Amdahl
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Constant problem size. Speedup of a program using multiple 
PEs is limited by sequential fraction of the program and does 
not depend on problem size. 
Solution time varies with number of PEs for a fixed total problem size. 

Gustafson
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Parallel fraction grows with number of PEs and is not bounded by sequential 
fraction of program. Problem is scaled according to number of nodes available. 
Speedup grows linearly with p. Sometimes called weak speedup.