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Subject: Re: [OMPI users] Fortran derived types
From: Prentice Bisbal (prentice_at_[hidden])
Date: 2010-05-05 11:50:53

Vedran Coralic wrote:
> Hello,
> In my Fortran 90 code I use several custom defined derived types.
> Amongst them is a vector of arrays, i.e. v(:)%f(:,:,:). I am wondering
> what the proper way of sending this data structure from one processor to
> another is. Is the best way to just restructure the data by copying it
> into a vector and sending that or is there a simpler way possible by
> defining an MPI derived type that can handle it?
> I looked into the latter myself but so far, I have only found the
> solution for a scalar fortran derived type and the methodology that was
> suggested in that case did not seem naturally extensible to the vector case.

It depends on how your data is distributed in memory. If the arrays are
evenly distributed, like what would happen in a multidimensional-array,
the derived datatypes will work fine. If you can't guarantee the spacing
between the arrays that make up the vector, then using
MPI_Pack/MPI_Unpack (or whatever the Fortran equivalents are) is the
best way to go.

I'm not an expert MPI programmer, but I wrote a small program earlier
this year that created a dynamically created array of dynamically
created arrays. After doing some research into this same problem, it
looked like packing/unpacking was the only way to go.

Using Pack/Unpack is easy, but there is a performance hit since the data
needs to be copied into the packed buffer before sending, and then
copied out of the buffer after the receive.