to be acquired. Questions as simple as, "Why 

 are tiiere two families of galaxies, spiral and 

 elliptical?" remain unanswered. 



The first step in understanding how gala.xies and 

 stars form is to understand the environment in 

 which their formation occurs. Scientists infer 

 that appro.ximately 90 percent of the mass of the 

 Universe is in the form of dark matter, that 

 which can be observed only indirectly, through 

 its gra\itational intluence on obsersable matter. 

 such as stars, gas clouds, and nebulae. 

 Simulations are used to study the shapes and 

 dynamics of dark matter halos that are known to 

 surround observed galaxies. 



One obstacle to the development of efficient 

 codes to simulate the galaxy formation process is 

 the sheer size of the problem. To obtain useful 

 information, the software must simulate the 

 interactions of millions of bodies. Improved 

 algorithms for N-body calculations have been 

 developed and adapted to this problem. Even 

 with the new algorithms, however, conventional 

 supercomputers lack the computational power to 

 perform simulations in a reasonable period of 

 time at an acceptable level of resolution. A few 

 hundred thousand bodies is the most that con- 

 ventional systems can model at a time, about two 

 orders of magnitude too few to obtain new scien- 

 tific results. Hence, scientists must turn to par- 

 allel computers - and very large ones at that - to 

 perform the computations. 



Parallel versions of simulation code are general- 

 ly more difficult to develop than those for 

 sequential computing systems because of the 

 inherent complexity of parallel systems. In this 

 particular case, code development was further 

 complicated by the requirements for internal 

 data organization in the program that needed to 

 adapt as the computation progressed to reflect 

 changes in the structure of the evolvinc universe. 



Concurrent Supercomputing Consortium 

 (CSCC) scientists overcame these programming 

 difficulties and recently developed a parallel 

 simulation code for the 512 node Intel 

 Touchstone Delta System, which achieved 

 speedups in excess of 400 over the single pro- 

 cessor speed. 



In March 1992. researchers ran a simulation with 

 almost 8.8 million bodies for 780 timesteps on 

 512 processors of the Touchstone Delta system. 

 The simulation was of a spherical region of 

 space of diameter 10 megaparsecs (or. about 30 

 million light years); a region large enough to 

 contain several hundred typical galaxies. The 

 simulation ran continuously for 16.7 hours, and 

 carried out 3.24 x 1,014 floating point opera- 

 tions, for a sustained rate of 5.4 gigaflops per 

 second. Algorithm improvements accounted for 

 nearly a 3,000-fold improvement in execution 

 time over traditional calculation methods. 

 Subsequently, the research team ran two 17.15 

 million body simulations using the Cold Dark 

 Matter model of the Universe. These simula- 

 tions, the largest N-body simulations ever run. 

 took into account recently acquired microwave 

 background measurements gathered by the 

 COBE satellite. 



SPONSORING AGENCIES AND 

 ORGANIZATIONS 



Concurrent Supercomputing Consortium 

 DOE 



NASA 



NSF 



PERFORMING ORGANIZATIONS 



California Institute of Technology 

 Center for Research on Parallel Computation 

 Los Alamos National Laboratory 

 University of California-Santa Barbara 



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