arXiv subjects

Simon Portegies Zwart

Publications and source records attributed to Simon Portegies Zwart.

At least 163 records · Page 9Linked to original sources

arXiv2011

High Performance Gravitational N-body Simulations on a Planet-wide Distributed Supercomputer

We report on the performance of our cold-dark matter cosmological N-body simulation which was carried out concurrently using supercomputers across the globe. We ran simulations on 60 to 750 cores distributed over a variety of supercomputers in Amsterdam (the Netherlands, Europe), in Tokyo (Japan, Asia), Edinburgh (UK, Europe) and Espoo (Finland, Europe). Regardless the network latency of 0.32 seconds and the communication over 30.000 km of optical network cable we are able to achieve about 87% of the performance compared to an equal number of cores on a single supercomputer. We argue that using widely distributed supercomputers in order to acquire more compute power is technically feasible, and that the largest obstacle is introduced by local scheduling and reservation policies.

cs.DC↗

arXiv2010

The distinction between star clusters and associations

In Galactic studies a distinction is made between (open) star clusters and associations. For barely resolved objects at a distance of several Mpc this distinction is not trivial to make. Here we provide an objective definition by comparing the age of the stars to the crossing time of nearby stellar agglomerates. We find that a satisfactory separation can be made where this ratio equals unity. Stellar agglomerates for which the age of the stars exceeds the crossing time are bound, and are referred to as star clusters. Alternatively, those for which the crossing time exceeds the stellar age are unbound and are referred to as associations. This definition is useful whenever reliable measurements for the mass, radius and age are available.

astro-ph.GA↗

arXiv2010

A Light-Weight Communication Library for Distributed Computing

We present MPWide, a platform independent communication library for performing message passing between computers. Our library allows coupling of several local MPI applications through a long distance network and is specifically optimized for such communications. The implementation is deliberately kept light-weight, platform independent and the library can be installed and used without administrative privileges. The only requirements are a C++ compiler and at least one open port to a wide area network on each site. In this paper we present the library, describe the user interface, present performance tests and apply MPWide in a large scale cosmological N-body simulation on a network of two computers, one in Amsterdam and the other in Tokyo.

cs.DC↗

arXiv2010

Gravitational tree-code on graphics processing units: implementation in CUDA

We present a new very fast tree-code which runs on massively parallel Graphical Processing Units (GPU) with NVIDIA CUDA architecture. The tree-construction and calculation of multipole moments is carried out on the host CPU, while the force calculation which consists of tree walks and evaluation of interaction list is carried out on the GPU. In this way we achieve a sustained performance of about 100GFLOP/s and data transfer rates of about 50GB/s. It takes about a second to compute forces on a million particles with an opening angle of $θ\approx 0.5$. The code has a convenient user interface and is freely available for use\footnote{\tt http://castle.strw.leidenuniv.nl/software/octgrav.html}.

astro-ph.IM↗

arXiv2010

On the temporal evolution of the stellar mass function in Galactic clusters

We show that we can obtain a good fit to the present day stellar mass functions (MFs) of a large sample of young and old Galactic clusters in the range 0.1 - 10 Msolar with a tapered power law distribution function with an exponential truncation of the form dN/dm \propto m^alpha [1 - exp-(m/m_c)^beta]. The average value of the power-law index alpha is -2, that of beta is 2.5, whereas the characteristic mass m_c is in the range 0.1 - 0.8 Msolar and does not seem to vary in any systematic way with the present cluster parameters such as metal abundance, total cluster mass or central concentration. However, m_c shows a remarkable correlation with the dynamical age of the cluster, namely m_c/Msolar ~ 0.15 + 0.5 tau_dyn^0.75, where tau_dyn is the dynamical age taken as the ratio of cluster age and dissolution time. The small scatter seen around this correlation is consistent with the uncertainties on the estimated value of tau_dyn. We attribute the observed trend to the onset of mass segregation via two-body relaxation in a tidal environment, causing the preferential loss of low-mass stars from the cluster and hence a drift of the characteristic mass m_c towards higher values. If dynamical evolution is indeed at the origin of the observed trend, it would seem plausible that high-concentration globular clusters, now with median m_c ~ 0.33 Msolar, were born with a stellar MF very similar to that measured today in the youngest Galactic clusters and with a value of m_c ~ 0.15 Msolar. This hypothesis is consistent with the absence of a turn-over in the MF of the Galactic bulge down to the observational limit at ~0.2 Msolar and, if correct, it would carry the implication that the characteristic mass is not set by the thermal Jeans mass of the cloud.

astro-ph.GA↗

arXiv2010

Young massive star clusters

Young massive clusters are dense aggregates of young stars that form the fundamental building blocks of galaxies. Several examples exist in the Milky Way Galaxy and the Local Group, but they are particularly abundant in starburst and interacting galaxies. The few young massive clusters that are close enough to resolve are of prime interest for studying the stellar mass function and the ecological interplay between stellar evolution and stellar dynamics. The distant unresolved clusters may be effectively used to study the star-cluster mass function, and they provide excellent constraints on the formation mechanisms of young cluster populations. Young massive clusters are expected to be the nurseries for many unusual objects, including a wide range of exotic stars and binaries. So far only a few such objects have been found in young massive clusters, although their older cousins, the globular clusters, are unusually rich in stellar exotica. In this review we focus on star clusters younger than $\sim100$ Myr, more than a few current crossing times old, and more massive than $\sim10^4$ \Msun, irrespective of cluster size or environment. We describe the global properties of the currently known young massive star clusters in the Local Group and beyond, and discuss the state of the art in observations and dynamical modeling of these systems. In order to make this review readable by observers, theorists, and computational astrophysicists, we also review the cross-disciplinary terminology.

astro-ph.GA↗

arXiv2010

Simulating the universe on an intercontinental grid of supercomputers

Understanding the universe is hampered by the elusiveness of its most common constituent, cold dark matter. Almost impossible to observe, dark matter can be studied effectively by means of simulation and there is probably no other research field where simulation has led to so much progress in the last decade. Cosmological N-body simulations are an essential tool for evolving density perturbations in the nonlinear regime. Simulating the formation of large-scale structures in the universe, however, is still a challenge due to the enormous dynamic range in spatial and temporal coordinates, and due to the enormous computer resources required. The dynamic range is generally dealt with by the hybridization of numerical techniques. We deal with the computational requirements by connecting two supercomputers via an optical network and make them operate as a single machine. This is challenging, if only for the fact that the supercomputers of our choice are separated by half the planet, as one is located in Amsterdam and the other is in Tokyo. The co-scheduling of the two computers and the 'gridification' of the code enables us to achieve a 90% efficiency for this distributed intercontinental supercomputer.

astro-ph.CO↗

arXiv2009

Effects of Primordial Mass Segregation on the Dynamical Evolution of Star Clusters

In this paper we use N-body simulations to study the effects of primordial mass segregation on the early and long-term evolution of star clusters. Our simulations show that in segregated clusters early mass loss due to stellar evolution triggers a stronger expansion than for unsegregated clusters. Tidally limited, strongly segregated clusters may dissolve rapidly as a consequence of this early expansion, while segregated clusters initially underfilling their Roche lobe can survive the early expansion and have a lifetime similar to that of unsegregated clusters. Long-lived initially segregated clusters tend to have looser structure and reach core collapse later in their evolution than initially unsegregated clusters. We have also compared the effects of dynamical evolution on the global stellar mass function (MF) of low-mass main sequence stars. In all cases the MF flattens as the cluster loses stars. The amount of MF flattening induced by a given amount of mass loss in a rapidly dissolving initially segregated cluster is less than for an unsegregated cluster. The evolution of the MF of a long-lived segregated cluster, on the other hand, is very similar to that of an initially unsegregated cluster.

astro-ph.GA↗

arXiv2009

On the onset of runaway stellar collisions in dense star clusters - II. Hydrodynamics of three-body interactions

The onset of runaway stellar collisions in young star clusters is more likely to initiate with an encounter between a binary and a third star than between two single stars. Using the initial conditions of such three-star encounters from direct $N$-body simulations, we model the resulting interaction by means of Smoothed Particle Hydrodynamics (SPH). We find that, in the majority of the cases considered, all three stars merge together, and in such three star mergers, the hydrodynamic simulations reveal that: (1) mass lost as ejecta can be a considerable fraction of the total mass in the system (up to $\sim25$%); (2) due to asymmetric mass loss, the collision product can sometimes receive a kick velocity that exceeds 10 km/s, large enough to allow the collision product to escape the core of the cluster; and (3) the energy of the ejected matter can be large enough (up to $\sim 3\times 10^{50}$ erg) to remove or disturb the inter cluster gas appreciably.

astro-ph.SR↗

arXiv2009

SAPPORO: A way to turn your graphics cards into a GRAPE-6

We present Sapporo, a library for performing high-precision gravitational N-body simulations on NVIDIA Graphical Processing Units (GPUs). Our library mimics the GRAPE-6 library, and N-body codes currently running on GRAPE-6 can switch to Sapporo by a simple relinking of the library. The precision of our library is comparable to that of GRAPE-6, even though internally the GPU hardware is limited to single precision arithmetics. This limitation is effectively overcome by emulating double precision for calculating the distance between particles. The performance loss of this operation is small (< 20%) compared to the advantage of being able to run at high precision. We tested the library using several GRAPE-6-enabled N-body codes, in particular with Starlab and phiGRAPE. We measured peak performance of 800 Gflop/s for running with 10^6 particles on a PC with four commercial G92 architecture GPUs (two GeForce 9800GX2). As a production test, we simulated a 32k Plummer model with equal mass stars well beyond core collapse. The simulation took 41 days, during which the mean performance was 113 Gflop/s. The GPU did not show any problems from running in a production environment for such an extended period of time.

astro-ph.IM↗

arXiv2008

A multiphysics and multiscale software environment for modeling astrophysical systems

We present MUSE, a software framework for combining existing computational tools for different astrophysical domains into a single multiphysics, multiscale application. MUSE facilitates the coupling of existing codes written in different languages by providing inter-language tools and by specifying an interface between each module and the framework that represents a balance between generality and computational efficiency. This approach allows scientists to use combinations of codes to solve highly-coupled problems without the need to write new codes for other domains or significantly alter their existing codes. MUSE currently incorporates the domains of stellar dynamics, stellar evolution and stellar hydrodynamics for studying generalized stellar systems. We have now reached a "Noah's Ark" milestone, with (at least) two available numerical solvers for each domain. MUSE can treat multi-scale and multi-physics systems in which the time- and size-scales are well separated, like simulating the evolution of planetary systems, small stellar associations, dense stellar clusters, galaxies and galactic nuclei. In this paper we describe three examples calculated using MUSE: the merger of two galaxies, the merger of two evolving stars, and a hybrid N-body simulation. In addition, we demonstrate an implementation of MUSE on a distributed computer which may also include special-purpose hardware, such as GRAPEs or GPUs, to accelerate computations. The current MUSE code base is publicly available as open source at http://muse.li

astro-ph↗

arXiv2007

A runaway collision in a young star cluster as the origin of the brightest supernova

Supernova 2006gy in the galaxy NGC 1260 is the most luminous one recorded \cite{2006CBET..644....1Q, 2006CBET..647....1H, 2006CBET..648....1P, 2006CBET..695....1F}. Its progenitor might have been a very massive ($>100$ \msun) star \cite{2006astro.ph.12617S}, but that is incompatible with hydrogen in the spectrum of the supernova, because stars $>40$ \msun are believed to have shed their hydrogen envelopes several hundred thousand years before the explosion \cite{2005A&A...429..581M}. Alternatively, the progenitor might have arisen from the merger of two massive stars \cite{2007ApJ...659L..13O}. Here we show that the collision frequency of massive stars in a dense and young cluster (of the kind to be expected near the center of a galaxy) is sufficient to provide a reasonable chance that SN 2006gy resulted from such a bombardment. If this is the correct explanation, then we predict that when the supernova fades (in a year or so) a dense cluster of massive stars becomes visible at the site of the explosion.

astro-ph↗

arXiv2007

A parallel gravitational N-body kernel

We describe source code level parallelization for the {\tt kira} direct gravitational $N$-body integrator, the workhorse of the {\tt starlab} production environment for simulating dense stellar systems. The parallelization strategy, called ``j-parallelization'', involves the partition of the computational domain by distributing all particles in the system among the available processors. Partial forces on the particles to be advanced are calculated in parallel by their parent processors, and are then summed in a final global operation. Once total forces are obtained, the computing elements proceed to the computation of their particle trajectories. We report the results of timing measurements on four different parallel computers, and compare them with theoretical predictions. The computers employ either a high-speed interconnect, a NUMA architecture to minimize the communication overhead or are distributed in a grid. The code scales well in the domain tested, which ranges from 1024 - 65536 stars on 1 - 128 processors, providing satisfactory speedup. Running the production environment on a grid becomes inefficient for more than 60 processors distributed across three sites.

astro-ph↗

arXiv2007

Distributed N-body Simulation on the Grid Using Dedicated Hardware

We present performance measurements of direct gravitational N -body simulation on the grid, with and without specialized (GRAPE-6) hardware. Our inter-continental virtual organization consists of three sites, one in Tokyo, one in Philadelphia and one in Amsterdam. We run simulations with up to 196608 particles for a variety of topologies. In many cases, high performance simulations over the entire planet are dominated by network bandwidth rather than latency. With this global grid of GRAPEs our calculation time remains dominated by communication over the entire range of N, which was limited due to the use of three sites. Increasing the number of particles will result in a more efficient execution. Based on these timings we construct and calibrate a model to predict the performance of our simulation on any grid infrastructure with or without GRAPE. We apply this model to predict the simulation performance on the Netherlands DAS-3 wide area computer. Equipping the DAS-3 with GRAPE-6Af hardware would achieve break-even between calculation and communication at a few million particles, resulting in a compute time of just over ten hours for 1 N -body time unit. Key words: high-performance computing, grid, N-body simulation, performance modelling

astro-ph↗

arXiv2007

High Performance Direct Gravitational N-body Simulations on Graphics Processing Units -- II: An implementation in CUDA

We present the results of gravitational direct $N$-body simulations using the Graphics Processing Unit (GPU) on a commercial NVIDIA GeForce 8800GTX designed for gaming computers. The force evaluation of the $N$-body problem is implemented in ``Compute Unified Device Architecture'' (CUDA) using the GPU to speed-up the calculations. We tested the implementation on three different $N$-body codes: two direct $N$-body integration codes, using the 4th order predictor-corrector Hermite integrator with block time-steps, and one Barnes-Hut treecode, which uses a 2nd order leapfrog integration scheme. The integration of the equations of motions for all codes is performed on the host CPU. We find that for $N > 512$ particles the GPU outperforms the GRAPE-6Af, if some softening in the force calculation is accepted. Without softening and for very small integration time steps the GRAPE still outperforms the GPU. We conclude that modern GPUs offer an attractive alternative to GRAPE-6Af special purpose hardware. Using the same time-step criterion, the total energy of the $N$-body system was conserved better than to one in $10^6$ on the GPU, only about an order of magnitude worse than obtained with GRAPE-6Af. For $N \apgt 10^5$ the 8800GTX outperforms the host CPU by a factor of about 100 and runs at about the same speed as the GRAPE-6Af.

astro-ph↗

arXiv2007

The formation of higher-order hierarchical systems in star clusters

We simulate open clusters containing up to 182 stars initially in the form of singles, binaries and triples. Due to the high interaction rate a large number of stable quadruples, quintuples, sextuples, and higher-order hierarchies form during the course of the simulations. For our choice of initial conditions, the formation rate of quadruple systems after about 2 Myr is roughly constant with time at $\sim 0.008$ per cluster per Myr. The formation rate of quintuple and sextuple systems are about half and one quarter, respectively, of the quadruple formation rate, and both rates are also approximately constant with time. We present reaction channels and relative probabilities for the formation of persistent systems containing up to six stars. The reaction networks for the formation and destruction of quintuple and sextuple systems can become quite complicated, although the branching ratios remain largely unchanged during the course of the cluster evolution. The total number of quadruples is about a factor of three smaller than observed in the solar neighbourhood.

astro-ph↗

arXiv2007

High Performance Direct Gravitational N-body Simulations on Graphics Processing Unit I: An implementation in Cg

We present the results of gravitational direct $N$-body simulations using the commercial graphics processing units (GPU) NVIDIA Quadro FX1400 and GeForce 8800GTX, and compare the results with GRAPE-6Af special purpose hardware. The force evaluation of the $N$-body problem was implemented in Cg using the GPU directly to speed-up the calculations. The integration of the equations of motions were, running on the host computer, implemented in C using the 4th order predictor-corrector Hermite integrator with block time steps. We find that for a large number of particles ($N \apgt 10^4$) modern graphics processing units offer an attractive low cost alternative to GRAPE special purpose hardware. A modern GPU continues to give a relatively flat scaling with the number of particles, comparable to that of the GRAPE. The GRAPE is designed to reach double precision, whereas the GPU is intrinsically single-precision. For relatively large time steps, the total energy of the N-body system was conserved better than to one in $10^6$ on the GPU, which is impressive given the single-precision nature of the GPU. For the same time steps, the GRAPE gave somewhat more accurate results, by about an order of magnitude. However, smaller time steps allowed more energy accuracy on the grape, around $10^{-11}$, whereas for the GPU machine precision saturates around $10^{-6}$ For $N\apgt 10^6$ the GeForce 8800GTX was about 20 times faster than the host computer. Though still about a factor of a few slower than GRAPE, modern GPUs outperform GRAPE in their low cost, long mean time between failure and the much larger onboard memory; the GRAPE-6Af holds at most 256k particles whereas the GeForce 8800GTX can hold 9 million particles in memory.

astro-ph↗

arXiv2007

The present day mass function in the central region of the Arches cluster

We study the evolution of the mass function in young and dense star clusters by means of direct N-body simulations. Our main aim is to explain the recent observations of the relatively flat mass function observed near the centre of the Arches star cluster. In this region, the power law index of the mass function for stars more massive than about 5-6 solar mass, is larger than the Salpeter value by about unity; whereas further out, and for the lower mass stars, the mass function resembles the Salpeter distribution. We show that the peculiarities in the Arches mass function can be explained satisfactorily without primordial mass segregation. We draw two conclusions from our simulations: 1) The Arches initial mass function is consistent with a Salpeter slope down to ~1 solar mass, 2) The cluster is about half way towards core collapse. The cores of other star clusters with characteristics similar to those of the Arches are expected to show similar flattening in the mass functions for the high mass (>5 solar mass) stars.

astro-ph↗