This paper presents a database containing 'ground truth' segmentations produced by humans for images of a wide variety of natural scenes. We define an error measure which quantifies the consistency between segmentations of differing granularities and find that different human segmentations of the same image are highly consistent. Use of this dataset is demonstrated in two applications: (1) evaluating the performance of segmentation algorithms and (2) measuring probability distributions associated with Gestalt grouping factors as well as statistics of image region properties.

/pdf/a-database-of-human-segmented-natural-images-and-its-2ve1vk356s.pdf

A database of human segmented natural images and its application to evaluating segmentation algorithms and measuring ecological statistics

https://deim.urv.cat/~alexandre.arenas/publicacions/pdf/review.pdf

Synchronization in complex networks

This survey covers rollback-recovery techniques that do not require special language constructs. In the first part of the survey we classify rollback-recovery protocols into checkpoint-based and log-based.Checkpoint-based protocols rely solely on checkpointing for system state restoration. Checkpointing can be coordinated, uncoordinated, or communication-induced. Log-based protocols combine checkpointing with logging of nondeterministic events, encoded in tuples called determinants. Depending on how determinants are logged, log-based protocols can be pessimistic, optimistic, or causal. Throughout the survey, we highlight the research issues that are at the core of rollback-recovery and present the solutions that currently address them. We also compare the performance of different rollback-recovery protocols with respect to a series of desirable properties and discuss the issues that arise in the practical implementations of these protocols.

/pdf/a-survey-of-rollback-recovery-protocols-in-message-passing-bnrpate93a.pdf

A survey of rollback-recovery protocols in message-passing systems

Parallel discrete event simulation (PDES), sometimes called distributed simulation, refers to the execution of a single discrete event simulation program on a parallel computer. PDES has attracted a considerable amount of interest in recent years. From a pragmatic standpoint, this interest arises from the fact that large simulations in engineering, computer science, economics, and military applications, to mention a few, consume enormous amounts of time on sequential machines. From an academic point of view, parallel simulation is interesting because it represents a problem domain that often contains substantial amounts of parallelism (e.g., see [59]), yet paradoxically, is surprisingly difficult to parallelize in practice. A sufficiently general solution to the PDES problem may lead to new insights in parallel computation as a whole. Historically, the irregular, data-dependent nature of PDES programs has identified it as an application where vectorization techniques using supercomputer hardware provide little benefit [14].A discrete event simulation model assumes the system being simulated only changes state at discrete points in simulated time. The simulation model jumps from one state to another upon the occurrence of an event. For example, a simulator of a store-and-forward communication network might include state variables to indicate the length of message queues, the status of communication links (busy or idle), etc. Typical events might include arrival of a message at some node in the network, forwarding a message to another network node, component failures, etc.We are especially concerned with the simulation of asynchronous systems where events are not synchronized by a global clock, but rather, occur at irregular time intervals. For these systems, few simulator events occur at any single point in simulated time; therefore parallelization techniques based on lock-step execution using a global simulation clock perform poorly or require assumptions in the timing model that may compromise the fidelity of the simulation. Concurrent execution of events at different points in simulated time is required, but as we shall soon see, this introduces interesting synchronization problems that are at the heart of the PDES problem.This article deals with the execution of a simulation program on a parallel computer by decomposing the simulation application into a set of concurrently executing processes. For completeness, we conclude this section by mentioning other approaches to exploiting parallelism in simulation problems.Comfort and Shepard et al. have proposed using dedicated functional units to implement specific sequential simulation functions, (e.g., event list manipulation and random number generation [20, 23, 47]). This method can provide only a limited amount of speedup, however. Zhang, Zeigler, and Concepcion use the hierarchical decomposition of the simulation model to allow an event consisting of several subevents to be processed concurrently [21, 98]. A third alternative is to execute independent, sequential simulation programs on different processors [11, 39]. This replicated trials approach is useful if the simulation is largely stochastic and one is performing long simulation runs to reduce variance, or if one is attempting to simulate a specific simulation problem across a large number of different parameter settings. However, one drawback with this approach is that each processor must contain sufficient memory to hold the entire simulation. Furthermore, this approach is less suitable in a design environment where results of one experiment are used to determine the experiment that should be performed next because one must wait for a sequential execution to be completed before results are obtained.

/pdf/parallel-discrete-event-simulation-m759whzicd.pdf

Parallel discrete event simulation

A number of library based parallel and sequential network simulators have been designed. The paper describes a library, called GloMoSim (Global Mobile system Simulator), for parallel simulation of wireless networks. GloMoSim has been designed to be extensible and composable: the communication protocol stack for wireless networks is divided into a set of layers, each with its own API. Models of protocols at one layer interact with those at a lower (or higher) layer only via these APIs. The modular implementation enables consistent comparison of multiple protocols at a given layer. The parallel implementation of GloMoSim can be executed using a variety of conservative synchronization protocols, which include the null message and conditional event algorithms. The paper describes the GloMoSim library, addresses a number of issues relevant to its parallelization, and presents a set of experimental results on the IBM 9076 SP, a distributed memory multicomputer. These experiments use models constructed from the library modules.

GloMoSim: a library for parallel simulation of large-scale wireless networks

Widespread deployment of sensors in roadways and vehicles is creating new challenges in effectively exploiting the wealth of real-time transportation system data. However, the precision of the real-time data varies depending on the level of data aggregation. For example, minute-by-minute data are more precise than hourly average data. This paper explores the ability to create an accurate estimate of the evolving state of transportation systems using real-time roadway data aggregated at various update intervals. It is found that simulation based on inflow data aggregated over a short time interval is capable of providing a superior representation of the real world over longer aggregate intervals. However, the perceived improvements are minimal under congested conditions and most pronounced under un-congested conditions. In addition, outflow constraints should be considered during congested flow periods, otherwise significant deviation from the real world performance may arise.

/pdf/an-investigation-of-real-time-dynamic-data-driven-57gca9dq02.pdf

An investigation of real-time dynamic data driven transportation simulation

Advances in computing and wireless communication technologies have increased interest in "smart" vehicles---vehicles equipped with significant computing, communication and sensing capabilities to provide services to travelers. Smart vehicles can be exploited to improve driving safety and comfort as well as optimize surface transportation systems. Wireless communications among vehicles and between vehicles and roadside infrastructures represent an important class of vehicle communications. 
One can envision creating an integrated radio network leveraging various wireless technologies that work together in a seamless fashion. Based on cost-performance tradeoffs, different network configurations may be appropriate for different environments. An understanding of the properties of different vehicular network architectures is absolutely necessary before services can be successfully deployed. Based on this understanding, efficient data services (e.g., data dissemination services) can be designed to accommodate application requirements. 
This thesis examines several research topics concerning both the evaluation and design of vehicular networks. 
We explore the properties of vehicle-to-vehicle (v2v) communications. We study the spatial propagation of information along the road using v2v communications. Our analysis identifies the vehicle traffic characteristics that significantly affect information propagation. We also evaluate the feasibility of propagating information along a highway. 
Several design alternatives exist to build infrastructure-based vehicular networks. Their characteristics have been evaluated in a realistic vehicular environment. Based on these evaluations, we have developed some insights into the design of future broadband vehicular networks capable of adapting to varying vehicle traffic conditions. 
Based on the above analysis, opportunistic forwarding that exploit vehicle mobility to overcome vehicular network partitioning appears to be a viable approach for data dissemination using v2v communications for applications that can tolerate some data loss and delay. We introduce a methodology to design enhanced opportunistic forwarding algorithms. Practical algorithms derived from this methodology have exhibited different performance/overhead tradeoffs. 
An in-depth understanding of wireless communication performance in a vehicular environment is necessary to provide the groundwork for realizing reliable mobile communication services. We have conducted an extensive set of field experiments to uncover the performance of short-range communications between vehicles and between vehicles and roadside stations in a specific highway scenario.

/pdf/analysis-and-design-of-vehicular-networks-4zap4yyvek.pdf

Analysis and design of vehicular networks

The MIMDIX Environment for Parallel Simulation

This paper presents a novel networking architecture designed for communication intensive parallel applications running on clusters of workstations (COWs) connected by high speed network. This architecture permits: (1) the transfer of selected communication-related functionality the host machine to the network interface coprocessor and (2) the exposure of this functionality directly to applications as instructions of a Virtual Communication Machine (VCM) implemented by the coprocessor. The user-level code interacts directly with the network coprocessor as the host kernel only 'connects' the application to the VCM and does not participate in the data transfers. The distinctive feature of our design is its flexibility: the integration of the network with the application can be varied to maximize performance. The resulting communication architecture is characterized by a very low overhead on the host processor by latency and bandwidth close to the hardware limits, and by an application interface which enables zero-copy messaging and eases the port of some shared-memory parallel applications to COWs. The architecture admits low cost implementations based only on off-the-shelf hardware components. Additionally, its current ATM-based implementation can be used to communicate with any ATM-enabled host.

Supporting parallel applications on clusters of workstations: The intelligent network interface approach

In this paper we describe an approach to exploit temporal uncertainty in parallel and distributed simulation by utilizing time intervals rather than precise time stamps. Unlike previously published work that propose new message ordering semantics, our approach is based on conservative, time stamp order execution and enhancing the lookahead of the simulation by pre-drawing random numbers from a distribution that models temporal uncertainty. The advantages of this approach are that it allows time intervals to be exploited using a conventional Time Stamp Order (TSO) delivery mechanism, and it offers the modeler greater statistical control over the assigned time stamps. An implementation of this approach is described and initial performance measurements are presented.

Richard M. Fujimoto

Papers

An investigation of real-time dynamic data driven transportation simulation

Analysis and design of vehicular networks

The MIMDIX Environment for Parallel Simulation

Supporting parallel applications on clusters of workstations: The intelligent network interface approach

Pre-sampling as an approach for exploiting temporal uncertainty