Re: [Boost-users] Boost MPI + Serialization

Subject: Re: [Boost-users] Boost MPI + Serialization
From: John Phillips (phillips_at_[hidden])
Date: 2008-11-10 13:15:51

Raymond Wan wrote:
> 
> Hi all,
> 
> I am using Boost MPI and would like to send user-defined objects between 
> processes.  As the Boost MPI tutorial / documentation suggests 
> (http://www.boost.org/doc/libs/1_37_0/doc/html/mpi/tutorial.html), I 
> should use serialization and in this document, they show an example 
> class gps_position which is a friend with boost::serialization::access.
> 
> So far, I understand what's going on, but I don't know what is the next 
> step.  The example in the documentation stops there and  says that the 
> example continues in the Serialization document.  It also just mentions 
> that one could store it in many forms, such as XML on disk, in addition 
> to sending it to processes.  If all I want to do is send it between 
> processes, which one of the formats listed in boost/archive/*.hpp should 
> I use?  In the Serialization document, it continues by creating a 
> boost::archive::text_oarchive object.
> 
> But, if I am sending things like vectors of unsigned integers and 
> strings, a binary format should be more compact?  Or does this not 
> matter and sticking with the Serialization example is fine?
> 
> Thank you in advance!
> 
> Ray

   Ray,

   The reason that the tutorial doesn't talk about archive selection is 
because it is not a concern. The design of the Serialization library 
separates the archive description away from the rules for serializing 
the class, so the class doesn't ever need to know what format the 
archive is using. The same serialization rules work, no matter what 
format the archive is in.

   The MPI library defines the "archive" formats it uses to transmit 
your class in the detail folder of the library, and you need not worry 
about them. The library makes the right choice of archive based on what 
you put in your class definition and use. The reason the documentation 
points out other archive formats is to let you know that the ability to 
use these formats to save your data in other ways comes for free once 
you have added serialization functionality to your class.

   A cruise through the examples for the library will show you a use of 
serializing a custom class, and show that you need never specify the 
archive type. There is an efficiency choice you need to consider. That 
is, do your types have a fixed layout in memory. As the tutorial points 
out, this can provide significant savings. (Note, there is also a useful 
savings if you can assure a homogeneous cluster. There is a macro 
definition for this one.)

   I hope that helps. And, of course, if I got anything wrong or the 
authors have a better way to talk about it, I'm sure a correction will 
show up, soon.

                        John