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Interesting points. And to contribute to this discussion from a technological point of view. There is a small thing of Multiprocessor vs. Cluster.
I've done a few experiment runs with Mosix and MPI/PVM type of stuff. For those who don't know what I'm talking about. An overview:
Mosix - An augmentation to the Linux kernel to migrate processes from node to node.
MPI - An API for programming multiprocessor/cluster Architectures.
Now as I seen it. I think that mosix is the best tool a "perl"-programmer can use to do cluster stuff. Using simple fork's and if a script runs up processor usage it gets migrated. But I wouldn't call it exactly totaly efficient.
And the relation with MPI/PVM stuff to threading is close. It is a "threading" api that is designed to conserve bandwith.(the first version of MPI didn't have any support for shared variables that I could see)
One common thing among all these things is that communication between processes/threads isn't encouraged by the "standards". To conserve bandwith. So accessing global variables through different threads isn't exactly a good idea for more than one reason...
I wont rant anymore...