Scripted HTML version of Foils prepared 12 November 1996

Foil 15 Parallel Execution Time -I

From CPS615-End of N-Body Discussion and Beginning of Numerical Integration Delivered Lectures of CPS615 Basic Simulation Track for Computational Science -- 15 October 96. by Geoffrey C. Fox *
Secs 79.2
Consider time for Runge Kutta invocation of function Grav
Shifting particles communicates one set of particle information - all processors communicate at the same time giving estimate:
9 * tcomm (factor should be 7 as need only 1 not 3 masses as we used in simple implementation earlier)
  • we are ignoring latency which in practice means best implementation transfers several (not N-1 as in naive data parallel algorithm) particles at a time
Floating point calculations: roughly 3(x,y,z) of -, *, sum, sqrt, exp, /, *, +, *, + which can be summarized as estimate: > 30 tfloat
Each communicated particle is interacted with the N/Nproc particles in the local partition of that processor and each step has one shift giving a total time for (N-1)/2 steps in Grav:

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