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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.

-- 
Prentice