Optimal bounds for decision problems on the CRCW PRAM

  • Authors:
  • Paul Beame;Johan Hastad

  • Affiliations:
  • Univ. of Washington, Seattle;Royal Institute of Technology, Stockholm, Sweden

  • Venue:
  • Journal of the ACM (JACM)
  • Year:
  • 1989

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Abstract

Optimal &OHgr;(log n/log log n) lower bounds on the time for CRCW PRAMS with polynomially bounded numbers of processors or memory cells to compute parity and a number of related problems are proven. A strict time hierarchy of explicit Boolean functions of n bits on such machines that holds up to &Ogr;(log n/log log n) time is also exhibited. That is, for every time bound T within this range a function is exhibited that can be easily computed using polynomial resources in time T but requires more than polynomial resources to be computed in time T - 1. Finally, it is shown that almost all Boolean functions of n bits require log n - log log n + &OHgr;(1) time when the number of processors is at most polynomial in n. The bounds do not place restrictions on the uniformity of the algorithms nor on the instruction sets of the machines.