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Nuno Santos, Paulo Flores, and José Monteiro.
"MPBO - A Distributed PBO
Solver".
Engineering Applications of Artificial Intelligence, The International
Journal of Intelligent Real-Time Automation, 2014.
(em fase de submissão).
Abstract Parallel
computing has been the subject of intensive research in the last decade and
is increasingly being chosen as a solution for developing applications that
require high computational power, such as the Boolean Satisfiability (SAT)
and Pseudo-Boolean Optimization (PBO) problems. Research in SAT solvers has
obtained relevant results in the last years, achieving significant reductions
in execution times. Unfortunately, hard instances of SAT require large
computational power and even efficient SAT solvers take huge execution times
to obtain their solution. Therefore, SAT solvers adaptation to parallel
computing systems began to be the subject of considerable research and there
already exist several distributed versions of popular SAT solvers. However,
the absence of distributed solvers for the PBO problem is notorious. This
work intends to contribute and encourage the research into distributed solu-
tions to solve the PBO problem. The goal of this work is to propose a
distributed Pseudo-Boolean Optimization Solver, named MPBO solver, based on
MPI (Message Passing Interface ) and focused on an efficient search space
partition, more specifically the partition of the problem optimization search
space. The proposed solver achieved significant reductions in the time to
solve hard PBO instances, when compared to the MiniSat+ and pwbo solvers.
Keywords: Pseudo-Boolean Optimization (PBO),
Boolean Satisfiability (SAT), Message Passing Interface (MPI), Distributed
Systems (GRID)
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Levent Aksoy, Eduardo Costa, Paulo Flores, and José Monteiro.
"Exploration of Tradeoffs in
the Design of Integer Cosine Transforms for Image Compression".
In Proceedings of European Conference on Circuit Theory and Design
(ECCTD), September 8-12, 2013.
Abstract In image data
compression, integer cosine transforms (ICTs) have been preferred to discrete
cosine transforms (DCTs) due to their similar transform efficiency and lower
implementation cost. However, there exist many alternative ICTs with
different performance measures and implementation costs. In this work, we
explore all possible ICTs, compute their performance measures, and find their
implementation costs in terms of the number of adders/subtractors, where a
state-of-art technique is used to realize ICTs under a shift-adds
architecture. We also investigate the tradeoff between performance and
implementation cost, present the pareto-optimal points of this tradeoff, and
introduce promising ICTs that were not considered before.
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Ricardo Marques, Luis Guerra e Silva, Paulo Flores, and L. Miguel Silveira.
"Improving SAT Solver
Efficiency using a Multi-core Approach".
In The 26th International Florida Artificial Intelligence Research
Society Conference (FLAIRS), pages 94-99, May 2013.
Abstract Many
practical problems in multiple fields can be converted to a SAT problem, or a
sequence of SAT problems, such that their solution immediately implies a
solution to the original problem. Despite the enormous progress achieved over
the last decade in the development of SAT solvers, there is strong demand for
higher algorithm efficiency to solve harder and larger problems. The
widespread availability of multi-core, shared memory parallel environments
provides an opportunity for such improvements. In this paper we present our
results on improving the effectiveness of standard SAT solvers on such
architectures, through a portfolio approach. Multiple instances of the same
basic solver using different heuristic strategies, for search-space
exploration and problem analysis, share information and cooperate towards the
solution of a given problem. Results from the application of our methodology
to known problems from SAT competitions show relevant improvements over the
state of the art and yield the promise of further advances.
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Ruben Martins, Vasco Manquinho, and Inês Lynce.
"Clause Sharing in
Deterministic Parallel Maximum Satisfiability".
In Proceedings of the 19th RCRA workshop on Experimental Evaluation of
Algorithms for Solving Problems with Combinatorial Explosion (RCRA
2012)., June 2012.
Abstract Multicore
processors are becoming the dominant platform in modern days. As a result,
parallel Maximum Satisfiability (MaxSAT) solvers have been developed to
exploit this new architecture. Sharing learned clauses in parallel MaxSAT is
expected to help to further prune the search space and boost the performance
of a parallel solver. Yet, so far it has not been made clear which learned
clauses should be shared among the different threads. This paper studies the
impact of clause sharing heuristics. Evaluating these heuristics can be a
hard task in parallel MaxSAT because existing solvers suffer from
non-determinism behavior, i.e. several runs of the same solver can lead to
different solutions. This is a clear downside for applications that require
solving the same problem instance more than once, such as the evaluation of
heuristics. For a fair evaluation, this paper presents the first
deterministic parallel MaxSAT solver that ensures reproducibility of results.
By using a deterministic solver one can independently evaluate the gains
coming from the use of different heuristics rather than the non-determinism
of the solver. Experimental results show that sharing learned clauses
improves the overall performance of parallel MaxSAT solvers. Moreover, the
performance of the new deterministic solver is comparable to the
corresponding nondeterministic version.
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J.F. Villena and L. Silveira.
"Exploiting Parallelism for
Improved Automation of Multidimensional Model Order Reduction".
IEEE Transactions on Computer-Aided Design of Integrated Circuits and
Systems (TCAD), 31(1):37-49, January 2012.
Special Issue PAR-CAD: Parallel CAD Algorithms and CAD for Parallel
Architectures/Systems.
(doi:10.1109/TCAD.2011.2167510)
Abstract This paper
addresses the issue of automatically generating reduced order models of very
large multidimensional systems. To tackle this problem we introduce an
efficient parallel projection based model order reduction framework for
parameterized linear systems. The underlying methodology is based on an
automated multidimensional sample selection procedure that maximizes
effectiveness in the generation of the projection basis. The parallel nature
of the algorithm is efficiently exploited using both shared and distributed
memory architectures. This leads to a highly scalable, automatic, and
reliable parallel reduction scheme, able to handle very large systems
depending on multiple parameters. In addition, the framework is general
enough to provide a good approximation regardless of the model's
representation or underlying nature, as will be demonstrated on a variety of
benchmark examples. The method provides the potential to tackle, in an
automatic fashion, extremely challenging models that would be otherwise
difficult to address with existing sequential approaches.
Keywords: linear systems;memory
architecture;multidimensional systems;reduced order systems;sampling
methods;automated multidimensional sample selection procedure;distributed
memory architectures;multidimensional model order reduction;multidimensional
sampling method;parallel projection based model order reduction
framework;parallel reduction scheme;parameterized linear systems;sequential
approaches;very large multidimensional systems;Approximation
methods;Automation;Computational modeling;Computer architecture;Sparse
matrices;Vectors;Multidimensional sampling;parallel model order
reduction;parameterized systems
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Ruben Martins, Vasco Manquinho, and Inês Lynce.
"Parallel Search for Maximum
Satisfiability".
Journal of Artificial Intelligence Communications, 25(2):75-95,
January 2012.
(doi:10.3233/AIC-2012-0517)
Abstract The
predominance of multicore processors has increased the interest in developing
parallel Boolean Satisfiability (SAT) solvers. As a result, more parallel SAT
solvers are emerging. Even though parallel approaches are known to boost
performance, parallel approaches developed for Boolean optimization are
scarce. This paper proposes parallel search algorithms for Maximum
Satisfiability (MaxSAT) and introduces PWBO, a new parallel solver for
partial MaxSAT. Using two threads, an unsatisfiability-based algorithm is
used to search on the lower bound of the optimal solution, while at the same
time a linear search is performed on the upper bound of the optimal solution.
Moreover, learned clauses are shared between threads during the search. For a
larger number of threads two different strategies are proposed. The first
strategy performs a portfolio approach by searching on the lower and upper
bound values of the optimal solution using different encodings of cardinality
constraints for each thread. The second strategy splits the search space
considering different upper bound values of the optimal solution for each
thread. Experimental results show that the techniques proposed in the paper
enable PWBO to improve when increasing the number of threads
Keywords: Parallel search, maximum satisfiability,
cardinality constraints
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J.F. Villena and L.M. Silveira.
"3POR - Parallel
projection based parameterized order reduction for multi-dimensional linear
models".
In Proceedings of IEEE/ACM International Conference on Computer-Aided
Design (ICCAD), pages 536-542, November 7-11, 2010.
(doi:10.1109/ICCAD.2010.5653851)
Abstract This paper
introduces a distributed and shared memory parallel projection based model
order reduction framework for parameterized linear systems. The proposed
methodology is based on a sampling scheme followed by a projection to build
the reduced model. It exploits the parallel nature of the sampling methods to
improve the efficiency of the basis generation. The sample selection scheme
uses the residue as a proxy for the model error in order to improve
automation and maximize the effectiveness of the sampling step. This yields
an automatic and reliable methodology, able to handle large systems depending
on the frequency and multiple parameters. The framework can be used in shared
and distributed memory architectures separately or in conjunction. It is able
to deal with different system representations and models of different
characteristics, is highly scalable and the parallelization is very
effective, as will be demonstrated on a variety of industrial benchmarks,
with super linear speed-ups in certain cases. The methodology provides the
potential to tackle large and complex models, depending on multiple
parameters in an automatic fashion.
Keywords: reduced order systems;sampling
methods;3POr;distributed parallel projection;multidimensional linear
models;parallel projection based parameterized order reduction;sample
selection scheme;sampling methods;shared memory parallel
projection;Arrays;Indexes;Memory management;Sparse matrices;Transfer
functions;Vectors
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R. Martins, V. Manquinho, and I. Lynce.
"Improving Search Space
Splitting for Parallel SAT Solving".
In Proceedings of the IEEE International Conference on Tools with
Artificial Intelligence (ICTAI), volume 1, pages 336-343, October
27-29, 2010.
(Realizado no ambito do projecto, mas financiado por outros projectos, artigo
não contabilizado).
(doi:10.1109/ICTAI.2010.56)
Abstract The last
two decades progresses have led Propositional Satisfiability (SAT) to be a
competitive practical approach to solve a wide range of industrial and
academic problems. Thanks to these advances, the size and difficulty of the
SAT instances have grown significantly. The demand for more computational
power led to the creation of new computer architectures and paradigms
composed by multiple machines connected by a network to act as one machine,
like clusters and grids. However, extra computing power is not coming anymore
from higher processor frequencies, but rather from a growing number of
computing cores and processors. It becomes clear that exploiting this new
architecture is essential for the evolution of SAT solvers. Search space
splitting is probably the most commonly used strategy to explore the
parallelism provided by the search space. However, it is not clear how to
find the relevant set of variables to divide the search space. This paper
extends a method based on the VSIDS heuristic to find the initial set of
partition variables. A drawback of search space splitting is load balancing.
To overcome this problem, we propose the use of a hybrid approach between
search space splitting and portfolio. Preliminary results show that both
these techniques improve the performance of the solver and reveal that
combining search space splitting and portfolio approaches can lead to better
results.
Keywords: computability;search problems;VSIDS
heuristic;extra computing power;parallel SAT solving;partition
variable;portfolio;propositional satisfiability;search space
splitting;Heuristic algorithms;Instruction sets;Load
management;Master-slave;Multicore
processing;Portfolios;Switches
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