Wolfgang Blochinger

PaSAT-parallel sat-checking with lemma exchange : Implementation and applications

Abstract We present PaSAT, a parallel implementation of a Davis-Putnam-style prepositional satisfiability checker incorporating dynamic search space partitioning, intelligent backjumping, as well as lemma generation and exchange; the main focus of our implementation is on speeding up SAT-checking of prepositional encodings of real-world combinatorial problems. We investigate and analyze the speed-ups obtained by parallelization in conjunction with lemma exchange and describe the effects we observed during our experiments. Finally, we present performance measurements from the application of our prover in the areas of formal consistency checking of industrial product documentation, cryptanalysis, and hardware verification. We would like to thank Jurgen Ellinger for help on carrying out the experiments.

Parallel propositional satisfiability checking with distributed dynamic learning

We address the parallelization and distributed execution of an algorithm from the area of symbolic computation: propositional satisfiability (SAT) checking with dynamic learning. Our parallel programming models are strict multithreading for the core SAT checking procedure, complemented by mobile agents realizing a distributed dynamic learning process. Individual threads treat dynamically created subproblems, while mobile agents collect and distribute pertinent knowledge obtained during the learning process. The parallel algorithm runs on top of our parallel system platform Distributed Object-Oriented Threads System, which provides support for our parallel programming models in highly heterogeneous distributed systems. We present performance measurements evaluating the performance gains by our approach in different application domains with practical significance.

Parallel techniques for physically based simulation on multi-core processor architectures

As multi-core processor systems become more and more widespread, the demand for efficient parallel algorithms also propagates into the field of computer graphics. This is especially true for physically based simulation, which is notorious for expensive numerical methods. In this work, we explore possibilities for accelerating physically based simulation algorithms on multi-core architectures. Two components of physically based simulation represent a great potential for bottlenecks in parallelisation: implicit time integration and collision handling. From the parallelisation point of view these two components are substantially different. Implicit time integration can be treated efficiently using static problem decomposition. The linear system arising in this context is solved using a data-parallel preconditioned conjugate gradient algorithm. The collision handling stage, however, requires a different approach, due to its dynamic structure. This stage is handled using multi-threaded programming with fully dynamic task decomposition. In particular, we propose a new task splitting approach based on a reasonable estimation of work, which analyses previous simulation steps. Altogether, the combination of different parallelisation techniques leads to a concise and yet versatile framework for highly efficient physical simulation.

Structured collaborative workflow design

Workflow design is often an effort of distributed and heterogeneous teams, thus making tool support for collaboration a necessity. We present a novel concept of collaborative workflow design which combines cooperation and workflow model analysis. Workflow analysis is simplified using workflow metrics, which help identifying problematic aspects of the workflow model. Our findings are implemented in a collaborative workflow design system, which is easily accessible on the Web, but provides a desktop-like user experience.

ZetaSAT - Boolean SATisfiability solving on Desktop Grids

ZetaSAT is a research effort to enable efficient parallel Boolean satisfiability (SAT) solving on the Desktop Grid. ZetaSAT is based on the Desktop Grid platform Zeta-Grid. Our work particularly addresses specific issues arising when executing constraint satisfaction problems of the kind of SAT in Desktop Grids, like dynamic problem decomposition, load balancing, termination detection, and domain specific fault tolerance. We report on performance measurements indicating the usefulness of our approach.

An object-oriented platform for distributed high-performance symbolic computation

We describe the distributed object-oriented threads system (DOTS), a programming environment designed to support object-oriented fork/join parallel programming in a heterogeneous distributed environment. A mixed network of Windows NT PCs and UNIX workstations is transformed by DOTS into a homogeneous pool of anonymous compute servers forming together a multicomputer. DOTS is a complete redesign of the distributed threads system (DTS) using the object-oriented paradigm both in its internal implementation and in the programming paradigm it supports. It has been used for the parallelization of applications in the field of computer algebra and in the field of computer graphics. We also give a brief account of applications in the domain of symbolic computation that were developed using DTS.

Physically based simulation of cloth on distributed memory architectures

Physically based simulation of cloth in virtual environments is a computationally demanding problem. It involves modeling the internal material properties of the textile (physical modeling) and also treating interactions with the surrounding scene (collision handling). In this paper, we present an approach to parallel cloth simulation designed for distributed memory parallel architectures, particularly clusters built of commodity components. We discuss parallel techniques for the physical modeling phase as well as for the collision handling phase which can significantly reduce the respective computation times. To deal with the very fine granularity of the physical modeling phase we apply a static data decomposition approach based on graph partitioning. In order to cope with the high irregularity of the collision handling phase we employ task-parallel techniques based on fully dynamic problem decomposition. We show how both techniques can be integrated into a robust parallel cloth simulation method which can deal with considerably complex scenes.

Parallel implicit integration for cloth animations on distributed memory architectures

We present a parallel cloth simulation engine designed for distributed memory parallel architectures, in particular clusters built of commodity components. We focus on efficient parallel processing of irregularly structured and real-world sized problems typically occurring in the simulation of garments. We report on performance measurements showing a high degree of parallel efficiency and scalability indicating the usefulness of our approach.

Capability-Aware Information Aggregation in Peer-to-Peer Grids

Information aggregation is the process of summarizing information across the nodes of a distributed system. We present a hierarchical information aggregation system tailored for Peer-to-Peer Grids which typically exhibit a high degree of volatility and heterogeneity of resources. Aggregation is performed in a scalable yet efficient way by merging data along the edges of a logical self-healing tree with each inner node providing a summary view of the information delivered by the nodes of the corresponding subtree. We describe different tree management methods suitable for high-efficiency and high-scalability scenarios that take host capability and stability diversity into account to attenuate the impact of slow and/or unstable hosts. We propose an architecture covering all three phases of the aggregation process: Data gathering through a highly extensible sensing framework, data aggregation using reusable, fully isolated reduction networks, and application-sensitive data delivery using a broad range of propagation strategies. Our solution combines the advantages of approaches based on Distributed Hash Tables (DHTs) (i.e., load balancing and self-maintenance) and hierarchical approaches (i.e., respecting administrative boundaries and resource limitations). Our approach is integrated into our Peer-to-Peer Grid platform Cohesion. We substantiate its effectiveness through performance measurements and demonstrate its applicability through a graphical monitoring solution leveraging our aggregation system.

Collaborative BPEL Design with a Rich Internet Application

Workflow design is often an effort of distributed and inhomogeneous teams. We present a collaborative workflow design tool, which is implemented as a Rich Internet Application. The tool provides a desktop like user experience, but can easily be embedded into Web sites. It enables synchronous as well as asynchronous cooperation of design team members and encourages a well coordinated design process.