Jeff,thanks a lot for replying.
My LAM version is 7.0.2.
When I said "blowing up", I mean t1 is actually set to
"NaN" by the function calling MPI_Allreduce. And this
will quickly make the code abort.
Thanks.
Shi
--- Jeff Squyres <jsquyres_at_[hidden]> wrote:
> On Apr 3, 2005, at 12:03 PM, Shi Jin wrote:
>
> > I recently had a very wired problem. I inherited
> somebody 's MPI code
> > but I only want to ran it with single process
> since the problem size
> > is too small to have any speedup with
> parallization. But I still
> > compile the code using mpif90 and run it with
> "lamboot localhost"
> > first. I ran it directly by ./Codename since it is
> equavalent to
> > "mpirun -np 1 ./Codename".
> >
> > But my code blew up at some point and my major
> suspection is in the
> > code, I have two lines at the end of one function
> as:
> > call
> MPI_ALLREDUCE(energy,t1,1,dtype2,MPI_SUM,comm,ierr)
> > call
>
MPI_ALLREDUCE(localbanden,t2,1,dtype2,MPI_SUM,comm,ierr)
> > I suspect that the function returns before
> MPI_ALLREDUCE actually set
> > the correct number to t1 and t2. So I did a simple
> remedy by adding a
> > MPI_BARRIER after each MPI_ALLREDUCE and the code
> runs fine forever.
>
> This is, indeed, quite odd -- what version of LAM
> are you using?
>
> When the communicator only contains one process, the
> ALLREDUCE should
> effectively be a local memory copy (and nothing
> else). And it cannot
> return until energy==t1 and localbander==t2 --
> you're right that MPI
> does not guarantee synchronization through
> collectives, but it does
> guarantee that you're supposed to get the right
> answers.
>
> When you say that your code "blew up", what,
> exactly, do you mean? Do
> you get wrong answers? Or does LAM abort your
> process with some error?
> Or something else?
>
> --
> {+} Jeff Squyres
> {+} jsquyres_at_[hidden]
> {+} http://www.lam-mpi.org/
>
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