Jeff and Sven, Thank you for your helpful responses to my grumpy, uninformed, inquiry. I like option 3 best, because while I currently do not require OMPI's C++ bindings, I might in the future, in which case I would probably spend ours trying to figure out why I can't find the C++ headers.
I added -DOMPI_SKIP_MPICXX to my LAMMPS Makefile, and it dropped the compile time by 4x, so it now compiles just as fast as the serial version. Very nice. Aidan > > ------------------------------ > > Message: 3 > Date: Thu, 6 Sep 2007 10:56:54 +0200 > From: Sven Stork <st...@hlrs.de> > Subject: Re: [OMPI users] Open MPI compiler is slowed down by > including unnecessary header files > To: us...@open-mpi.org > Message-ID: <200709061056.55017.st...@hlrs.de> > Content-Type: text/plain; charset="iso-8859-1" > > On Thursday 06 September 2007 02:29, Jeff Squyres wrote: >> 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. > > maybe this could be a third option: > > 3. just add -DOMPI_SKIP_MPICXX to you compilation flags to skip the inclusion > of the mpicxx.h. > > -- Sven > >> See "./configure --help" for a full list of configure flags that are >> available.
