Monica Brockmeyer

Simulation-verification: biting at the state explosion problem

Simulation and verification are two conventional techniques for the analysis of specifications of real-time systems. While simulation is relatively inexpensive in terms of execution time, it only validates the behavior of a system for one particular computation path. On the other hand, verification provides guarantees over the entire set of computation paths of a system, but is, in general, very expensive due to the state-space explosion problem. We introduce a new technique: simulation-verification combines the best of both worlds by synthesizing an intermediate analysis method. This method uses simulation to limit the generation of a computation graph to that set of computations consistent with the simulation. This limited computation graph, called a simulation-verification graph, can be one or more orders of magnitude smaller than the full computation graph. A tool, XSVT, is described which implements simulation-verification graphs. Three paradigms for using the new technique are proposed. The paper illustrates the application of the proposed technique via an example of a robot controller for a manufacturing assembly line.

QoSMap: Achieving Quality and Resilience through Overlay Construction

We describe QoSMap, an overlay construction mechanism which computes high quality overlay networks for applications having stringent constraints on hop-degrading QoS metrics and provides resilience against the Internet’s unpredictable network behavior. QoSMap implements three features to achieve the QoS and resiliency goals. In order to provide high QoS, it constructs overlay communication edges with short but efficient underlay paths. In case of QoS violations due to changing network conditions, QoSMap utilizes supplemental backup paths, which are specifically constructed in order to extend overlay lifetimes. QoSMap also avoids nodes that have experienced recent QoS failure, further improving the resilience of the overlays. PlanetLab experiments prove the ability of QoSMap to construct efficient and resilient overlays for applications with stringent QoS constraints of latency and packet loss for a variety of topologies. QoSMap-constructed overlays significantly outperform overlays constructed without the three QoSMap features. The results also reveal the usability and effectiveness of QoSMap to construct near-optimal solution in a short amount of time, compared to the optimal solution which has a very high time complexity.

Efficient data aggregation middleware for wireless sensor networks

This paper describes a data aggregation and consensus algorithm for object location and tracking applications deployed on wireless sensor networks. This consensus algorithm permits ad-hoc, in-network, group formation in response to a detected event. By reaching a consensus in the network, only a single message indicating the detected event needs to be forwarded to the tracking application at a base station, leading to significant savings in communication costs and prolonging of the life of the network. Our ad-hoc approach offers superior detecting power to other approaches, which statically partition the network into detection regions. The use of a quorum mechanism to achieve consensus leads to higher availability under failure conditions. Moreover, the algorithm exploits the broadcast communication and synchrony properties of wireless sensor networks. Our experimental results show that the algorithm exhibits low overhead despite faults and converges quickly to a single consensus even if multiple nodes initiate the consensus protocol.

QoSMap: QoS aware Mapping of Virtual Networks for Resiliency and Efficiency

We describe QoSMap, an efficient and flexible mechanism for constructing virtual networks on a shared Internet substrate for applications having stringent QoS and resiliency requirements. Applications specify desired overlay topology and weighted network characteristics which serve as resource constraints desired by the application in meeting the QoS expectations. QoSMap uses these constraints to select an overlay consisting of high quality direct paths between nodes from a pool of candidate nodes and paths. In addition to the required overlay topology constructed from direct paths between nodes, QoSMap provides path resiliency by constructing alternate one-hop overlay routes via intermediary nodes having paths that meet or exceed the resource constraints. As a case study, we utilized QoSMap to form an overlay for an application requiring constraints on message delay and loss rates. We describe the design of QoSMap and show that it leads to higher quality and more resilient overlays than does a mechanism which addresses only the minimum QoS requirements of the application.

Experimental Analysis of Application Specific Energy Efficiency of Data Centers with Heterogeneous Servers

Energy efficiency is an important issue for data centers given the amount of energy they consume yearly. However, there is still a gap of understanding of how exactly the application type and the heterogeneity of servers and their configuration impact the energy efficiency of data centers. To this end, we introduce the notion of Application Specific Energy Efficiency (ASEE) in order to rank energy efficiency of heterogeneous servers based on the hosted applications. We conducted extensive sets of experiments using three benchmarks: TPC-W, BS Seeker, and Matrix Stress mark. We observed that each server has different ASEE value based on the type of application running, the size of the virtual machine, the application load, and the scalability factor. In some cases, we witnessed 70% of ASEE improvement by changing the virtual machine size within the same node while keeping an identical load. In different cases, we witnessed up to 86% of ASEE improvement by running the same application with the same load within the same size of virtual machine but on different nodes. Our observation has many implications which include but are not limited to improving virtual machine scheduling based on the ASEE rank of the node. Another implication stresses on the importance of accurate prediction of application load and selecting the appropriate virtual machine size in order to improve the ASEE.

PaxonDHT: achieving consensus in distributed hash tables

Despite the scalability, availability and resiliency potential suggested by distributed hash tables, their lack of support for replication of mutable data and for strong consistency models stands in the way of their becoming widely accepted as a viable basis for the future generation Internet. This paper presents PaxonDHT, a Paxos-based middleware service, which ensures with high probability that a dynamic set of replicas reach total order agreement. PaxonDHT works well in the highly dynamic environment of DHTs, preserving the properties of the underlying DHT layer, and provides simultaneous support for heterogeneous services. Our algorithm has been deployed on Pastry, using the common API for interoperability with other DHTs. Experimental results in a simulated network show that PaxonDHT is scalable with replica set size and system load, and that it works well in highly dynamic environments.

BatteryExtender: an adaptive user-guided tool for power management of mobile devices

The battery life of mobile devices is one of their most important resources. Much of the literature focuses on accurately profiling the power consumption of device components or enabling application developers to develop energy-efficient applications through fine-grained power profiling. However, there is a lack of tools to enable users to extend battery life on demand. What can users do if they need their device to last for a specific duration in order to perform a specific task? To this extent, we developed BatteryExtender, a user-guided power management tool that enables the reconfiguration of the devices resources based on the workload requirement, similar to the principle of creating virtual machines in the cloud. It predicts the battery life savings based on the new configuration, in addition to predicting the impact of running applications on the battery life. Through our experimental analysis, BatteryExtender decreased the energy consumption between 10.03% and 20.21%, and in rare cases by up to 72.83%. The accuracy rate ranged between 92.37% and 99.72%.

GRACE: enabling collaborations in wide-area distributed systems

The globalization of businesses and the cooperation between organizations have brought on an ever increased need for providing support for distributed collaborations. In this paper, we argue that existing peer-to-peer frameworks, although suitable for collaborative environments, fail to provide the support for multiple consistency semantics required by modern complex collaborations. We present GRACE, a middleware platform that enables distributed and mobile collaborations, by combining the capabilities of distributed hash tables (DHTs) with a layered architecture that supports a wide range of semantics. The DHT substrate provides support for mobility, high scalability, availability and load balancing of data, as well as self-repairing capabilities and ease of management, while the layered architecture takes into account the different semantic needs of the applications. We evaluate the efficiency of the algorithm for routing requests to the correct replica in an implementation of GRACE built on Pastry.

TACON: tactical construction of overlay networks

Internet-based overlay networks are likely to demonstrate poor performance when deployed on the Internet without consideration of topological and link properties. We introduce a new method for tactical construction of unstructured overlay networks which exploits a minimal landmark infrastructure to align the overlay with the underlying network. Our framework results in well-connected overlays which can better meet the growing demands of current applications. We demonstrate, using simulation, that our approach yields reduced latency, better connectivity and higher availability of resources than conventional overlay systems and other landmark-based approaches, thereby providing a scalable and efficient approach for overlay construction.

Efficient and dependable overlay networks

We describe QoSMap, a novel approach for mapping overlay networks with stringent QoS requirements. QoSMap computes high quality overlay paths by utilizing direct underlay paths that fulfill the applications topology and link-specific QoS requirements. It also improves application dependability by increasing resiliency against QoS failures under dynamic network conditions. The increased resiliency is achieved through the use of supplemental indirect paths which have a single intermediate node and which also meet application QoS requirements. For evaluation, we compared QoSMap with a simple QoS approach for overlay mapping. We compared the two approaches for fulfilling varying application requests with strict QoS requirements of latency and packet loss on PlanetLab. We observed that QoSMap yielded high quality overlays. Further, the use of indirect paths upon the QoS failure of the direct path avoided overlay reconfiguration and resulted in increased dependability and longer lifetime for the application overlays.