[Beowulf] ramdisk with tmpfs

[Mark Hahn]

> No, don't use /dev/shm - this is a API mount for posix shared memory.  Mount
> another tmpfs somehwere else (we mount one at /tmp).  tmpfs is very nice, just
> remember that is competes with the RAM demands of your processes.

ramdisks are icky and tmfs is indeed much nicer.  tmpfs competes for ram,
but can be swapped, so is relatively polite.  tmpfs also has options
for limiting the size and number of inodes (set these for safety.)  and since
it's basically allocating ram for you, it also has NUMA parameters - 
see Documentation/filesystems/tmpfs.txt for details.  (I wonder whether 
allocating files mmaped from numa-tweaked tmpfs would provide the kind of
persistent numa binding to pages that some openmpi people want...)