The original issue, still reflected by the subject heading of this e-mail,
was that a message overran its receive buffer. That was fixed by using
tags to distinguish different kinds of messages (res, jacob, row, and col).
I thought the next problem was the small (10^-10) variations in results
when np>2. In my mind, a plausible explanation for this is that you're
adding the "res_cpu" contributions from all the various processes to the
"res" array in some arbitrary order. The contribution from rank 0 is added
in first, but all the others come in in some nondeterministic order. Since
you're using finite-precision arithmetic, this can lead to tiny round-off
If you want to get rid of those minor variations, you have to perform
floating-point arithmetic in a particular order.