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From: Peter Dimov (pdimov_at_[hidden])
Date: 2005-11-23 04:53:56

Matthias Troyer wrote:

> Indeed this sounds like a lot of work and that's why this mechanism
> for message passing was rarely used in the past. The hard part is to
> manually build up the custom MPI datatype, i.e. to inform MPI about
> what the offsets and types of the various data members in a struct
> are.
> This is where the serialization library fits in and makes the task
> extraordinarily easy. Saving a data member with such an MPI archive
> will register its address, type (as well as the number of identical
> consecutive elements in an array) with the MPI library. Thus the
> serialization library does all the hard work already!

I still don't see the 10x speedup in the subject. For a X[], the two
approaches are:

1. for each x in X[], "serialize" into an MPI descriptor
2. serialize X[0] into an MPI descriptor, construct an array descriptor from

Conceptual issues with (2) aside (the external format of X is determined by
X itself and you have no idea whether the structure of X[0] also describes
X[1]), I'm not sure that there will be such a major speedup compared to the
naive (1).

Robert's point also deserves attention; a portable binary archive that
writes directly into a socket eliminates the MPI middleman and will probably
achieve a similar performance as your two-pass MPI approach. It also
supports versioned non-PODs and other nontrivial types. As an example, I
have a type which is saved as

save( x ):

    save( ); // std::string

and loaded as

load( x ):

    string tmp;
    load( tmp );
    x.set_animation( tmp );

Not everything is field-based.

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