All,
I'm not too sure if this is an MPI issue, a Fortran issue, or something
else but I thought I'd ask the MPI gurus here first since my web search
failed me.
There is a chance in the future I might want/need to query an environment
variable in a Fortran program, namely to figure out what switch a currently
running process is on (via SLURM_TOPOLOGY_ADDR in my case) and perhaps make
a "per-switch" communicator.[1]
So, I coded up a boring Fortran program whose only exciting lines are:
call MPI_Get_Processor_Name(processor_name,name_length,ierror)
call get_environment_variable("HOST",host_name)
write (*,'(A,X,I4,X,A,X,I4,X,A,X,A)') "Process", myid, "of", npes, "is
on processor", trim(processor_name)
write (*,'(A,X,I4,X,A,X,I4,X,A,X,A)') "Process", myid, "of", npes, "is
on host", trim(host_name)
I decided to try out with the HOST environment variable first because it is
simple and different per node (I didn't want to take many, many nodes to
find the point when a switch is traversed). I then grabbed two nodes with 4
processes per node and...:
(1046) $ echo "$SLURM_NODELIST"
borgj[020,036]
(1047) $ pdsh -w "$SLURM_NODELIST" echo '$HOST'
borgj036: borgj036
borgj020: borgj020
(1048) $ mpifort -o hostenv.x hostenv.F90
(1049) $ mpirun -np 8 ./hostenv.x | sort -g -k2
Process 0 of 8 is on host borgj020
Process 0 of 8 is on processor borgj020
Process 1 of 8 is on host borgj020
Process 1 of 8 is on processor borgj020
Process 2 of 8 is on host borgj020
Process 2 of 8 is on processor borgj020
Process 3 of 8 is on host borgj020
Process 3 of 8 is on processor borgj020
Process 4 of 8 is on host borgj020
Process 4 of 8 is on processor borgj036
Process 5 of 8 is on host borgj020
Process 5 of 8 is on processor borgj036
Process 6 of 8 is on host borgj020
Process 6 of 8 is on processor borgj036
Process 7 of 8 is on host borgj020
Process 7 of 8 is on processor borgj036
It looks like MPI_Get_Processor_Name is doing its thing, but the HOST one
seems to only be reflecting the first host. My guess is that OpenMPI
doesn't export every processes' environment separately to every process so
it is reflecting HOST from process 0.
So, I guess my question is: can this be done? Is there an option to Open
MPI that might do it? Or is this just something MPI doesn't do? Or is my
Google-fu just too weak to figure out the right search-phrase to find the
answer to this probable FAQ?
Matt
[1] Note, this might be unnecessary, but I got to the point where I wanted
to see if I *could* do it, rather than *should*.
--
Matt Thompson
Man Among Men
Fulcrum of History
