Error when setting MPI_Info: value is too long - c

Question

I am configuring my MPI_Info with the hosts where the processes will be spawned (MPI_Comm Spawn).
The problem is that when I use a big amount of processes the setting is not performed correctly, throwing this error:
Fatal error in MPI_Info_set: Invalid MPI_Info value , error stack:
MPI_Info_set(117): MPI_Info_set(info=0x9c000000, key=hosts, value=cuda00,cuda00,cuda00,cuda00,cuda00,cuda00,cuda00,cuda00,cuda01,cuda01,cuda01,cuda01,cuda01,cuda01,cuda01,cuda01,cuda02,cuda02,cuda02,cuda02,cuda02,cuda02,cuda02,cuda02,cuda03,cuda03,cuda03,cuda03,cuda04,cuda0
MPI_Info_set(97).: Value is too long
The field "value" is the hostlist and this hostlist is assigned to the MPI_Info structure with:
MPI_Info_set(info, "hosts", mpihosts);
where "mpihosts" is a char*.
Can I change the capacity of MPI_Info?

Answer 1

Indeed, there an hard-coded maximal length for the string describing the values in MPI_INFO. It is defined in mpi.h and it is named MPI_MAX_INFO_VAL. According to the MPI standards, at the beginning of chapter 9 (p397):
values have an implementation-defined maximum length
of MPI_MAX_INFO_VAL... ... If either key or value are larger than the allowed maximums, the errors MPI_ERR_INFO_KEY or MPI_ERR_INFO_VALUE are raised, respectively.
Let's print this value: by trying printf("MPI_MAX_INFO_VAL is %d\n",MPI_MAX_INFO_VAL);, my openmpi value is 256... There is a line #define MPI_MAX_INFO_VAL 1024 in the file mpich3-2/src/include/mpi.h.in of MPICH2. As a result, the list of the hostname can be too long to be stored as the value of a key in an MPI_INFO structure.
As an alternative, the list of the hosts can be stored in a file and the name of the file can be provided to MPI_Comm_spawn() as the value of the key hostfile of the MPI_INFO structure, as detailed in the documentation of OpenMPI

Answer 2

Taking #francis' comments into account, I decided to give other approach to the problem.
Since MPI_MAX_INFO_VAL cannot be altered and MPI_Info_set(info, "hostfile", filename) is ignored, I must use MPI_Comm_spawn_multiple().
MPI_Comm_spawn_multiple() expects a number of nodes and several arrays of that length.
For this solution I keep focus on the MPI_Info field, that now is an array which can be set like that:
MPI_info *infos = malloc(sizeof (MPI_Info) * nodecnt);
for (i = 0; i < nodecnt; i++) {
MPI_Info_create(&infos[i]);
MPI_Info_set(infos[i], "hosts", hostnames[i]);
}
nodecnt is the total amount of nodes to spawn the new processes.
hostnames is an array of strings which have the host name repeated (separated by commas) as many times as processes will be spawned in that host.
So, in our call to MPI_Comm_spawn_multiple() we will use the variable infos.
Mine is similar to this:
MPI_Comm_spawn_multiple(nodecnt, commands, MPI_ARGVS_NULL, maxprocs, infos, 0, MPI_COMM_WORLD, &newcomm, MPI_ERRCODES_IGNORE);
By the way, it is true that this does not solve the problem completely, due to with a very long list of host names, we would get the same problem. However, this solution give us more room have for manoeuvre.