Re: [boost] [MPI] Review comments

From: Ian McCulloch (ianmcc_at_[hidden])
Date: 2006-09-18 12:35:45

Hi,

Matthias Troyer wrote:
> On Sep 17, 2006, at 7:56 PM, Robert Ramey wrote:
[...]
>> Note that you've used packed_archive - I would use mpi_archive
>> instead. I
>> think this is a better description of what it is.
> 
> I still prefer mpi::packed_archive, since there can also be other MPI
> archives. One possible addition to speed up things on homogeneous
> machines might be just an mpi::binary_archive, using a binary buffer.

Yes, this is a realistic idea; almost all MPI programs are run on
homogeneous clusters anyway.  Even in a heterogeneous environment there
remains the question as to whether one can do better than MPI_Pack/Unpack
by using some kind of 'portable' archive (although 'transportable' might be
a better word).  In principle the answer is definitely yes, as the
conversion functions can be inlined.

[...]

> 
>> Really its only a name change - and "packed archive" is already
>> inside an
>> mpi namespace so its not a huge issue.  BUT I'm wondering if the
>> idea of
>> rendering C++ data structures as MPI primitives should be more
>> orthogonal to
>> MPI prototcol itself.  That is, might it not be sometimes
>> convenient to save
>> such serializations to disk?  Wouldn' this provide a portable
>> binary format
>> for free?  (Lots of people have asked for this but no one as been
>> sufficiently interested to actually invest the required effort).
> 
> As Doug Gregor pointed out this is not possible since the format is
> implementation-defined, and can change from one execution to another.

This is only true for MPI-1.1.  MPI-2 supports multiple data representations
and adds the functions MPI_Pack_external and MPI_Unpack_external to convert
to/from the "external32" format defined in section 9.5.2 of the MPI-2
standard.  The intent of this is to be able to transfer data between MPI
implementations.  Also, as part of the file I/O interface, MPI-2 also
allows user-defined representations so in principle it would be possible to
make some kind of adaptor to read a different archive format via the MPI-2
file I/O.  Not that MPI file I/O seems to be used much anyway...

Cheers,
Ian