Delivering Consistent Network Performance in Multi-tenant Data Centers

Data centers are growing rapidly in size and have recently begun acquiring a new role as cloud hosting platforms, allowing outside developers to deploy their own applications on large scales. As a result, today\u27s data centers are multi-tenant environments that host an increasingly diverse set of applications, many of which have very demanding networking requirements. This has prompted research into new data center architectures that offer increased capacity by using topologies that introduce multiple paths between servers. To achieve consistent network performance in these networks, traffic must be effectively load balanced among the available paths. In addition, some form of system-wide traffic regulation is necessary to provide performance guarantees to tenants. To address these issues, this thesis introduces several software-based mechanisms that were inspired by techniques used to regulate traffic in the interconnects of scalable Internet routers. In particular, we borrow two key concepts that serve as the basis for our approach. First, we investigate packet-level routing techniques that are similar to those used to balance load effectively in routers. This work is novel in the data center context because most existing approaches route traffic at the level of flows to prevent their packets from arriving out-of-order. We show that routing at the packet-level allows for far more efficient use of the network\u27s resources and we provide a novel resequencing scheme to deal with out-of-order arrivals. Secondly, we introduce distributed scheduling as a means to engineer traffic in data centers. In routers, distributed scheduling controls the rates between ports on different line cards enabling traffic to move efficiently through the interconnect. We apply the same basic idea to schedule rates between servers in the data center. We show that scheduling can prevent congestion from occurring and can be used as a flexible mechanism to support network performance guarantees for tenants. In contrast to previous work, which relied on centralized controllers to schedule traffic, our approach is fully distributed and we provide a novel distributed algorithm to control rates. In addition, we introduce an optimization problem called backlog scheduling to study scheduling strategies that facilitate more efficient application execution

On Modeling Weak Sinks in MODPATH

Regional groundwater flow systems often contain both strong sinks and weak sinks. A strong sink extracts water from the entire aquifer depth, while a weak sink lets some water pass underneath or over the actual sink. The numerical groundwater flow model MODFLOW may allow a sink cell to act as a strong or weak sink, hence extracting all water that enters the cell or allowing some of that water to pass. A physical strong sink can be modeled by either a strong sink cell or a weak sink cell, with the latter generally occurring in low-resolution models. Likewise, a physical weak sink may also be represented by either type of sink cell. The representation of weak sinks in the particle tracing code MODPATH is more equivocal than in MODFLOW. With the appropriate parameterization of MODPATH, particle traces and their associated travel times to weak sink streams can be modeled with adequate accuracy, even in single layer models. Weak sink well cells, on the other hand, require special measures as proposed in the literature to generate correct particle traces and individual travel times and hence capture zones. We found that the transit time distributions for well water generally do not require special measures provided aquifer properties are locally homogeneous and the well draws water from the entire aquifer depth, an important observation for determining the response of a well to non-point contaminant inputs

Engineering Extracellular Secretion Pathways In Escherichia Coli

Extracellular secretion is highly desirable in preparative protein production. The bacterium Escherichia coli is a commonly used for both laboratory- and industrial- scale biosynthesis of proteins, but it lacks many of the pathways for exporting proteins out of cells. This lack of a dedicated extracellular secretion system represents a major bottleneck across many biotechnology disciplines, in particular the bioprocessing of plant biomass where extracellular secretion of cellulase is required. Furthermore, the study and engineering of extracellular secretion systems is limited due to a lack of high-throughput screen to identify rare genetic conditions that affect secretion activity. Recently, it was discovered in E. coli that the YebF protein is secreted efficiently into the supernatant when over expressed, and YebF has been employed to carry heterologous proteins into the supernatant via C-terminal genetic fusions. Here, we harness the YebF pathway to simultaneously co-secrete active cellulases into the culture medium, which enabled non-cellulolytic E. coli cells to utilize and convert cellulose to bioenergy products. We also developed a universal approach to study and engineer YebF and other extracellular secretion pathways. This high-throughput screening platform was used to screen a genome-wide transposon insertion library for the isolation of gene deletions that upregulate the secretion of YebF and YebF fusions. We also developed an alternative strategy for engineering extracellular secretion systems by way of a genetic selection where the nonsecretory phenotype is lethal. Finally, we describe some of the physiological consequences to the bacterial host caused by heterologous protein secretion, in particular an envelope stress response that triggers CRISPR RNA-mediated DNA silencing. ii

Sliver: A BPEL Workflow Process Execution Engine for Mobile Devices

The Business Process Execution Language (BPEL) has become the dominant means for expressing traditional business processes as workflows. The widespread deployment of mobile devices like PDAs and mobile phones has created a vast computational and communication resource for these workflows to exploit. However, BPEL so far has been deployed only on relatively heavyweight server platforms such as Apache Tomcat, leaving the potential created by these lower-end devices untapped. This paper presents Sliver, a BPEL workflow process execution engine that supports a wide variety of devices ranging from mobile phones to desktop PCs. We discuss the design decisions that allow Sliver to operate within the limited resources of a mobile phone or PDA. We also evaluate the performance of a prototype implementation of Sliver

Supporting Collaborative Behavior in MANETs using Workflows

Groupware activities provide a powerful representation for many collaborative tasks. Today, the technologies that support typical groupware applications often assume a stable wired network infrastructure. The potential for collaboration in scenarios that lack this fixed infrastructure remains largely untapped. Such scenarios include activities on construction sites, wilderness exploration, disaster recovery, and rapid intervention teams. Communication in these scenarios can be supported using wireless ad hoc networks, an emerging technology whose full potential is yet to be understood and realized. In this paper, we consider the fundamental technical issues that need to be addressed in order to introduce groupware concepts into mobile ad hoc networks. Starting with a simple workflow model, we examine the process of allocating its actions to physically-mobile agents in a manner that accommodates transient communication and runtime errors

MobiWork: Mobile Workflow for MANETs

The workflow model is well suited for scenarios where many entities work collaboratively towards a common goal, and is used widely today to model complex business processes. However, the fundamental workflow model is very powerful and can be applied to a wider variety of application domains. This paper represents an initial investigation into the possibility of using workflows to model collaboration in an ad hoc mobile environment. Moving to a mobile setting introduces many challenges as the mobility of the participants in a workflow imposes constraints on allocation of workflow tasks, coordination among participants, and marshaling of results. We present an algorithm that heuristically allocates tasks to participants based on their capabilities and mobility and discuss the architecture and implementation of MobiWork, our prototype system that allocates and executes workflows in an ad hoc mobile environment. An evaluation of the performance of our heuristic algorithm is also presented