16570884
FPGA-Based High-Order Finite Difference Algorithm for 2D Acoustic Wave Propagation Problems
Finite Difference Time Domain (FDTD) method is the most commonly used numerical algorithm for simulating linear wave propagation phenomena in geophysics, electromagnetics, and aeroor marineacoustics applications. Unfortunately, evaluating timeevolution for millions to trillions of discrete spatial points is computational intensive, especially when the geometrical size under study is much larger than the wavelength of excitations or long time integration is needed. In this paper, we proposed an FPGA-based hardware implementation to accelerate these simulation tasks. By adopting high-order temporal and spatial finite difference numerical algorithms along with efficient on-chip memory architecture, we alleviate the bandwidth bottleneck between the FPGA chip and onboard memories at the cost of increased computational burdens, which are absorbed effectively into the pipelined computing engine without speed penalty. The speed of our design implemented on Xilinx ML401 Virtex-4 evaluation platform is about 7~20 times faster than a pure software implementation of the same algorithm running on a 3.0GHz DELL workstation. Moreover, the desirable properties of simplicity and scalability make our design compatible with most commercial reconfigurable coprocessor platforms and correspondingly, the performance would be proportional to their available onboard memory bandwidth. 1. Motivation and related work Time domain numerical simulation can improve people’s understanding on dynamic behaviors of complex time-evolution problems, so plays an important role in scientific research and engineering design. In the past decades, efforts of simulating linear wave phenomena, including acoustic, electromagnetic, and elastic waves, have grown rapidly with the performance improvement of digital computers. The standard 2-order accurate FD method (including the famous Yee’s FDTD method on staggered grid in computational electromagnetic applications) is the most commonly used numerical algorithm because of its simplicity and flexibility. It is also well known that the propagation of waves over long distances requires long-time integration with minimal numerical dissipation and dispersion, and the performance of high-order FD schemes is superior to the standard one. However, their extraordinarily computational costs lead to relatively low algorithm efficiency, which limit their utilization in practice. Although pure software acceleration methods, from low-level instruction reordering to high-level process parallelism are all exhausted to speedup these applications, a typical 2D or 3D simulation task still costs hours to weeks using even the simplest 2-order FDTD method on High Performance Computing (HPC) environments. Recently, as the programmable hardware resources inside FPGA chips have increased greatly, people start showing their interests in accelerating time domain numerical simulations for wave-like equations on FPGA-based reconfigurable hardware platforms. Comparing with pure software procedures running on general-purpose computers or fully-customized VLSI hardware platforms, FPGA technology provide people a compromise between software’s best flexibility and the highest performance of hardware implementations. The idea of accelerating acoustic wave simulations using hardware platforms for geophysical applications can be traced back to 1990s [1]. The first effort of designing and implementing such a stand-alone system was described in [2]. In [3], a reconfigurable coprocessor platform using high density FPGA was proposed to speedup seismic migration problems. For computational electromagnetic applications, several authors proposed their FPGA-based solutions to accelerating Yee’s Finite-Difference TimeDomain (FDTD) algorithm from the early 1990s [4-5]. Recent work in this field can be found in [6-9]. While most previous implementations concentrated on integrating more hardware arithmetic units into FPGA so that the aggressive computational speed of their designs could exceed generic computers, we have to point out that the massive data volume is the fundamental hindrance of simulating wave propagation problems numerically in 2D or 3D space. Specifically, memory bandwidth available between the computing engine (FPGA) and onboard memory modules has been proven a bottleneck preventing people taking full advantage of FPGA’s computational potentials [3, 8, 9]. In this paper, we try to alleviate this bottleneck from two approaches: First, we adopt high-order spatial FD operators together with high-order time integration schemes to greatly reduce the number of discrete gird points inside the simulation area and the number of time-marching steps. Second, we propose a new on-chip memory architecture design for our high-order FD numerical algorithms. This new design can be mapped into most conventional FPGAbased reconfigurable coprocessor platforms efficiently and sustained high computational throughput can be achieved. The rest of this paper is organized as follow: In section 2, we introduce the high-order FD numerical algorithms we adopted in this work and show their advantages over the standard 2-order method using a simple one-dimensional acoustic wave propagation experiment. In section 3, we first propose our memory architecture design for the (2-4) FD scheme in 2D space, and then extend it to 4-order time-marching scheme to show its good scalability. Section 4 provides the performance comparisons between our hardware accelerator design implemented on Xilinx ML401 evaluation platform and its pure software counterpart on a P4 workstation for a simple 2D seismic modeling problem. Finally, conclusions and a discussion of future research direction are presented in section 5. 2. Acoustic wave equation and its High-order FD approximations 2.1 2-order accurate FD scheme Seismic modeling (forwarding) problems governed by acoustic or elastic wave equations in geophysics field are a class of numerical methods that simulate the scattering field arising when an impulsive source excites an underground region with known physical properties like density, velocity, anisotropy, elasticity, etc. We consider here the simplest 2D scalar acoustic case in the form of second-order hyperbolic Partial Differential Equation (PDE) as, ) , , ( ) , , ( ) , ( ) , , ( 2
3254470
Providing absolute differentiated services with statistical guarantees in static-priority scheduling networks
The authors propose and analyze a methodology for providing absolute differentiated services with statistical performance guarantees for real time applications in networks that use static priority schedulers. We extend previous work on delay analysis and develop a method that can be used to derive statistical delay guarantees in a flow-unaware fashion. While traditional delay analysis methods depend on flow population information, we have developed a method that relies on network (re-)configuration time analysis, and that allows us to analyze the delays without depending on the dynamic status of flow population. With this new method, we are able to reduce the runtime admission control to a simple utilization test. No explicit delay computation is necessary at admission time, making this approach scalable to large systems.
35665703
Implementation of QoS-Provisioning system for voice over IP
In this paper, we address issues related to implementing Voice-over-IP (VoIP) services in packet switching networks. VoIP has been identified as a critical real-time application in the network QoS research community and has been implemented in commercial products. To provide competent quality of service for VoIP, the call admission control (CAC) mechanism has to be introduced to prevent packet losing and over-queuing. Several well-designed CAC mechanisms, such as the Site- Utilization-Based CAC and the link-utilization-based CAC mechanisms, are in place. However, the existing commercial VoIP systems have not been able to adequately apply and support these CAC mechanisms, and hence unable to provide QoS guarantees to VoIP. We have designed and implemented a QoS-Provisioning system that can be seamlessly integrated to the existing VoIP system to overcome its weakness in offering QoS guarantees. As a result, our system has been realized at Internet2 Voice Over IP Testbed in Texas A&M University.
14217231
Scheduling Tasks with Resource Requirements in Hard Real-Time Systems
This paper describes a heuristic approach for solving the problem of dynamically scheduling tasks in a real-time system where tasks have deadlines and general resource requirements. The crux of our approach lies in the heuristic function used to select the task to be scheduled next. The heuristic function is composed of three weighted factors. These factors explicitly consider information about real-time constraints of tasks and their utilization of resources. Simulation studies show that the weights for the various factors in the heuristic function have to be fine-tuned in order to obtain a degree of success in the range of 75-88 percent of that obtained via exhaustive search. However, modifying the approach to use limited backtracking improves the degree of success substantially to as high as 99.5 percent. This improvement is observed even when the initial set of weights are not tailored for a particular set of tasks. Simulation studies also show that in most cases the schedule determined by the heuristic algorithm is optimal or close to optimal.
38045083
Guarantee protocols for communication in distributed hard real-time systems
A classification scheme is proposed which divides real-time messages into three different classes according to the impact on system performance when the deadline of a message is missed. A study is made of several multiaccess protocols that take into account the time constraints of messages as well as their classifications and that are able to given an early notice when an important message will miss its deadline. The performances of these protocols are evaluated and compared through simulation.<<ETX>>
3436856
Distributed Privacy Preserving Information Sharing
In this paper, we address issues related to sharing information in a distributed system consisting of autonomous entities, each of which holds a private database. Semi-honest behavior has been widely adopted as the model for adversarial threats. However, it substantially underestimates the capability of adversaries in reality. In this paper, we consider a threat space containing more powerful adversaries that includes not only semi-honest but also those malicious adversaries. In particular, we classify malicious adversaries into two widely existing subclasses, called weakly malicious and strongly malicious adversaries, respectively. We define a measure of privacy leakage for information sharing systems and propose protocols that can effectively and efficiently protect privacy against different kinds of malicious adversaries.
12760716
Making Web services dependable
Web services offer great promise for integrating and automating software applications within and between enterprises over the Internet. However, ensuring that Web services are dependable, and can satisfy their clients' requests when the clients need them is a real challenge because, typically, a business activity involves multiple Web services and a Web service involves multiple components, each of which must be dependable. In this paper, we describe fault tolerance techniques, including replication, checkpointing, and message logging, in addition to reliable messaging and transaction management for which Web services specifications exist. We discuss how those techniques can be applied to the components of the Web services involved in the business activities to render them dependable.
446110
Controlling the mobility of multiple data transport ferries in a delay-tolerant network
As technology rapidly progresses, more devices will combine both communication and mobility capabilities. With mobility in devices, we envision a new class of proactive networks that are able to adapt themselves, via physical movement, to meet the needs of applications. To fully realize these opportunities, effective control of device mobility and the interaction between devices is needed. In this paper, we consider the message ferrying (MF) scheme which exploits controlled mobility to transport data in delay-tolerant networks, where end-to-end paths may not exist between nodes. In the MF scheme, a set of special mobile nodes called message ferries are responsible for carrying data for nodes in the network. We study the use of multiple ferries in such networks, which may be necessary to address performance and robustness concerns. We focus on the design of ferry routes. With the possibilities of interaction between ferries, the route design problem is challenging. We present algorithms to calculate routes such that the traffic demand is met and the data delivery delay is minimized. We evaluate these algorithms under a variety of network conditions via simulations. Our goal is to guide the design of MF systems and understand the tradeoff between the incurred cost of multiple ferries and the improved performance. We show that the performance scales well with the number of ferries in terms of throughput, delay and resource requirements in both ferries and nodes.
17989665
Message ferrying: proactive routing in highly-partitioned wireless ad hoc networks
An ad hoc network allows devices with wireless interfaces to communicate with each other without any pre-installed infrastructure. Due to node mobility, limited radio power, node failure and wide deployment area, ad hoc networks are often vulnerable to network partitioning. A number of examples are in battlefield, disaster recovery and wide area surveillance. Unfortunately, most existing ad hoc routing protocols will fail to deliver messages under these circumstances since no route to the destination exists. In this work we propose the Message Ferrying or MF scheme that provides efficient data delivery in disconnected ad hoc networks. In the MF scheme, nodes move proactively to send or receive messages. By introducing non-randomness in a node's proactive movement and exploiting such nonrandomness to deliver messages, the MF scheme improves data delivery performance in a disconnected network. In this paper, we propose the basic design of the MF scheme and develop a general framework to classify variations of MF systems. We also study ferry route design problem in stationary node case which is shown to be NP-hard and provide an efficient algorithm to compute ferry route.
20298520
Increasing the reliability of three-tier applications
In this paper we describe an infrastructure that provides increased reliability for three-tier applications, transparently, using commercial off-the-shelf application servers and database systems. In this infrastructure the application servers are actively replicated to protect the business logic processing. Replicating the transaction coordinator renders the two-phase commit protocol non-blocking and, thus, avoids potentially long service disruptions caused by coordinator failure. A thin interpositioning library provides client-side automatic failover, so that clients know the outcome of their requests. The interaction between the application servers and the database servers is handled through replicated gateways that prevent duplicate requests from reaching the database servers. Aborted transactions, caused by process or communication faults, are automatically retried on the client's behalf.
9870709
End-to-end latency of a fault-tolerant CORBA infrastructure
This paper presents an evaluation of the end-to-end latency of a fault-tolerant CORBA infrastructure that we have implemented. The fault-tolerant infrastructure replicates the server applications using active, passive and semi-active replication, and maintains strong replica consistency of the server replicas. By analyses and by measurements of the running fault-tolerant infrastructure, we characterize the end-to-end latency under fault-free conditions. The main determining factor of the run-time performance of the fault-tolerant infrastructure is the Totem group communication protocol, which contributes to the end-to-end latency primarily in two ways: the delay in sending messages and the processing cost of the rotating token.To reduce the delay in sending messages for passive and semi-active replication, the position of the primary server replica on the Totem ring, the token rotation time, the processing time at the client, and the processing time at the server must be considered. For active replication, the presence of duplicate messages adversely affects the performance. However, if an effective sending-side duplicate suppression mechanism is implemented, active replication is more advantageous than both passive and semi-active replication because of the automatic selection of the most favorable position of the server replica that sends the first non-duplicate reply.