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

Subject: Re: [boost] [Boost-users] What's so cool about Boost.MPI?
From: David Abrahams (dave_at_[hidden])
Date: 2010-11-10 14:11:56

At Wed, 10 Nov 2010 02:05:43 -0500,
Sid Sacek wrote:
> 
> 
> On Sat, Nov 6, 2010 at 3:37 AM, Matthias Troyer <troyer_at_[hidden]> wrote:
> >
> > Nice summary!
> > Dave Abrahams
> 
> I've been meaning to ask you... ( after reading your web page in a rushed manner )
> 
> How is your library different from RPC?  Unless I missed the point,
> the library efficiently delivers data to a receiver so that it can
> process it offline. If you can, please, point out some important
> points that I may not have grokked, (I mean other than the network
> and OS abstraction-layer).

RPC is a *paradigm* in which you make Remote Procedure Calls.  The
idea is that you call some function over here

  foo(bar)

and it invokes a function called foo on some representation of bar's
value on a remote machine.  

MPI (which is not my library) is an *API* with a bunch of available
implementations that offers *Message Passing*.  That's just about
movement of data, and the computation patterns aren't limited to those
you can create with an imperative interface like RPC.  Massively
parallel computations tend to do well with a Bulk Synchronous Parallel
(BSP) model, and MPI supports that paradigm very nicely.

Boost.MPI is a library that makes it much nicer to do MPI programming
in C++.

> Also, do you believe that efficiency in data transmission is
> important? 

I'm sure it depends on the underlying technology and the problem
you're applying it to.

> Usually, headers of serialized data adds about 5%
> overhead, unless data packets are small. The internet and local
> networks are about 10-gigabit, and are pushing into the 100-gigabit
> range now. By the time you get a lot of programmers coding to the
> library, the networks and CPU's will be so fast, I hardly think the
> small overheads will make any difference. One thing I've noticed for
> a decade now is that networks are an order or two magnitudes faster
> than computers, I mean, networks deliver data way faster than a
> computer can process it.

That's not commonly the case with systems built on MPI.  Communication
costs tend to be very significant in a system where every node needs
to talk to an arbitrary set of other nodes, and that's a common
pattern for HPC problems.

-- 
Dave Abrahams
BoostPro Computing
http://www.boostpro.com