[Beowulf] disabling swap on cluster nodes?

[Jason Riedy]

And Bogdan Costescu writes:
> Your description fits *light* swap usage. However, in quite some years
> of combined experience on both sides (user and sysadmin) I have never
> seen such swap usage on a compute node: [...]

You know all those system daemons that aren't doing anything while a
compute job is running?  Those get pushed out to swap if the thing
doing real work needs the memory.  They are swapped back into memory
between jobs (ideally not during).

It'd be nice simply not to have those daemons or to force them into
out-of-memory hibernation, but that's much more effort.  (Also, it'd
be nice for boot times on 1+TiB nodes to be less than 15 minutes
before even touching the OS.  ugh.  If only there were a fast scan
of a small portion where the OS will be loaded, then the OS could
scan the rest...)

> Overcommitting is unfortunately still needed for the older Fortran
> applications which statically allocate large arrays.

Also for modern runtime systems that allocate a large heap they
manage themselves.  Or C code that does similar things to avoid
copying pages while remaining portable (e.g. UMFPACK).