Basic HTML version of Foils prepared 19 September 1997

Foil 50 Why isn't GRAPE a Perfect Solution?

From Master Set of Foils for 1997 Session of CPS615 CPS615 Basic Simulation Track for Computational Science -- Fall Semester 97. by Geoffrey C. Fox
GRAPE will execute the classic O(N^2) (parallel) N body algorithm BUT this is not the algorithm used in most such computations
Rather there is the O(N) or O(N)logN so called "fast-multipole" algorithm which uses hierarchical approach
  • On one million stars, fast multipole is a factor of 100-1000 faster than GRAPE algorithm
  • fast multipole works in most but not all N-body problems (in globular clusters, extreme heterogenity makes direct O(N^2) method most attractive)
So special purpose devices cannot usually take advantage of new nifty algorithms!

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