Status of Vectorized Monte Carlo for Particle Transport Analysis

The conventional particle transport Monte Carlo algorithm is ill suited for modem vector supercomputers because the random nature of the particle transport process in the history based algorithm in hibits construction of vectors. An alterna tive, event-based algorithm is suitable for vectorization and has been used recently to achieve impressive gains in perfor mance on vector supercomputers. This re view describes the event-based algorithm and several variations of it Implementa tions of this algorithm for applications in particle transport are described, and their relative merits are discussed. The imple mentation of Monte Carlo methods on multiple vector parallel processors is con sidered, as is the potential of massively parallel processors for Monte Carlo par ticle transport simulations.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/67177/2/10.1177_109434208700100203.pd

Can COBE see the shape of the universe?

In recent years, the large angle COBE--DMR data have been used to place constraints on the size and shape of certain topologically compact models of the universe. Here we show that this approach does not work for generic compact models. In particular, we show that compact hyperbolic models do not suffer the same loss of large angle power seen in flat or spherical models. This follows from applying a topological theorem to show that generic hyperbolic three manifolds support long wavelength fluctuations, and by taking into account the dominant role played by the integrated Sachs-Wolfe effect in a hyperbolic universe.Comment: 16 Pages, 5 Figures. Version published in Phys. Rev.

Quadrupole and octupole collectivity in Ba 143

The neutron-rich barium nuclei have been the subject of intense interest due to the enhanced octupole correlations they are predicted to exhibit. The observation of enhanced octupole collectivity in Ba144,146 as measured in sub-barrier Coulomb excitation, consistent with static octupole deformation, has further heightened this interest. In the present work, these studies are extended to the neighboring odd-mass Ba143 to investigate the interplay between single-particle and collective octupole degrees of freedom. A new measurement of the first 92 - state lifetime is also presented. Reflection-Asymmetric Triaxial Particle Rotor Model calculations indicate that the negative-parity bands in Ba143 can be understood as a decoupled structure of νh9/2 parentage, while the positive-parity bands are built on a decoupled octupole phonon. No evidence for E3 excitation is observed in this work, but an upper limit is placed on the E3 matrix element to the lowest octupole band

Measurement of memory access contentions in multiple vector processor systems

The purpose of this paper is to study delays encountered when several vector processors access a common memory. Such delays may be caused by access conflicts at the memory or by contentions within the network connection processors to common memory. Because such delays are potentially the limiting performance bottleneck of common memory architectures, it is important to understand how they depend on design parameters, in particular the number of processors serviced by the common memory, the degree of memory interleaving, and the speed of the memory chips. Such understanding will make it possible to assess the performance of systems with increasing number of processors and new memory technologies. It is essential for any assessment of the future potential of common memory systems. There are three possible approaches to the study of this problem, analytical models, simulations, and measurements on existing machines. All three approaches are necessary because each has advantages and limitations. Analytical models owe their effectiveness to simplifying assumptions that need to be checked by the other two methods. In particular, stochastic methods and queueing theory offer not techniques to deal with regular memory access patterns that are common in scientific and engineering applications. Simulations can generally approximate reality more closely, however, even simulations do not replace real measurements. They are often very slow and contain assumptions that need to be tested. This paper presents a set of measurements of memory access delay performed on several computers of the Cray family under carefully controlled conditions. 9 refs., 12 figs., 1 tab

Comparative performance evaluation of two supercomputers: CDC Cyber-205 and CRI Cray-1

This report compares the performance of Control Data Corporation's newest supercomputer, the Cyber-205, with the Cray Research, Inc. Cray-1, currently the Laboratory's largest mainframe. The rationale of our benchmarking effort is discussed. Results are presented of tests to determine the speed of basic arithmetic operations, of runs using our standard benchmark programs, and of runs using three codes that have been optimized for both machines: a linear system solver, a model hydrodynamics code, and parts of a plasma simulation code. It is concluded that the speed of the Cyber-205 for memory-to-memory operations on vectors stored in consecutive locations is considerably faster than that of the Cray-1. However, the overall performance of the machine is not quite equal to that of the Cray for tasks of interest to the Laboratory as represented by our benchmark set

I/O performance measurement on Cray-1 and CDC 7600 computers

Disk I/O transfer rates and overhead CPU times were measured as functions of buffer size and number of logically independent I/O channels for several operating systems and 16 I/O routines on the Cray-1 and CDC 7600 computers. By parameterizing the codes for a variable number of channels, buffer sizes, and words transmitted, the effect of these variables is observed for buffered, nonbuffered, and random-access I/O transmissions. To measure CPU-overlapped performance, I/O was performed concurrently with a pretimed compute loop. Rates, sector overhead, and CPU transmission speeds were calculated upon completion of I/O. Effects of memory blocking due to vector operations were observed. Methods and results are presented. 4 figures, 1 table

Kaposi sarcoma risk in HIV-infected children and adolescents on combination antiretroviral therapy from sub-Saharan Africa, Europe, and Asia

BACKGROUND  The burden of Kaposi sarcoma (KS) in human immunodeficiency virus (HIV)-infected children and adolescents on combination antiretroviral therapy (cART) has not been compared globally. METHODS  We analyzed cohort data from the International Epidemiologic Databases to Evaluate AIDS and the Collaboration of Observational HIV Epidemiological Research in Europe. We included HIV-infected children aged <16 years at cART initiation from 1996 onward. We used Cox models to calculate hazard ratios (HRs), adjusted for region and origin, sex, cART start year, age, and HIV/AIDS stage at cART initiation. RESULTS  We included 24 991 children from eastern Africa, southern Africa, Europe and Asia; 26 developed KS after starting cART. Incidence rates per 100 000 person-years (PYs) were 86 in eastern Africa (95% confidence interval [CI], 55-133), 11 in southern Africa (95% CI, 4-35), and 81 (95% CI, 26-252) in children of sub-Saharan African (SSA) origin in Europe. The KS incidence rates were 0/100 000 PYs in children of non-SSA origin in Europe (95% CI, 0-50) and in Asia (95% CI, 0-27). KS risk was lower in girls than in boys (adjusted HR [aHR], 0.3; 95% CI, .1-.9) and increased with age (10-15 vs 0-4 years; aHR, 3.4; 95% CI, 1.2-10.1) and advanced HIV/AIDS stage (CDC stage C vs A/B; aHR, 2.4; 95% CI, .8-7.3) at cART initiation. CONCLUSIONS  HIV-infected children from SSA but not those from other regions, have a high risk of developing KS after cART initiation. Early cART initiation in these children might reduce KS risk