Re: [Boost-users] What's so cool about Boost.MPI?

Subject: Re: [Boost-users] What's so cool about Boost.MPI?
From: Matthias Troyer (troyer_at_[hidden])
Date: 2010-11-06 06:37:36

On 5 Nov 2010, at 20:04, Riccardo Murri wrote:

> On Fri, Nov 5, 2010 at 3:51 PM, Peter Foelsche <foelsche_at_[hidden]> wrote:
>> Can somebody explain to me, what is so great about mpi or boost/mpi?
> 
> Daunting task, but I'll give it a try :-)
> 
> MPI is a library interface[1] to ease some common communication
> patterns in scientific/HPC software.  In particular, MPI provides
> library functions for:
> 
>  1. Reliable delivery of messages between two concurrent processes;
>     there are functions for both synchronous and asynchronous
>     communication.
> 
>  2. Collective communication of the running processes: e.g., one
>     process broadcasting a message to all others; all processes
>     broadcasting to all; scattering a large array to all running
>     processes, etc.
> 
>  3. "Remote shared memory": a process can expose a memory buffer that
>     all others can directly read/write.
> 
>  4. Parallel I/O, where many processes access (possibly interleaved)
>     portions of a file independently.
> 
> Despite being non-trivial features to implement on top of a standard
> TCP socket interface, MPI implementations are mostly into performance:
> they can usually take advantage of special hardware (e.g., Infiniband
> or Myrinet interconnects) and use the native protocols (instead of
> TCP/IP) to provide faster communication speed.
> 
> A "message" in usual socket programming is just a flat array of bytes.
> A "message" in MPI is a (potentially non-contiguous portion) of a C or
> FORTRAN data structure; MPI can handle C structures and
> multi-dimensional C-style arrays, and multi-dimensional arrays of C
> structures (containing multi-dimensional arrays, etc.).  However,
> since MPI is implemented as a library, you have to tdescribe to MPI
> the in-memory layout of the data structures you want to send as a
> message; thus "duplicating" work that is done by the compiler.  You
> can imagine that this quickly gets tedious and unwieldy once you start
> having non-trivial data structures, possibly with members of unknown
> length (e.g., a list).
> 
> This is where Boost.MPI comes to the rescue: if you can provide
> Boost.Serialization support for your classes, you can send them as MPI
> messages with no extra effort: no need for lengthy calls to
> MPI_Type_create_struct() etc.  And it does this with minimal or even
> no overhead!  (Plus you also get a nice C++ interface to MPI
> functionality, whereas the MPI C calls are quite low-level.)
> 
> 
> .. [1] MPI proper is a specification; there are several
>   implementations of the spec (e.g., OpenMPI, MPICH, MVAPICH, plus
>   many vendor/proprietary one), but they are all compatible at the
>   source level.
> 
> Best regards,
> Riccardo

Nice summary!

Matthias