Performance Analysis of Speculative Parallel Adaptive Local Timestepping for Conservation Laws

Efficient computer simulation of complex physical phenomena has long been challenging due to their multiphysics and multiscale nature. In contrast to traditional time-stepped execution methods, the authors describe an approach using optimistic parallel discrete event simulation (PDES) and reverse computation techniques to execute plasma physics codes. They show that reverse computation-based optimistic parallel execution can significantly reduce the execution time of an example plasma simulation without requiring a significant amount of additional memory compared to conservative execution techniques. The authors describe an application-level reverse computation technique that is efficient and suitable for complex scientific simulations.

In an optimistic (time-warp) parallel simulation, local clocks with different logical processes must advance at the same rate in order to reduce the number of rollbacks. In this paper, we propose two algorithms for dynamic load balancing which reduce the number of rollbacks in an optimistic parallel discrete event simulation (PDES) system. The first algorithm is based on the load transfer mechanism between logical processes, while the second algorithm, which is based on the principle of an evolutionary strategy, migrates logical processors between several pairs of physical processors. We have implemented both of these algorithms on a cluster of heterogeneous workstations and studied their performance. The experimental results show that the algorithm based on the load transfer is effective when the grain size is larger than 10 ms, and the algorithm based on the process migration yields good performance for grain sizes of 20 ms or larger. In both of these cases, the average speed-up ranges between 1 and 2 using four processors, when the computation grain-size is within the range 7 to 50 ms. The reduction in rollback messages as a percentage of the total number of messages due to the algorithms is, however, around 4 to 8%.

Adaptive total variation stable local timestepping for conservation laws

We introduce Backstroke, a new open source framework for the automatic generation of reverse code for functions written in C++. Backstroke enables reverse computation for optimistic parallel discrete event simulations. It is built using the ROSE open-source compiler infrastructure, and handles complex C++ features including pointers and pointer types, arrays, function and method calls, class types, inheritance, polymorphism, virtual functions, abstract classes, templated classes and containers. Backstroke also introduces new program inversion techniques based on advanced compiler analysis tools built into ROSE. We explore and illustrate some of the complex language and semantic issues that arise in generating correct reverse code for C++ functions.

The large size and complexity of the modern digital hardware impose great challenges to design and validation. Hardware Description Languages (HDLs) and System-Level Description Languages (SLDLs) rely on sequential discrete event semantics. Parallel Discrete Event Simulation (PDES) has recently gained extensive attention for parallelizing these languages due to the ever-increasing complexity of embedded and cyber physical systems. However, PDES application has not yet reached acceptable maturity and pervasiveness for accelerating computer architecture problems. This is due to inherent complexity of hardware components that require using different advanced PDES techniques. In this paper, we look at the main problem from a radically different angle. First, we suppose there is only a single universal discrete event model of computation for simulation purely defined by distributed optimistic PDES, i.e., logical-process-based event scheduling worldview. Second, we construct a new parallel system-level simulation language called OSML for ESL. Third, we propose a unified Cloud-based CAD tool called Troodon to automatically parallelize existing hardware languages atop OSML. To the best of our knowledge, OSML is the first work on optimistic synchronization applied to hardware-specific SLDLs and hardware models at different levels of abstraction in ESL, which contain complex data structures by proposing a hybrid checkpointing scheme.

Describes a nonblocking checkpointing mode in support of optimistic parallel discrete event simulation. This mode allows real concurrency in the execution of state saving and other simulation specific operations (e.g, event list update, event execution) with the aim of removing the cost of recording state information from the completion time of the parallel simulation application. We present an implementation of a C library supporting nonblocking checkpointing on a myrinet based cluster, which demonstrates the practical viability of this checkpointing mode on standard off-the-shelf hardware. By the results of an empirical study on classical parameterized synthetic benchmarks, we show that, except for the case of minimal state granularity applications, nonblocking checkpointing allows improvement of the speed of the parallel execution, as compared to commonly adopted, optimized checkpointing methods based on the classical blocking mode. A performance study for the case of a personal communication system (PCS) simulation is additionally reported to point out the benefits from nonblocking checkpointing for a real world application.

Parallel discrete-event simulation (PDES) is an important tool in the codesign of extreme-scale systems because PDES provides a cost-effective way to evaluate designs of high-performance computing systems. Optimistic synchronization algorithms for PDES, such as Time Warp, allow events to be processed without global synchronization among the processing elements. A rollback mechanism is provided when events are processed out of timestamp order. Although optimistic synchronization protocols enable the scalability of large-scale PDES, the performance of the simulations must be tuned to reduce the number of rollbacks and provide an improved simulation runtime. To enable efficient large-scale optimistic simulations, one has to gain insight into the factors that affect the rollback behavior and simulation performance. We developed a tool for ROSS model developers that gives them detailed metrics on the performance of their large-scale optimistic simulations at varying levels of simulation granularity. Model developers can use this information for parameter tuning of optimistic simulations in order to achieve better runtime and fewer rollbacks. In this work, we instrument the ROSS optimistic PDES framework to gather detailed statistics about the simulation engine. We have also developed an interactive visualization interface that uses the data collected by the ROSS instrumentation to understand the underlying behavior of the simulation engine. The interface connects real time to virtual time in the simulation and provides the ability to view simulation data at different granularities. We demonstrate the usefulness of our framework by performing a visual analysis of the dragonfly network topology model provided by the CODES simulation framework built on top of ROSS. The instrumentation needs to minimize overhead in order to accurately collect data about the simulation performance. To ensure that the instrumentation does not introduce unnecessary overhead, we perform a scaling study that compares instrumented ROSS simulations with their noninstrumented counterparts in order to determine the amount of perturbation when running at different simulation scales.

An efficient local time-stepping scheme for solution of nonlinear conservation laws

The flowfield over a sharp-tipped double cone axisymmetric configuration is carried out by solving time dependent axisymmetric laminar compressible Navier-Stokes equations for freestream Mach number range of 2.0 - 6.0. The fluid dynamics equations are discretized in spatial coordinates using integral formulation in conjunction with finite volume method which reduces the governing equations to semi-discretized ordinary differential equations. Temporal integration is performed employing multistage Runge-Kutta time-stepping scheme. A local time step is used to achieve steady-state solution. The numerical computation is carried out on a mono-block with structured grids. The flowfield features over the sharp-tipped double cone configuration such as conical shock wave, separation region, separation and reattachment shock wave and bow shock wave in the cone region, expansion fan, and recirculation flow in the base region are well captured at Mach number 3 and 6 which are identical to the Edney VI type shock interaction. The velocity vector, Mach contours plots and variations of surface pressure coefficient along the sharp-tipped double cone configuration are analyzed at various Mach numbers. The fore-body aerodynamic drag is calculated employing computed pressure distribution. The paper presents the influence of the freestream Mach number on the flows with shock interactions over the sharp-tipped double cone geometry.

In conventional parallel processing, the main objective of scheduling is to reduce the processor's idle time. However, in Time Warp (TW), which is an optimistic parallel discrete event simulation approach, keeping the processors busy does not necessarily lead to good performance, since the processors may be performing erroneous computations that must be eventually rolled back. Hence, the existing knowledge on scheduling is not easily transferable to TW simulations. The main objective of this work is to study the impact of scheduling policies on performance of TW. We compare a dynamic scheduling scheme with a static one. Our experimental results indicate that the dynamic policy reduces rollback costs for small shared memory multiprocessors.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

There have been P2P systems with simultaneous millions of nodes on Internet. But existing simulators and techniques cannot simulate such a large scale. Even parallelized simulators have not solved the scale problem. They provide large memory and hold a large number of nodes but the speed of simulation degrades significantly than sequential simulation due to much overhead of inter-server synchronization. We propose a simulation technique for large-scale P2P systems based on an optimistic parallel discrete event simulation model. The technique employs low cost synchronization techniques that are effective for P2P simulation.

Developments of a flood inundation model based on the cellular automata approach: Testing different methods to improve model performance

Reentry capsule configurations significantly differ from each other due to entry conditions and mission requirements. This paper describes numerical simulations of the viscous flow over the Beagle and the OREX (Orbital Reentry EXperiments) configurations for freestream Mach numbers in the range of 1.2 - 5.0. The flow fields over the reentry modules are obtained by solving time-dependent axisymmetric compressible laminar Navier-Stokes equations. The fluid mechanics equations are discretized in spatial coordinates employing a finite volume method, which reduces the governing equations to semi-discretized ordinary differential equations. Temporal integration is carried out using a two-stage Runge-Kutta time-stepping scheme. A local time-stepping is employed to get the steady state solution. The numerical simulation is performed on a single-block structured grid. The flow field features around the reentry capsules such as bow shock wave, sonic line, expansion fan and recirculation flow in the base-shell region are well captured by the present numerical computation. The effects of the geometrical parameters of the module, such as aspect ratio, frontal segment bluntness, fore body cone angle and shoulder rounding radius on the wake throat width, distance from the wake throat to the base of the model, flow departure angle and aerodynamic drag are analyzed using the numerically simulated flow field.

Duality principle of conservation laws in dislocation continuum

Advancements in enhancing flood evolution and urban inundation predictions: A study of local time stepping algorithm and GPU-accelerated hydrodynamic model