Enabling/disabling Myrinet shared memory with mpiexec

[Patrick Geoffray]

Hi Chris,

Chris Samuel wrote:
> 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 ?

If the env variable GMPI_SHMEM is not set or is set to 1, shared memory 
is enabled. If GMPI_SHMEM is set to 2, then shared memory is used only 
for local processes using the same Myrinet board (CC-NUMA mode, where 
you don't want to use shared memory between distant processes in large 
SMPs). If GMPI_SHMEM is set to anything except 1 or 2, then shared 
memory is disabled.

GMPI_SHMEM_PREFIX allows to change the location of the shared memory 
file (we don't use IPC because it's a mess to cleanup, instead a file is 
mmap()ed by all process and immediately unlink()ed). The default is 
"/tmp/gmpi_shmem-" and it's just fine for most config, except when /tmp 
is NFS mounted like on diskless machines.

In short, you don't need to change GMPI_SHMEM_PREFIX if /tmp is not NFS 
mounted, and set GMPI_SHMEM to 0 to turn shared memory off (on by default).

Hope it helps.

Patrick
-- 

Patrick Geoffray
Myricom, Inc.
http://www.myri.com