Shell Ying Huang

Travel time analysis of a new automated storage and retrieval system

In conventional automated storage and retrieval systems (AS/RS), stacker cranes are used to access (store or retrieve loads into/from) the storage cells. The stacker cranes can travel simultaneously in the vertical and horizontal directions. However, because the combined motions generally require heavy machineries, the stacker cranes are inadequate for extra heavy loads such as sea container cargo. For such applications, we present a new kind of storage/retrieval (S/R) mechanism, designed with input from AS/RS manufacturers. Unlike stacker cranes, the new S/R mechanism has one vertical platform and N horizontal platforms to serve N tiers of an AS/RS rack. The vertical platform provides the vertical link among different tiers of the AS/RS rack, whereas the horizontal platforms access the storage cells on a given tier. The vertical platform and the horizontal platforms may move independently and concurrently; and the separation of the mechanisms for vertical/horizontal movements also makes the platforms lighter and hence they can operate at a higher speed than the conventional design. We then present a travel-time model under the stay dwell point policy, i.e. the platforms remain where they are after completing a storage/retrieval operation. The model is validated by computer simulations. The results show that our analytical model is reliable for the design and analysis of the new kind of AS/RS. We also present guidelines for the optimal design of a rectangular-in-time AS/RS rack with the new S/R mechanism.

Load balancing for conservative simulation on shared memory multiprocessor systems

Load balancing is a crucial factor in achieving good performance for parallel discrete event simulations. We present a load balancing scheme that combines both static partitioning and dynamic load balancing. The static partitioning scheme maps simulation objects to logical processes before simulation starts while the dynamic load balancing scheme attempts to balance the load during runtime. The static scheme involves two steps. First, the simulation objects that contribute to small lookahead are merged together by using a merging algorithm. Then a partitioning algorithm is applied. The merging is needed to ensure a consistent performance for our dynamic scheme. Our dynamic scheme is tailor-made for an asynchronous simulation protocol that does not rely on null messages. The performance study on a supply chain simulation shows that the partitioning algorithm and dynamic load balancing are important in achieving good performance.

Dynamic yard crane dispatching in container terminals with predicted vehicle arrival information

The performance of a container terminal depends on many aspects of operations. This paper focuses on the optimal sequencing of a yard crane (or YC for short) for serving a fleet of vehicles for delivery and pickup jobs. The objective is to minimize the average vehicle waiting time. While heuristic algorithms could not guarantee an optimal solution, a conventional mathematical formulation such as mixed integer program would require too much computing time. We present two new algorithms to efficiently compute YC dispatching sequences that are provably optimal within the planning window. The first algorithm is based on the well-known A^* search along with an admissible heuristics. We also incorporate this heuristics into a second backtracking algorithm which uses a prioritized search order to accelerate the computation. Experimental results show that both new algorithms perform very well for realistic YC jobs. Specifically, both are able to find within seconds optimal solutions for heavy workload scenarios with over 2.4x10^1^8 possible dispatching sequences. Moreover, even when the vehicle arrival times are not accurately forecasted, the new algorithms are still robust enough to produce optimal or near-optimal sequences, and they consistently outperform all the other algorithms evaluated.

Dynamic Space and Time Partitioning for Yard Crane Workload Management in Container Terminals

We propose a new hierarchical scheme for yard crane (YC) workload management in container terminals. We also propose a time partitioning algorithm and a space partitioning algorithm for deploying YCs to handle changing job arrival patterns in a row of yard blocks. The main differences between our approach and most of the methods in literature are (1) the average vehicle job waiting time instead of the number of jobs is used to balance YC workload and to evaluate the quality of a partition, (2) the YC working zone assignment is not in units of yard blocks and our space partitioning algorithm generates more flexible divisions of the workload from all blocks, and (3) the YC deployment frequency is not fixed but is decided by our time partitioning algorithm with the objective of minimizing average vehicle waiting times. The scheme combines simulation and optimization to achieve our objective for a row of yard blocks. Experimental results show that the proposed binary partitioning algorithm TP2 makes substantial improvements in job waiting times over the basic partitioning scheme and another existing algorithm (Ng, W. C. 2005. Crane scheduling in container yards with intercrane interference. Eur. J. Oper. Res.164(1) 64--78) in all tested job arrival scenarios.

Yard crane dispatching based on real time data driven simulation for container terminals

This paper studies the problem of real time yard crane dispatching in container terminals. Many technologies, including transponders, RFID and GPS have been used in the container terminal setting for real-time tracking of terminal equipment. A judicious integration of real-time data into the yard crane management system will allow better utilization of terminal resources to improve overall terminal productivity. We propose a yard crane dispatching algorithm based on real time data driven simulation to solve the problem of yard crane job sequencing to minimize average vehicle waiting time. The algorithm will produce optimal operation sequence for each planning window. Several policies to select jobs to form the planning window are also proposed. Our simulation results show that dispatching yard crane based on real time data driven simulation is of great value in improving yard crane performance in 3 scenarios with different vehicle arriving patterns and our results are 10% worse off a loosely estimated overall optimal performance result.

Capacity analysis of container terminals using simulation techniques

Simulation based analysis is particularly useful for designing new container terminals and evaluating the benefits of new resources or impacts of operation policies and modifications. We present a computer simulation model that integrates all the activities of a container terminal and captures the essential interactions among the subsystems which affect terminal capacity. The system incorporates methods to generate realistic vessel arrival patterns and can track the moves of millions of containers in a terminal. We further present case studies of the handling capacities of three container terminals in the Southeast Asia region, the issues involved, our approach and the findings.

Performance prediction tools for parallel discrete-event simulation

We have developed a set of performance prediction tools which help to estimate the achievable speedups from parallelizing a sequential simulation. The tools focus on two important factors in the actual speedup of a parallel simulation program: the simulation protocol used; and the inherent parallelism in the simulation model. The first two tools are a performance/parallelism analyzer for a conservative, asynchronous simulation protocol, and a similar analyzer for a conservative, synchronous (super-step) protocol. Each analyzer allows us to study how the speedup of a model changes with increasing number of processors, when a specific protocol is used. The third tool-a critical path analyzer-gives on ideal upper bound to the models speedup. This paper gives an overview of the prediction tools, and reports the predictions from applying the tools to a discrete-event wafer fabrication simulation model. The predictions are close to speedups from actual parallel implementations. These tools help us to set realistic expectations of the speedup from a parallel simulation program, and to focus our work on issues which are more likely to yield performance improvement.

An Empirical Comparison of Runtime Systems for Conservative Parallel Simulation

A main consideration when implementing a parallel simulation application is the choice of the parallel simulation protocol (conservative vs. optimistic). Given a particular protocol, the application programmer then has to determine a suitable parallel runtime system to implement the application. If the choice is an optimistic protocol, there are several parallel simulation libraries intended for application programmers (e.g. GTW, Warped). For a conservative protocol, the most effective approach is for the programmer to use a general parallel runtime library, and implement optimizations specific to the simulation application and/or model. In this paper, we selected four general parallel runtime libraries potentially relevant to parallel simulations, and implemented a conservative protocol on each of them. We study the four libraries on three main aspects: (a) programmability; (b) performance, and (c) mechanisms for performance tuning. Our target platforms are machines supporting shared address spaces (e.g. SGI Origin200, Sun Enterprise 3000), and we obtained performance figures from a 4-CPU Ultra2 Sun Enterprise 3000. From our experiments, we find that POSIX, though an industry standard, still has relatively high overheads, and cannot efficiently support a protocol with fine-grain LPs. The research libraries all show speedups on 4 processors, but to different extents. Cilk speedup curves improves with larger thread granularity, while Active threads show relatively good speedup even for small thread granularity. BSP processes are naturally coarse-grained, and thus good speedup is achieved in our simulation application.

A General Framework for Parallel BDI Agents

The traditional BDI agent has 3 basic computational components that generate beliefs, generate intentions and execute intentions. They run in a sequential and cyclic manner. This may introduce several problems. Among them, the inability to watch the environment continuously in dynamic environments may be disastrous. There is also no support for goal and intention reconsideration and consideration of relationships between goals at the architecture level. A parallel BDI agent architecture was proposed in [15] and evaluated in [16]. Based on the work in [15] and [16], we propose in this paper, a general framework for the parallel BDI agent model. Under this general framework, parallel BDI agents with different configurations depending on the availability of physical resources may be built. These agents have a number of advantages over the sequential one: 1. changes in the agents environment can be detected immediately; 2. emergencies will be dealt with immediately; 3. the support is provided at the architecture level for reconsideration of desires/intentions and the consideration of goal relationships when a new belief/desire is generated. We show some example parallel BDI agents with different configurations under the framework and their performance in a set of experiments.

A methodology for automating the parallelization of manufacturing simulations

One of the main reasons why parallel discrete event simulation has not been adopted more widely in industry is that the terminology used by the parallel simulation community differs from that of industrial simulation practitioners. The paper shows how the gap between these two communities can be bridged by presenting a methodology for automating the parallelization of manufacturing simulations. Our approach provides a way of automatically generating a mapping from a sequential simulation model to an efficient parallel implementation. The results of this mapping can be expressed in a form which is independent of any particular parallel simulation system or language. Since it is easy to generate code for different simulation systems, it is possible to evaluate alternative parallel simulation protocols at an early stage of development. A prediction of the performance can thus be obtained by studying the behaviour of an abstraction of the simulation model with various strategies or on different computing platforms.