Brendan Cully

SecondSite: disaster tolerance as a service

This paper describes the design and implementation of SecondSite, a cloud-based service for disaster tolerance. SecondSite extends the Remus virtualization-based high availability system by allowing groups of virtual machines to be replicated across data centers over wide-area Internet links. The goal of the system is to commodify the property of availability, exposing it as a simple tick box when configuring a new virtual machine. To achieve this in the wide area, we have had to tackle the related issues of replication traffic bandwidth, reliable failure detection across geographic regions and traffic redirection over a wide-area network without compromising on transparency and consistency.

Execution mining

Operating systems represent large pieces of complex software that are carefully tested and broadly deployed. Despite this, developers frequently have little more than their source code to understand how they behave. This static representation of a system results in limited insight into execution dynamics, such as what code is important, how data flows through a system, or how threads interact with one another. We describe Tralfamadore, a system that preserves complete traces of machine execution as an artifact that can be queried and analyzed with a library of simple, reusable operators, making it easy to develop and run new dynamic analyses. We demonstrate the benefits of this approach with several example applications, including a novel unified source and execution browser.

Retroactive aspects: programming in the past

We present a novel approach to the problem of dynamic program analysis: writing analysis code directly into the program source, but evaluating it against a recording of the original programs execution. This approach allows developers to reason about their program in the familiar context of its actual source, and take full advantage of program semantics, data structures, and library functionality for understanding execution. It also gives them the advantage of hindsight, letting them easily analyze unexpected behavior after it has occurred. Our position is that writing offline analysis as retroactive aspects provides a unifying approach that developers will find natural and powerful.

Understanding Performance for Two 802.11 Competing Flows

It is well known that 802.11 suffers from both inefficiency and unfairness in the face of competition and interference. This paper provides a detailed analysis of the impact of topology and traffic type on network performance when two flows compete with each other for airspace. We consider both TCP and UDP flows and a comprehensive set of node topologies. We vary these topologies to consider all combinations of the following four node-to-node interactions: (1) nodes unable to read or sense each other, (2) nodes able to sense each other but not able to read each others packets and nodes able to communicate with (3) weak and with (4) strong signal. We evaluate all possible cases through simulation and show that the cases can be reduced to 9 UDP and 10 TCP 802.1 Ig models with similar efficiency/fairness characteristics. We also validate our simulation results with extensive experiments conducted in a laboratory testbed. These more detailed models improve on previous work such as hidden-Zexposed-terminal categorization and are thus better suited as a basis for adaptive techniques to improve performance in 802.11 multi-hop WLAN or Mesh Networks.