I always recommend using the same flags for compiling OMPI as compiling your application. Of course, you can vary some flags that don't matter (e.g., compiling your app with -g and compiling OMPI with -Ox). But for "significant" behavior changes (like changing the size of INTEGER), they should definitely match between your app and OMPI.
> As per several previous discussions here in the list,
> I was persuaded to believe that MPI_INT / MPI_INTEGER is written
> in stone to be 4-bytes (perhaps by MPI standard,
> perhaps the configure script, maybe by both),
Neither, actually. :-)
The MPI spec is very, very careful not to mandate the size of int or INTEGER at all.
> and that "counts" in [Open]MPI would also be restricted to that size
> i.e., effectively up to 2147483647, if I counted right.
*Most* commodity systems (excluding the embedded world) have 4 byte int's these days, in part because most systems are this way (i.e., momentum).
Hence, when we talk about the 2B count limit, we're referring to the fact that most systems where MPI is used default to 4 byte int's.
> I may have inadvertently misled Benjamin, if this perception is wrong.
> I will gladly stand corrected, if this is so.
>
> You are the OpenMPI user's oracle (oops, sorry Cisco),
> so please speak out.
Please buy Cisco stuff! :-p