Roy Levin

Policy/mechanism separation in Hydra

The extent to which resource allocation policies are entrusted to user-level software determines in large part the degree of flexibility present in an operating system. In Hydra the determination to separate mechanism and policy is established as a basic design principle and is implemented by the construction of a kernel composed (almost) entirely of mechanisms. This paper presents three such mechanisms (scheduling, paging, protection) and examines how external policies which manipulate them may be constructed. It is shown that the policy decisions which remain embedded in the kernel exist for the sole purpose of arbitrating conflicting requests for physical resources, and then only to the extent of guaranteeing fairness.

Caching function calls using precise dependencies

This paper describes the implementation of a purely functionalprogramming language for building software systems. In this language,external tools like compilers and linkers are invoked by function calls. Because some function calls are extremely expensive, it is obviously important to reuse the results of previous function calls whenever possible. Caching a function call requires the language interpreter to record all values on which the function call depends. For optimal caching, it is important to record precise dependencies that are both dynamic and fine-grained. The paper sketches how we compute such dependencies, describes the implementation of an efficient function cache, and evaluates our implementations performance.

Synchronization primitives for a multiprocessor: a formal specification

Formal specifications of operating system interfaces can be a useful part of their documentation. We illustrate this by documenting the Threads synchronization primitives of the Taos operating system. We start with an informal description, present a way to formally specify interfaces in concurrent systems, give a formal specification of the synchronization primitives, briefly discuss the implementation, and conclude with a discussion of what we have learned from using the specification for more than a year.

Overview of the Hydra Operating System development

An overview of the hardware and philosophic context in which the Hydra design was done is discussed. The construction methodology is discussed together with some data which suggests the success of this methodological approach.

Interactive route search in the presence of order constraints

A route search is an enhancement of an ordinary geographic search. Instead of merely returning a set of entities, the result is a route that goes via entities that are relevant to the search. The input to the problem consists of several search queries, and each query defines a type of geographical entities. When visited, some of the entities succeed in satisfying the user while others fail to do so; however, only the probability of success is known prior to arrival. The main task is to find a route that visits at least one satisfying entity of each type. In an interactive search, the route is computed in steps. In each step, only the next entity of the route is given to the user, and after visiting that entity, the user provides a feedback specifying whether the entity satisfies her. This paper investigates interactive route search in the presence of order constraints that specify that some types of entities should be visited before others. We present heuristic algorithms for interactive route search for two cases, depending on whether the constraints define a complete order or a partial one. The main challenge is to utilize the feedback in order to compute a route that is shorter and has a higher degree of success, compared to routes that are computed non-interactively. We also discuss how to compare the results of the algorithms and introduce suitable measures for doing so. Experiments on real-world data illustrate the efficiency and effectiveness of our algorithms.

Complementing missing and inaccurate profiling using a minimum cost circulation algorithm

Edge profiling is a very common means for providing feedback on program behavior that can be used statically by an optimizer to produce highly optimized binaries. However collecting full edge profile carries a significant runtime overhead. This overhead creates addition problems for real-time applications, as it may prevent the system from meeting runtime deadlines and thus alter its behavior. In this paper we show how a low overhead sampling technique can be used to collect inaccurate profile which is later used to approximate the full edge profile using a novel technique based on the Minimum Cost Circulation Problem. The outcome is a machine independent profile gathering scheme that creates a slowdown of only 2%-3% during the training set, and produces an optimized binary which is only 0.6% less than a fully optimized one.

TARS: traffic-aware route search

In a traffic-aware route search (TARS), the user provides start and target locations and sets of search terms. The goal is to find the fastest route from the start location to the target via geographic entities (points of interest) that correspond to the search terms, while taking into account variations in the travel speed due to changes in traffic conditions, and the possibility that some visited entities will not satisfy the search requirements. A TARS query may include temporal constraints and order constraints that restrict the order by which entities are visited. Since TARS generalizes the Traveling-Salesperson Problem, it is an NP-hard problem. Thus, it is unlikely to find a polynomial-time algorithm for evaluating TARS queries. Hence, we present in this paper three heuristics to answer TARS queries—a local greedy approach, a global greedy approach and an algorithm that computes a linear approximation to the travel speeds, formulates the problem as a Mixed Integer Linear Programming (MILP) problem and uses a solver to find a solution. We provide an experimental evaluation based on actual traffic data and show that using a MILP solver to find a solution is effective and can be done within a limited running time in many real-life scenarios. The local-greedy approach is the least effective in finding a fast route, however, it has the best running time and it is the most scalable.

TARSIUS: a system for traffic-aware route search under conditions of uncertainty

This demo presents TARSIUS---a system for traffic-aware route search. In a traffic-aware route search (TARS), the user provides start location, target location and search terms, which specify types of geographical entities that should be visited along the route. A TARS query may include additional temporal constraints and limitations on the order by which entities are visited. The goal is to find the fastest route from the start location to the target, via entities of the specified types, while taking into account variations in the travel speed, due to changes in traffic conditions. Planning a route under conditions of uncertainty requires the system to also take into account the possibility that some visited entities will not satisfy the user requirements so that the route may need to go via several entities of the same type. In the demonstration we present the system. We demonstrate a web-based user interface that facilitates the formulation of TARS queries. We show how queries are posed and evaluated over a database that contains real traffic data. Since answering a TARS query is NP-hard, we present three heuristics to the problem. Using the system, we illustrate the routes that are computed by these heuristics.

Grapevine: An exercise in distributed computing

Grapevine is a distributed, replicated system running on a large internet within the Xerox research and development community. The internet extends from coast to coast in the USA, to Canada and to Europe, and contains more than 50 Ethernet local networks linked by leased telephone lines. Over 1500 computers are attached to the internet. Most computers are used an personal workstations, but some are used as servers providing access to shared facilities such as printers, large-scale secondary storage, or data bases. Computers on the internet are uniformly addressable using the PUP family of protocols.

Islands in the Stream: A Study of Item Recommendation within an Enterprise Social Stream

Social streams allow users to receive updates from their network by syndicating social media activity. These streams have become a popular way to share and consume information both on the web and in the enterprise. With so much activity going on, filtering and personalizing the stream for individual users is a key challenge. In this work, we study the recommendation of enterprise social stream items through a user survey with 510 participants, conducted within a globally distributed organization. In the survey, participants rated their level of interest and surprise for different items from the stream and could also indicate whether they were already familiar with the item. Thus, our evaluation goes beyond the common accuracy measure and examines aspects of serendipity and novelty. We also inspect how various features of the recommended item, its author, and reader, influence its ratings. Our results shed light on the key factors that make a stream item valuable to its reader within the enterprise.