Look-ahead Information Research Articles

Comparing an ACO algorithm with other heuristics for the single machine scheduling problem with sequence-dependent setup times

We compare several heuristics for solving a single machine scheduling problem. In the operating situation modelled, setup times are sequence-dependent and the objective is to minimize total tardiness. We describe an Ant Colony Optimization (ACO) algorithm having a new feature using look-ahead information in the transition rule. This feature shows an improvement in performance. A comparison with a genetic algorithm, a simulated annealing approach, a local search method and a branch-and-bound algorithm indicates that the ACO that we describe is competitive and has a certain advantage for larger problems.

Local versus global lookahead in conservative parallel simulations

This paper presents an algorithm, which we refer to as SGTNE, to efficiently obtain lookahead information from a cluster of processors in a parallel simulation in order to unblock (logical) processes (LP) in a given processor. The SGTNE algorithm is based on a TNE conservative synchronization scheme that relies on an independent execution of a shortest path algorithm in individual processors in order to provide lookahead to the resident LPs. Because TNE is executed on individual processors, it is susceptible to inter-processor deadlocks, which must be detected and broken at some cost. SGTNE (Semi-Global TNE) avoids these deadlocks by executing a shortest path algorithm over a snapshot of the LPs in a cluster of processors. An experimental study of SGTNE was conducted on an Intel Paragon A4. The study compared SGTNE to TNE and to an optimized version of Chandy–Misra (CM) null message algorithms. We also investigated several scheduling algorithms for SGTNE and determined factors influencing its performance, most notably the influence of partitioning. Our results indicate that SGTNE provides good speedup relative to the fastest sequential algorithm and that it out-performs TNE for the population level examined, SGTNE was 3–5 times as fast as the CM-algorithm.

Dag consistent parallel simulation

We present a novel approach to parallel discrete event simulation based on the Cilk model of multithreaded computation. Cilk's runtime system not only manages the low-level aspects of program execution, but also provides the user with an algorithmic model of performance which can be used to predict the execution time of a parallel simulation. Moreover, a Cilk application can ``scale down'' to run on a single processor with nearly the same performance as that of serial code.A conservative parallel discrete event simulation algorithm has been developed in which communication between logical processes is achieved using Cilk's virtual memory model, dag consistent shared memory. The simulation executes in cycles, where each cycle involves a divide and conquer computation. Although local lookahead information can be exploited, the algorithm is robust in that it also calculates a global simulation time for each cycle. It can therefore be used for applications where zero lookahead may occur.

Modelling And Distributed Simulation On A Network Of Workstations

This paper addresses the issues of applying parallel simulation techniques to accelerate simulation on a network of workstations. The Omega multistage interconnection network is used as an example application. The conservative parallel simulation synchronization scheme using conventional null-message approach to resolve deadlock problem is based on a lookahead mechanism. For some application domains, unfortunately, the lookahead information is not available. Consequently, parallel simulation using null messages can result in live lock. We propose a deadlock/livelock free scheme using null messages, but without the lookahead assumption, to coordinate the simulation. In addition, we investigate different partitioning and transformation techniques for mapping a simulation program onto a network of workstations. A flushing mechanism to address the combinatoric explosion of using null-message in conservative simulation is also discussed. Our analysis shows that the proposed flushing mechanism effectively reduces the number of null messages from exponential to linear.

A parsing method for context-free languages using bottom-up lookahead computation

A new parsing algorithm of context-free languages using bottom-up computation of lookahead information is presented. Earley's algorithm uses a great number of items during enumeration to recognize a context-free language. If items with lookahead fields are used, the number of items can be reduced, but item spacing is considerably increased. The method presented here may reduce the number of items during enumeration without using lookahead fields of items. The computation of lookahead information takes place in a bottom-up manner. It is not necessary to compute lookahead information until it is required during enumeration.

LR-regular grammars—an extension of LR( k) grammars

LR-regular grammars are defined similarly to Knuth's LR( k ) grammars, with the following exception: arbitrarily long look-ahead is allowed before making a parsing decision during the bottom-up syntactical analysis; however, this look-ahead is restricted in that the essential “look-ahead information” can be represented by a finite number of regular sets, thus can be computed by a finite state machine. LR-regular grammars can be parsed deterministically in linear time by a rather simple two-scan algorithm. Efficient parsers are constructed for given LR-regular grammars. The family of LR-regular languages is studied; it properly includes the family of deterministic CF languages and has similar properties. Necessary and sufficient conditions for a grammar to be LR-regular are derived and then utilized for developing parser generation techniques for arbitrary grammars.