  This paper reinterprets Amdahl's law in terms of execution time and applies
this simple model to supercomputing. The systematic discussion results in
practical formulas enabling to calculate expected running time using large
number of processors from experimental runs using low number of processors,
delivers a quantitative measure of computational efficiency of supercomputing
applications. Through separating non-parallelizable contribution to fractions
according to their origin, Amdahl's law enables to derive a timeline for
supercomputers (quite similar to Moore's law) and describes why Amdahl's law
limits the size of supercomputers. The paper validates that Amdahl's 50-years
old model (with slight extension) correctly describes the performance
limitations of the present supercomputers. Using some simple and reasonable
assumptions, the absolute performance bound of supercomputers is concluded,
furthermore that serious enhancements are still necessary to achieve the
exaFLOPS dream value.
