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Subject: [OMPI users] parallel I/O on 64-bit indexed arays
From: Troels Haugboelle (haugboel_at_[hidden])
Date: 2011-06-06 05:26:59


The problem I face is not open-mpi specific, but I hope still the MPI
wizards on the list can help me nonetheless.

I am running and developing a large-scale scientific code written in
Fortran90. One type of objects are global 1-D vectors, which contains
data for particles in the application. I want to use MPI commands for
saving the particle data but the global 1D array holding the data can
reach up to 100 billion elements, and array offsets and global sizes
have to be 64-bit.

We use MPI_TYPE_CREATE_SUBARRAY for making a custom type and then
MPI_FILE_SET_VIEW and MPI_FILE_WRITE_ALL for saving the 3D field data.
This works with good performance on even very large installations /
runs, but arguments to MPI_TYPE_CREATE_SUBARRAY are 32 bit integers, and
that is not sufficient for the 1D-particle array. It needs 64-bit
offsets and 64-bit global sizes. Local sizes for each thread are 32-bit

What MPI call could I use to make a custom MPI type that describes the
above data, which has 64-bit indices / global size ?

As an example, for 3 threads the type layout would be :

Thread 0: n0 reals, n1 holes, n2 holes
Thread 1: n0 holes, n1 reals, n2 holes
Thread 2: n0 holes, n1 holes, n2 reals

The problem is I have to generalize that to 100 billion elements and
250k threads.

As a remark; given that data keeps on becoming bigger: It would be very
nice if the arguments to MPI_TYPE_CREATE_SUBARRAY, and arguments to
other similar routines could be 64-bit.