José Gracia

SYNTHETIC SYNCHROTRON EMISSION MAPS FROM MHD MODELS FOR THE JET OF M87

We present self-consistent global steady state MHD models and synthetic optically thin synchrotron emission maps for the jet of M87. The model consists of two distinct zones: an inner relativistic outflow, which we identify with the observed jet, and an outer cold disk wind. While the former does not self-collimate efficiently due to its high effective inertia, the latter fulfills all the conditions for efficient collimation by the magnetocentrifugal mechanism. Given the right balance between the effective inertia of the inner flow and the collimation efficiency of the outer disk wind, the relativistic flow is magnetically confined into a well-collimated beam and matches the measurements of the opening angle of M87 over several orders of magnitudes in spatial extent. The synthetic synchrotron maps reproduce the morphological structure of the jet of M87, i.e., center bright profiles near the core and limb bright profiles away from the core. At the same time, they also show a local increase of brightness at some distance along the axis associated with a recollimation shock in the MHD model. Its location coincides with the position of the optical knot HST-1. In addition, our best fitting model is consistent with a number of observational constraints such as the magnetic field in the knot HST-1 and the jet-to-counterjet brightness ratio.

DASH: Data Structures and Algorithms with Support for Hierarchical Locality

DASH is a realization of the PGAS (partitioned global address space) model in the form of a C++ template library. Operator overloading is used to provide global-view PGAS semantics without the need for a custom PGAS (pre-)compiler. The DASH library is implemented on top of our runtime system DART, which provides an abstraction layer on top of existing one-sided communication substrates. DART contains methods to allocate memory in the global address space as well as collective and one-sided communication primitives. To support the development of applications that exploit a hierarchical organization, either on the algorithmic or on the hardware level, DASH features the notion of teams that are arranged in a hierarchy. Based on a team hierarchy, the DASH data structures support locality iterators as a generalization of the conventional local/global distinction found in many PGAS approaches.

DART-MPI: An MPI-based Implementation of a PGAS Runtime System

A Partitioned Global Address Space (PGAS) approach treats a distributed system as if the memory were shared on a global level. Given such a global view on memory, the user may program applications very much like shared memory systems. This greatly simplifies the tasks of developing parallel applications, because no explicit communication has to be specified in the program for data exchange between different computing nodes. In this paper we present DART, a runtime environment, which implements the PGAS paradigm on large-scale high-performance computing clusters. A specific feature of our implementation is the use of one-sided communication of the Message Passing Interface (MPI) version 3 (i.e. MPI-3) as the underlying communication substrate. We evaluated the performance of the implementation with several low-level kernels in order to determine overheads and limitations in comparison to the underlying MPI-3.

Tools for High Performance Computing 2012

Current advances in High Performance Computing (HPC) increasingly impact efficient software development workflows. Programmers for HPC applications need to consider trends such as increased core counts, multiple levels of parallelism, reduced memory per core, and I/O system challenges in order to derive well performing and highly scalable codes. At the same time, the increasing complexity adds further sources of program defects. While novel programming paradigms and advanced system libraries provide solutions for some of these challenges, appropriate supporting tools are indispensable. Such tools aid application developers in debugging, performance analysis, or code optimization and therefore make a major contribution to the development of robust and efficient parallel software. This book introduces a selection of the tools presented and discussed at the 7th International Parallel Tools Workshop, held in Dresden, Germany, September 3-4, 2013.

Leveraging MPI-3 Shared-Memory Extensions for Efficient PGAS Runtime Systems

The relaxed semantics and rich functionality of one-sided communication primitives of MPI-3 makes MPI an attractive candidate for the implementation of PGAS models. However, the performance of such implementation suffers from the fact, that current MPI RMA implementations typically have a large overhead when source and target of a communication request share a common, local physical memory. In this paper, we present an optimized PGAS-like runtime system which uses the new MPI-3 shared-memory extensions to serve intra-node communication requests and MPI-3 one-sided communication primitives to serve inter-node communication requests. The performance of our runtime system is evaluated on a Cray XC40 system through low-level communication benchmarks, a random-access benchmark and a stencil kernel. The results of the experiments demonstrate that the performance of our hybrid runtime system matches the performance of low-level RMA libraries for intra-node transfers, and that of MPI-3 for inter-node transfers.

Programmability and portability for exascale: Top down programming methodology and tools with StarSs

Abstract StarSs is a task-based programming model that allows to parallelize sequential applications by means of annotating the code with compiler directives. The model further supports transparent execution of designated tasks on heterogeneous platforms, including clusters of GPUs. This paper focuses on the methodology and tools that complements the programming model forming a consistent development environment with the objective of simplifying the live of application developers. The programming environment includes the tools TAREADOR and TEMANEJO, which have been designed specifically for StarSs. TAREADOR, a Valgrind-based tool, allows a top-down development approach by assisting the programmer in identifying tasks and their data-dependencies across all concurrency levels of an application. TEMANEJO is a graphical debugger supporting the programmer by visualizing the task dependency tree on one hand, but also allowing to manipulate task scheduling or dependencies. These tools are complemented with a set of performance analysis tools (Scalasca, Cube and Paraver) that enable to fine tune StarSs application.

Resistive jet simulations extending radially self-similar magnetohydrodynamic models

Numerical simulations with self-similar initial and boundary conditions provide a link between theoretical and numerical investigations of jet dynamics. We perform axisymmetric resistive magnetohydrodynamic (MHD) simulations for a generalized solution of the Blandford & Payne type, and compare them with the corresponding analytical and numerical ideal MHD solutions. We disentangle the effects of the numerical and physical diffusivity. The latter could occur in outflows above an accretion disc, being transferred from the underlying disc into the disc corona by MHD turbulence (anomalous turbulent diffusivity), or as a result of ambipolar diffusion in partially ionized flows. We conclude that while the classical magnetic Reynolds number R m measures the importance of resistive effects in the induction equation, a new introduced number, R β = (β/2)R m with β the plasma beta, measures the importance of the resistive effects in the energy equation. Thus, in magnetized jets with /3 < 2, when R β ≤ 1 resistive effects are non-negligible and affect mostly the energy equation. The presented simulations indeed show that for a range of magnetic diffusivities corresponding to R β ≥1, the flow remains close to the ideal MHD self-similar solution.

Stability and structure of analytical MHD jet formation models with a finite outer disk radius

Context. Finite radius accretion disks are a strong candidate for launching astrophysical jets from their inner parts and disk-winds are considered as the basic component of such magnetically collimated outflows. Numerical simulations are usually employed to answer several open questions regarding the origin, stability and propagation of jets. The inherent uncertainties, however, of the various numerical codes, applied boundary conditions, grid resolution, etc., call for a parallel use of analytical methods as well, whenever they are available, as a tool to interpret and understand the outcome of the simulations. The only available analytical MHD solutions to describe disk-driven jets are those characterized by the symmetry of radial self-similarity. Those exact MHD solutions are used to guide the present numerical study of disk-winds. Aims. Radially self-similar MHD models, in general, have two geometrical shortcomings, a singularity at the jet axis and the nonexistence of an intrinsic radial scale, i.e. the jets formally extend to radial infinity. Hence, numerical simulations are necessary to extend the analytical solutions towards the axis and impose a physical boundary at finite radial distance. Methods. We focus here on studying the effects of imposing an outer radius of the underlying accreting disk (and thus also of the outflow) on the topology, structure and variability of a radially self-similar analytical MHD solution. The initial condition consists of a hybrid of an unchanged and a scaled-down analytical solution, one for the jet and the other for its environment. Results. In all studied cases, we find at the end steady two-component solutions. The boundary between both solutions is always shifted towards the solution with reduced quantities. Especially, the reduced thermal and magnetic pressures change the perpendicular force balance at the “surface” of the flow. In the models where the scaled-down analytical solution is outside the unchanged one, the inside solution converges to a solution with different parameters. In the models where the scaled-down analytical solution is inside the unchanged one, the whole two-component solution changes dramatically to stop the flow from collapsing totally to the symmetry axis. Conclusions. It is thus concluded that truncated exact MHD disk-wind solutions that may describe observed jets associated with finite radius accretion disks, are topologically stable.

Performance Modeling of the HPCG Benchmark

The TOP 500 list is the most widely regarded ranking of modern supercomputers, based on Gflop/s measured for High Performance LINPACK (HPL). Ranking the most powerful supercomputers is important: Hardware producers hone their products towards maximum benchmark performance, while nations fund huge installations, aiming at a place on the pedestal. However, the relevance of HPL for real-world applications is declining rapidly, as the available compute cycles are heavily overrated. While relevant comparisons foster healthy competition, skewed comparisons foster developments aimed at distorted goals. Thus, in recent years, discussions on introducing a new benchmark, better aligned with real-world applications and therefore the needs of real users, have increased, culminating in a highly regarded candidate: High Performance Conjugate Gradients (HPCG).

Comparison of synthetic maps from truncated jet-formation models with YSO jet observations

Context. Significant progress has been made in the last years in the understanding of the jet formation mechanism through a combination of numerical simulations and analytical MHD models for outflows characterized by the symmetry of self-similarity. Analytical radially self-similar models successfully describe disk-winds, but need several improvements. In a previous article we introduced models of truncated jets from disks, i.e. evolved in time numerical simulations based on a radially self-similar MHD solution, but including the effects of a finite radius of the jet-emitting disk and thus the outflow. Aims. These models need now to be compared with available observational data. A direct comparison of the results of combined analytical theoretical models and numerical simulations with observations has not been performed as yet. This is our main goal. Methods. In order to compare our models with observed jet widths inferred from recent optical images taken with the Hubble Space Telescope (HST) and ground-based adaptive optics (AO) observations, we use a new set of tools to create emission maps in different forbidden lines, from which we determine the jet width as the full-width half-maximum of the emission. Results. It is shown that the untruncated analytical disk outflow solution considered here cannot fit the small jet widths inferred by observations of several jets. Furthermore, various truncated disk-wind models are examined, whose extracted jet widths range from higher to lower values compared to the observations. Thus, we can fit the observed range of jet widths by tuning our models. Conclusions. We conclude that truncation is necessary to reproduce the observed jet widths and our simulations limit the possible range of truncation radii. We infer that the truncation radius, which is the radius on the disk mid-plane where the jet-emitting disk switches to a standard disk, must be between around 0.1 up to about 1 AU in the observed sample for the considered disk-wind solution. One disk-wind simulation with an inner truncation radius at about 0.11 AU also shows potential for reproducing the observations, but a parameter study is needed.