Message delay and asynchronous DisCSP search

Distributed constraint satisfaction problems (DisCSPs) are composed of agents, each holding its own variables, that are connected by constraints to variables of other agents. Due to the distributed nature of the problem, message delay can have unexpected effects on the behavior of distributed search algorithms on DisCSPs. This has been shown in experimental studies of asynchronous backtracking algorithms [1, 9]. To evaluate the impact of message delay on the run of DisCSP search algorithms, a model for distributed performance measures is presented. The model counts the number of non concurrent constraints checks, to arrive at a solution, as a non concurrent measure of distributed computation. A simpler version measures distributed computation cost by the number of non-concurrent steps of computation. An algorithm for computing these distributed measures of computational effort is described. The realization of the model for measuring performance of distributed search algorithms is a simulator which includes the cost of message delays. The performance of two asynchronous search algorithms is measured on randomly generated instances of DisCSPs with delayed messages. The Asynchronous Weak Commitment (AW C) algorithm and Asynchronous Backtracking (ABT). The intrinsic reordering process of AW C dictates a need for a more complex count of non-concurrent steps of computation. The improved counting algorithm is also needed for Dynamic ordered ABT. The delay of messages is found to have a strong negative effect on AW C and a smaller effect on dynamically ordered ABT

Solving Employee Timetabling Problems by Generalized Local Search

Employee timetabling is the operation of assigning employees to task

Modelling and Solving Employee Timetabling Problems

Employee timetabling is the operation of assigning employees to tasks in a set of shifts during a xed period of time, typically a week. We present a general de nition of employee timetabling problems (ETPs) that captures many real-world problem formulations and includes complex constraints. The proposed model of ETPs can be represented in a tabular form that is both intuitive and ecient for constraint representation and processing. The constraint networks of ETPs include non-binary constraints and are dicult to formulate in terms of simple constraint solvers. We investigate the use of local search techniques for solving ETPs. In particular, we propose several versions of hill-climbing that make use of a novel search space that includes also partial assignments

Concurrent dynamic backtracking for distributed csps

Abstract. A distributed concurrent search algorithm for distributed constraint satisfaction problems (DisCSPs) is presented. Concurrent search algorithms are composed of multiple search processes (SPs) that operate concurrently and scan non-intersecting parts of the global search space. Search processes are generated dynamically, started by the initializing agent, and by any number of agents during search. In the proposed, ConcDB, algorithm, all search processes perform dynamic backtracking. As a consequence of dynamic backtracking, a search space scanned by one search process can be found unsolvable by a different search process. This enhances the efficiency of the ConcDB algorithm. Concurrent search is an asynchronous distributed algorithm and is shown to be faster than asynchronous backtracking (ABT). The network load of ConcDB is also much lower than that of ABT.