IDE. My pc has 
> > WinXP SP3.
> >
> > I'm trying to introduce parallel processing to my project, i've been
> > reading about OpenMP, so i make this litle code to test it:
> >
> > //------------------------------------------------------------------------------------------------- 
> >
> >     PROGRAM HELLO
> > !$      USE omp_lib
> >
> >        INTEGER n, i
> >        PARAMETER (n=505150)
> >        REAL X(n)
> >
> >
> > !$      call omp_set_num_threads(2)
> >        write(6, "(a, i3)") " OpenMP max threads: ",
> >     +        OMP_GET_MAX_THREADS()
> >
> > !$OMP PARALLEL
> >        write(6, "(2(a,i3))") " OpenMP: N_threads = ",
> >     +      OMP_GET_NUM_THREADS(), " thread = ", OMP_GET_THREAD_NUM()
> > !$OMP END PARALLEL
> >
> >
> > !$OMP   PARALLEL DO
> > !$OMP+  SHARED(X)
> > !$OMP+  PRIVATE(i)
> >              DO i = 1, n
> >                 X(i) = i
> > !$              !write(*, *) "  From Thread = ", OMP_GET_THREAD_NUM(), 
> > " i= ",i
> >              END DO
> > !$OMP   END PARALLEL DO
> >
> >      END PROGRAM
> > //------------------------------------------------------------------------------------------------- 
> >
> >
> > The program runs ok with n<= 505150
> > with 505150 > n >= 505170 The output is from one thread only
> > with n > 505170 i get a runtime error, the cause is stack overflow.
> >
> > i set windows environment variables
> > OMP_STACKSIZE=10000
> > GOMP_STACKSIZE=10000
> >
> > but i get the same behaviour...
> >
> > any ideas???
>
> It could be you are just too big for your machine.  Also, when you set 
> environment variables, they do not always 'take' to cygwin terminals 
> that are already open, so you may want to close and reopen everything 
> after setting environment variables.
>
> Although I do build a gfortran binary for Cygwin and I use Cygwin on 
> occasion for real work, I am not a big fan of using windows for serious 
> work.  At the Cygwin web site there are discussions about non-Cygwin 
> apps that do things that interfere called BLODA I think. Flip side, 
> Cygwin actually works pretty damn good most of the time.
>
> Also, check your question at comp.lang.fortran news list.  Those folks 
> have a lot of combined experience.
>
> Jerry
I haven't had any real success with gomp on Windows, including cygwin, 
even for tests just large enough to show OpenMP performance which run 
fine on my laptop under linux, and with ifort on Windows.
It seems to be too heavy a load for gcc developers to handle Windows 
along with the more amenable systems.  I try nearly every week to make a 
new build of gcc/g++/gfortran on cygwin, but the last successful one 
(excellent, apart from gomp) was 6 weeks ago.  It takes several days to 
build and test, compared with a couple hours on linux, and the test 
suite is perhaps unacceptably unreliable on Windows.
I kept persevering with cygwin in the so far mistaken idea that the 
market might consolidate on a good Windows version.  It seemed almost 
about to happen with Windows 2000.  Now Windows 7 x64 and 2008 R2 x64 
are the next candidates, but cygwin is sure not to catch up soon even if 
those do come into wide use.
Even the braggarts with axes to grind fall down when asked to 
demonstrate their claim that Windows is easier to use than linux.  It 
still takes weeks for the experts from Microsoft to set up a moderately 
large Windows MPI cluster, which can be done in a day by a linux 
sysadmin.  Then the job submission scripts for Windows contain double 
the obscurity of linux equivalents, which are bad enough.
I do expect to be presiding over a 32 core single motherboard Windows 
demonstration at SC09, in case you think I'm too prejudiced against 
Windows. Don't know if anyone tried cygwin yet on that machine.l
Tim