Message delay and DisCSP search algorithms

Abstract

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 recently shown in experimental studies of asynchronous backtracking algorithms (Bejar et al., Artif. Intell., 161:117-148, 2005; Silaghi and Faltings, Artif. Intell., 161:25-54, 2005). 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 non-concurrent number of 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. Two families of distributed search algorithms on DisCSPs are investigated. Algorithms that run a single search process, and multiple search processes algorithms. The two families of algorithms are described and associated with existing algorithms. The performance of three representative algorithms of these two families is measured on randomly generated instances of DisCSPs with delayed messages. The delay of messages is found to have a strong negative effect on single search process algorithms, whether synchronous or asynchronous. Multi

Nogood-based asynchronous forward checking algorithms

“…Communication load is measured by the total number of exchanged messages among agents during algorithm execution (#msg), including those of termination detection (system messages). Computation effort is measured by the number of non-concurrent constraint checks (#ncccs) [42]. #ncccs is the metric used in distributed constraint solving to simulate the computation time.…”

Dynamic Ordering for Asynchronous Backtracking on DisCSPs

“…Every message carries the value of the sending agent's counter. When an agent receives a message it stores the data received together with the corresponding counter.When the agent first uses the received counter it updates its counter to the largest value between its own counter and the stored counter value which was carried by the message [28]. By reporting the cost of the search as the largest counter held by some agent at the end of the search, a measure of non-concurrent search effort that is close to Lamports logical time is achieved [13].…”

“…By reporting the cost of the search as the largest counter held by some agent at the end of the search, a measure of non-concurrent search effort that is close to Lamports logical time is achieved [13]. If instead of steps of computation, the number of non-concurrent constraints check is counted (NCCCs), then the local computational effort of agents in each step is measured [15,29].…”

AgentZero: A Framework for Simulating and Evaluating Multi-agent Algorithms

“…Fundamental actions of the agents are automatically monitored by the system in order to provide useful statistics -this reduces to a minimum the need to write code that is unrelated to the actual agent behavior. -Powerful message passing mechanism -messages are passed by value and are conveniently handled in an RPC fashion; statistical message delays [11] can be applied in order to measure the application's behavior under different network conditions without slowing down the actual execution. Additional utilities included in the mechanism, e.g., automatic idle detection and message timestamps.…”