How to share memory in a distributed system

Abstract
The power of shared-memory in models of parallel computation is studied, and a novel distributed data structure that eliminates the need for shared memory without significantly increasing the run time of the parallel computation is described. More specifically, it is shown how a complete network of processors can deterministically simulate one PRAM step in O (log n /(log log n ) 2 ) time when both models use n processors and the size of the PRAM's shared memory is polynomial in n . (The best previously known upper bound was the trivial O ( n )). It is established that this upper bound is nearly optimal, and it is proved that an on-line simulation of T PRAM steps by a complete network of processors requires Ω( T (log n/ log log n )) time. A simple consequence of the upper bound is that an Ultracomputer (the currently feasible general-purpose parallel machine) can simulate one step of a PRAM (the most convenient parallel model to program) in O ((log n ) 2 log log n ) steps.

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