Souper Computer
James R. Van Zandt
jrv at vanzandt.mv.com
Thu Feb 25 20:53:55 EST 1999
Scott Lipcon <slipcon at cs.jhu.edu> writes:
>compare it to a uniprocessor system... "Speedup" is defined as speed of
>a uniprocessor divided by speed of the parallel machine. For an N
>processor machine, the maximum possible speedup is N. You wont get
>that in real life. A simple model is Amdahl's Law, which says the
>following:
>
>Assume that for a given program, x% can be run only on one processor.
>Also assume that the rest of the job is entirely parallelizable.
>Therefore (1 - x)% of the job can be run on all N processors. The
>speedup in this case is:
>N / ( 1 + (N - 1)x)
Several years later, another researcher (whose name I have
unfortunately forgotten, but you will probably hear about in your
course) "refuted" Amdahl's Law, in the following sense: Amdahl's Law
is based on solving a fixed size problem faster. It is often more
useful, and more feasible, to solve a bigger problem in about the same
time. That is, you can come closer to solving an N times bigger
problem in the same time than you could to solving a fixed size
problem in 1/N the time.
- Jim Van Zandt
-
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