[petsc-dev] Swarm tag error

Dave May dave.mayhem23 at gmail.com
Wed Nov 23 12:07:47 CST 2022

On Mon, 21 Nov 2022 at 12:37, Matthew Knepley <knepley at gmail.com> wrote:

> In data_ex.c, Swarm uses a distinct tag for each pair of processes. If the
> number of processes exceeds 1024, there are > 1024^2 tags which exceeds
> MPI_TAG_UB on Intel MPI.
> My solution is going to be to use that process pair number modulo
> MPI_TAG_UB. Does anyone have a slicker suggestion?

I think it should be possible to use the adjacency graph associated with
the neighbour ranks which is defined within
in the Mat object.

If Intel MPI cannot support tags greater than 1024, the proposition above
is going to be of limited value.
A job with 100 MPI ranks, with subdomains which each have 11 neighbour
ranks will exceed 1024.

>   Thanks,
>       Matt
> --
> What most experimenters take for granted before they begin their
> experiments is infinitely more interesting than any results to which their
> experiments lead.
> -- Norbert Wiener
> https://www.cse.buffalo.edu/~knepley/
> <http://www.cse.buffalo.edu/~knepley/>
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