Open MPI logo

Open MPI Development Mailing List Archives

  |   Home   |   Support   |   FAQ   |   all Development mailing list

Subject: Re: [OMPI devel] OpenMPI and R
From: Benedict Holland (benedict.m.holland_at_[hidden])
Date: 2012-04-06 11:21:23


Oh this actually does make a lot of sense. The kicker is that Rmpi doesn't
like to use OMPI and really loves to use LAM so I have to use R in batch
mode by running mpirun -np 12 and specify the host file. I have a bad
feeling that this is loading the library 12 times, once for each R
namespace. While this is annoying, not having Rmpi working was far more so
and ram is cheap. I do agree whole heatedly that dynamic libraries are far
superior to static ones but I wonder if setting the configuration by
default to build both would allow those applications which require the
static libraries to compile to compile but at the same time getting the
applications which use dynamic loading libraries to exist side by side. I
don't plan on writing applications which require -libmpi but I know that
there are developers not nearly in tune with this line of thinking or know
the difference between static and dynamic libraries.

Also this was tripping me up for weeks. I will let the Rmpi developer know
though and hopefully they can shift code to the dynamic libraries or
perhaps take part in development of the library to make it load dynamic
libraries.

Anyway Jeff, thank you for a wonderful explication. I wonder if this should
be posted anywhere on OpenMPI as a note for Rmpi developers which desire to
compile OMPI themselves. The packages which exist out there are multiple
years old at this point, at least for ubuntu and the released version was
actually one of your development releases and had several large bug fixes
since then.

BTW, I don't see myself not using OMPI for a while, are there any simple
projects which I might be able to work on to get to know the code base and
maybe move up the chain? Fixing some low hanging fruit bugs and learning
how to debug OMPI would be optimal.

Thanks again,
~Ben

On Fri, Apr 6, 2012 at 8:45 AM, Jeffrey Squyres <jsquyres_at_[hidden]> wrote:

> On Apr 5, 2012, at 9:07 PM, Benedict Holland wrote:
>
> > Oh how interesting and I hope this helps someone. Following another
> link, I had to use:
> >
> > ./configure --prefix /usr --enable-shared --enable-static
>
> This makes sense. You were falling victim to the fact that R dlopens
> libmpi as a dynamic library in a private namespace. Hence, when Open MPI
> then dlopens its own plugins, it can't find libmpi's symbols. This is a
> generic problem with any system that opens plugins that, themselves, open
> plugns. I wish there was a better solution to this -- the OS guys need to
> give us a better mechanism here. :-(
>
> OMPI's --enable-static option does two things:
>
> - it builds libmpi.a (vs. libmpi.so)
> - it slurps all of OMPI's plugins into libmpi.a (so it doesn't need to
> dlopen anything at run-time)
>
> It's the latter point that is saving you.
>
> Note that you could also just --disable-dlopen (vs. --disable-shared
> --enable-static), which just does the latter of the above things (meaning:
> OMPI still builds libmpi.so), and it should also work for you.
>
> > when compiling this for Rmpi. Just curious, why isn't --enable-static a
> default option?
>
> Among other reasons, shared libraries generally help save memory at run
> time. This is somewhat important as core counts go up. If you "mpirun -np
> 32" on a single, 32-core machine, would you rather have 32 independent
> copies of libmpi.a loaded into RAM, or just one copy that all processes
> share?
>
> Using libmpi.so enables the latter option.
>
> Make sense?
>
> --
> Jeff Squyres
> jsquyres_at_[hidden]
> For corporate legal information go to:
> http://www.cisco.com/web/about/doing_business/legal/cri/
>
>
> _______________________________________________
> devel mailing list
> devel_at_[hidden]
> http://www.open-mpi.org/mailman/listinfo.cgi/devel
>