Enabling/disabling Myrinet shared memory with mpiexec

[Chris Samuel]

Hi folks,

The Myrinet MPICH (MPICH-GM) has its own shared memory system (different to 
the --with-comm=shared) that can be disabled at runtime by the mpirun.ch_gm 
command using the flag --gm-no-shmem, according to the FAQ entry at:

	http://www.myri.com/fom-serve/cache/161.html

I did some looking at the Myrinet mpirun.ch_gm.pl script and found this Perl 
fragment that gets run:

# Initialize the shared memory flag.
if ($use_shmem > 0) {
  if (!$shmem_file) {
    $shmem_file = "/tmp/gmpi_shmem-$magic:[0-9]*.tmp";
  }

  print "Shared memory file: $shmem_file\n\n" if $verbose;
}
$varenv .= " GMPI_SHMEM=$use_shmem";
$varenv .= " GMPI_SHMEM_PREFIX=$shmem_prefix";

$use_shmem is set to 1 by default and to 2 if --gm-numa-shmem is passed 
through.  It's set to 0 if --gm-no-shmem is specified.

Looking at the mpiexec code it would appear that there's no way for this to be 
turned on by default a la mpirun.ch_gm, the only mention is that you can set 
the environment variable GMPI_SHMEM=0 to disable it, but as it and 
GMPI_SHEMEM_PREFIX are never set in the first place I can't see how it that 
would make any difference ?

I'd much prefer it if mpiexec handled this the same way that mpirun.ch_gm did, 
and I'll work on a patch to it today to do this, but I'd like to know whether 
or not I'd be wasting my time and have misunderstood something critical.. :-)

cheers!
Chris
-- 
 Christopher Samuel - (03)9925 4751 - VPAC Systems & Network Admin
 Victorian Partnership for Advanced Computing http://www.vpac.org/
 Bldg 91, 110 Victoria Street, Carlton South, VIC 3053, Australia

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