1. gravitySimulator Beyond the Million Body Problem Collaborators:Rainer Spurzem (Heidelberg) Peter Berczik (Heidelberg/Kiev) Simon Portegies Zwart (Amsterdam) Alessia Gualandris (Amsterdam) Hans-Peter Bischof (RIT) Stefan Harfst and David Merritt Rochester Institute of Technology

2. Modelling Dense Stellar Systems  one approach: direct N-body simulations  exact but very compute-intensive ~O(N 2 )  many problems require large N  e.g. the evolution of binary Black holes  “empty losscone” is artificially repopulated by two-body scattering unless N > 10 6

3. How to deal with large N  A standard Supercomputer  Special-purpose hardware  GRAvity PipEline (GRAPE) (J. Makino, T. Fukushige)  Customed-designed pipelines for force calculations  Very fast (~1 TFlops)  Limited particle numbers (< 1/4 million)  Cost: ~$50K + extras (GRAPE-6)

4. The GRAPE cluster mini-GRAPEs (GRAPE-6A) N < 131,072

5. GRAPE cluster RIT’s gravitySimulator is operational since Feb 2005   32 dual 3GHz-Xeon nodes   32 GRAPE-6A’s   14 Tbyte RAID   low-latency Infiniband interconnects (10Gbps)   Speed: 4 TFlops   N up to 4 Million particles   Cost: $0.5x10 6   Funding: NSF/NASA/RIT   Next largest:   24 nodes (University of Tokyo)   soon 32 nodes (Heidelberg)

6. The Code and Performance  new parallel direct-summation code  fourth-order Hermite integrator  individual, block time steps  achieves best performance  for small particle numbers communication dominates  efficiencies are between 60% (many processors) and 90% (few processors) For details see poster GRAPE PC store local particles select active particles collect all active particles compute local force and sum over all nodes

7. Visualization of N-Body Simulations  new software package “Spiegel”  GUI to plot N-body data and make movies  See Poster for details in collaboration with Hans-Peter Bischof (RIT)