Re: [boost] "I want everything" should build "out of the box" (on Windows)

Subject: Re: [boost] "I want everything" should build "out of the box" (on Windows)
From: John Phillips (phillips_at_[hidden])
Date: 2009-07-28 11:57:21

OvermindDL1 wrote:
> On Tue, Jul 28, 2009 at 8:45 AM, OvermindDL1<overminddl1_at_[hidden]> wrote:
>> Also, I have never used an MPI system, but from quick perusing of what
>> MPI based systems look like, why does Boost.MPI create a full MPI
>> system instead of just create a glue on top of it?  Especially since
>> all the MPI system I see seem to cost a rather ridiculous amounts of
>> money?  I am probably missing something about the MPI standard though,
>> but I do not see what from a quick reading that would warrant such
>> costs I see.
>>
> 
> why doesn't Boost.MPI create a full MPI*

   Christopher's answer is the basic reason, but there are some other 
facets.

   First, the MPI specification is big. It is documented at the MPI 
Forum, and is being revised as we speak. You can read the MPI 2.1 
specification documents on line, or buy the 608 page book. Meetings on 
2.2 and 3.0 are currently ongoing.

http://www.mpi-forum.org/docs/docs.html

   Second is that to be useful the implementations have to be carefully 
tuned and tested. Some of the comercial packages simplify this by being 
very platform dependent. A boost implementation would not want to do 
that, and so the implementation gets harder. However, MPI is used for 
ultra high performance codes, so even a few percent performance 
difference is reason for a user to choose something else. Boost doesn't 
have the testing resources to play in this game.

   Third, there are a few widely used and high quality open source 
packages already available. OpenMPI is the best known and most modern 
but others include LamMPI, LaMPI, MPICH and more.Many installations are 
locked in to certain systems because they have gone through the 
extensive process of installing, tuning and getting to know one 
implementation. You would have to supply a lot of value to get them to 
change. Boost.MPI is intentionally designed to be package agnostic. 
Whatever the installation chooses, Boost.MPI should work with it.

   Fourth, MPI includes Fortran and C APIs, as well as a C++ API that is 
different from what Boost.MPI provides. To make a useful version, these 
would have to be included. Now is a particularly bad time to start the 
C++ part of this work, however. News from the MPI Forum is that the 
members of the forum think Boost.MPI is a far better C++ interface than 
the one they provided. They have also noticed that Boost.MPI uses the C 
bindings under the hood and not the C++ bindings. This combination has 
them considering first depricating and then removing the C++ bindings 
from the MPI specification in favor of telling people to look to 
libraries like Boost.MPI.

   In all, I just don't think trying to implement the underlying MPI as 
part of boost is a great choice. Maybe you could do it and convince me 
I'm wrong, but I think there are better uses for your time. (Which might 
include contributing to one of the existing MPI packages.)

                        John