[Beowulf] A fairly simple question
Robert G. Brown
rgb at phy.duke.edu
Sat Aug 13 06:04:53 PDT 2005
Mark Hahn writes:
>> Simple solution is single pc with virtual PCs. it works properly as
> I disagree. you can always run multiple MPI worker tasks on a single
> CPU - you won't see speedup, of course, but you can work on your programming
> logic pretty reasonably. I don't see why virtual PCs would offer any
> advantage over this. the next step is to use a few of real PCs as RGB
> suggests - not only do you start to see speedup, but you can play with
> the reasons why speedup is not perfect. however, I think just two PCs
> is a useful cluster - there's no big reason to have a separate master,
> and even with 2 boxes, you can learn.
...although to see a nonlinear saturation of speedup -- a scaling
"curve" -- you need at least three point, which is why I suggested four
nodes (there is a long and possibly silly tradition of building clusters
with power-of-two numbers of nodes).
It IS possible to do this even with one node, but only with a toy
application -- one that substitutes null delays e.g. sleep() for real
work between real or simulated communication steps. sleep() executes in
parallel even on a single processor, obviously. However, you can't see
"real" parallel speedup on a single processor, or play much with
communications to computation ratios with only two.
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