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Subject: Re: [OMPI users] Scalability issue
From: Benjamin Toueg (btoueg_at_[hidden])
Date: 2010-12-24 09:50:43


I did some testing and felt like giving some feeback. When I started this
discussion I compiled openmpi like that:
./configure --prefix=/home/toueg/openmpi CXX=g++ CC=gcc F77=gfortran
FC=gfortran *FLAGS="-m64 -fdefault-integer-8 -fdefault-real-8
-fdefault-double-8" FCFLAGS="-m64 -fdefault-integer-8 -fdefault-real-8
-fdefault-double-8"* --disable-mpi-f90

Now I compile openmpi like this:
./configure --prefix=/home/toueg/openmpi CXX=g++ CC=gcc F77=gfortran
FC=gfortran --disable-mpi-f90

I still have the segmentation fault I had:
*** Process received signal ***
Signal: Segmentation fault (11)
Signal code: Address not mapped (1)
Failing at address: 0x2c2579fc0
[ 0] /lib/ [0x7f52d2930410]
[ 1] /home/toueg/openmpi/lib/openmpi/ [0x7f52d153fe03]
[ 2] /home/toueg/openmpi/lib/ [0x7f52d3504a1e]
[ 3] /home/toueg/openmpi/lib/

It seems it doesn't change anything to compile openmpi with or without the
options FLAGS="-m64 -fdefault-integer-8 -fdefault-real-8 -fdefault-double-8"
FCFLAGS="-m64 -fdefault-integer-8 -fdefault-real-8 -fdefault-double-8".
I'd like to stress that in both cases MPI_INTEGER size is 4-bytes long.

I'll follow my own intuition and Jeff's advice that is using the same flags
for compiling openmpi as compiling DRAGON.


I always recommend using the same flags for compiling OMPI as compiling your
> application. Of course, you can vary some flags that don't matter (e.g.,
> compiling your app with -g and compiling OMPI with -Ox). But for
> "significant" behavior changes (like changing the size of INTEGER), they
> should definitely match between your app and OMPI.
> > As per several previous discussions here in the list,
> > I was persuaded to believe that MPI_INT / MPI_INTEGER is written
> > in stone to be 4-bytes (perhaps by MPI standard,
> > perhaps the configure script, maybe by both),
> Neither, actually. :-)
> The MPI spec is very, very careful not to mandate the size of int or
> INTEGER at all.
> > and that "counts" in [Open]MPI would also be restricted to that size
> > i.e., effectively up to 2147483647, if I counted right.
> *Most* commodity systems (excluding the embedded world) have 4 byte int's
> these days, in part because most systems are this way (i.e., momentum).
> Hence, when we talk about the 2B count limit, we're referring to the fact
> that most systems where MPI is used default to 4 byte int's.
> > I may have inadvertently misled Benjamin, if this perception is wrong.
> > I will gladly stand corrected, if this is so.
> >
> > You are the OpenMPI user's oracle (oops, sorry Cisco),
> > so please speak out.
> Please buy Cisco stuff! :-p
> --
> Jeff Squyres
> jsquyres_at_[hidden]
> For corporate legal information go to:
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