Re: understanding 9grid and clustering?

if you're using C, plan 9's C provides a library thread(2) (<thread.h>)
that implements processes and coroutines inside a single address space,
with communication by message passing or shared memory.
see man 2 thread.
it works on multiprocessors, but only tightly-coupled ones.  there isn't automatic
process migration, let alone fork-across-network.

the message passing is considerably less elaborated than MPI's:
send, receive, alt, with blocking and non-blocking forms.

the plan9ports collection aids portability of
<thread.h> applications from plan 9 to a variety of Unix-like systems.

`cpu' might be regarded as a properly secured remote command execution
that can be made to appear transparent to parts of a larger application running
on many nodes, because a remote side has access to the local name space
of the cpu command.  it doesn't by itself farm out work to many nodes.