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From: Jeff Squyres (jsquyres_at_[hidden])
Date: 2007-09-05 20:29:51


Unfortunately, <iostream> is there for a specific reason. The
MPI::SEEK_* names are problematic because they clash with the
equivalent C constants. With the tricks that we have to play to make
those constants [at least mostly] work in the MPI C++ namespace, we
*must* include them. The comment in mpicxx.h explains:

// We need to include the header files that define SEEK_* or use them
// in ways that require them to be #defines so that if the user
// includes them later, the double inclusion logic in the headers will
// prevent trouble from occuring.
// include so that we can smash SEEK_* properly
#include <stdio.h>
// include because on Linux, there is one place that assumes SEEK_* is
// a #define (it's used in an enum).
#include <iostream>

Additionally, much of the C++ MPI bindings are implemented as inline
functions, meaning that, yes, it does add lots of extra code to be
compiled. Sadly, that's the price we pay for optimization (the fact
that they're inlined allows the cost to be zero -- we used to have a
paper on the LAM/MPI web site showing specific performance numbers to
back up this claim, but I can't find it anymore :-\ [the OMPI C++
bindings were derived from the LAM/MPI C++ bindings]).

You have two options for speeding up C++ builds:

1. Disable OMPI's MPI C++ bindings altogether with the --disable-mpi-
cxx configure flag. This means that <mpi.h> won't include any of
those extra C++ header files at all.

2. If you're not using the MPI-2 C++ bindings for the IO
functionality, you can disable the SEEK_* macros (and therefore
<stdio.h> and <iostream>) with the --disable-mpi-cxx-seek configure
flag.

See "./configure --help" for a full list of configure flags that are
available.

On Sep 4, 2007, at 4:22 PM, Thompson, Aidan P. wrote:

> This is more a comment that a question. I think the compile-time
> required
> for large applications that use Open MPI is unnecessarily long. The
> situation could be greatly improved by streamlining the number of C+
> + header
> files that are included. Currently, compiling LAMMPS
> (lammps.sandia.gov)
> takes 61 seconds to compile with a dummy MPI library and 262
> seconds with
> Open MPI, a 4x slowdown.
>
> I noticed that iostream.h is included by mpicxx.h, for no good
> reason. To
> measure the cost of this, I compiled the follow source file 1)
> without any
> include files 2) with mpi.h 3) with iostream.h and 4) with both:
>
> $ more foo.cpp
> #ifdef FOO_MPI
> #include "mpi.h"
> #endif
>
> #ifdef FOO_IO
> #include <iostream>
> #endif
>
> void foo() {};
>
> $ time mpic++ -c foo.cpp
> 0.04 real 0.02 user 0.02 sys
> $ time mpic++ -DFOO_MPI -c foo.cpp
> 0.58 real 0.47 user 0.07 sys
> $ time mpic++ -DFOO_IO -c foo.cpp
> 0.30 real 0.23 user 0.05 sys
> $ time mpic++ -DFOO_IO -DFOO_MPI -c foo.cpp
> 0.56 real 0.47 user 0.07 sys
>
> Including mpi.h adds about 0.5 seconds to the compile time and
> iostream
> accounts for about half of that. With optimization, the effect is even
> greater. When you have hundreds of source files, that really adds up.
>
> How about cleaning up your include system?
>
> Aidan
>
>
>
>
>
> --
> Aidan P. Thompson
> 01435 Multiscale Dynamic Materials Modeling
> Sandia National Laboratories
> PO Box 5800, MS 1322 Phone: 505-844-9702
> Albuquerque, NM 87185 FAX : 505-845-7442
> mailto:athomps_at_[hidden]
>
>
>
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-- 
Jeff Squyres
Cisco Systems