Events

Suppose that a group of voters wants to select k \ge 1 alternatives from a given set, and each voter indicates which of the alternatives are acceptable to her: the alternatives could be conference submissions, applicants for a scholarship or locations for a fast food chain. In this setting it is natural to require that the winning set represents the voters fairly, in the sense that large groups of voters with similar preferences have at least some of their approved alternatives in the winning set. We describe several ways to formalize this idea, and show how to use it to classify voting rules. For one of our axioms, the only known voting rule that satisfies it is not polynomial-time computable, and it was conjectured that no voting rule that satisfies this axiom can be polynomial-time computable; however, we will show that this is not the case.

Programmable analog devices are a powerful new computing substrate that are especially appropriate for performing computationally intensive simulations of dynamical systems. Until recently, the de-facto standard for programming these devices required hardware specialists to manually program the analog device to model the dynamical system of interest. In this talk, I will present Arco, a compiler that automatically configures analog devices to perform dynamical system simulation, and Jaunt, a compilation technique that that scales dynamical system parameters to change the speed of the simulation and render the resulting simulation physically realizable given the operating constraints of the analog hardware platform. These techniques capture the domain knowledge required to fully exploit the capabilities of reconfigurable analog devices, eliminating a key obstacle to the widespread adoption of these devices.

Software has bugs. What can we do to find as many of these as possible? In this talk, I show how to systematically test software by generating such tests automatically, starting with simple random "fuzzing" generators and then proceeding to more effective grammar-based and coverage-guided approaches. Being fully automatic and easy to deploy, such fuzzers run at little cost, yet are very effective in finding bugs: Our own Langfuzz grammar-based test generator for JavaScript runs around the clock for the Firefox, Chrome, and Edge web browsers and so far has found more than 2,600 confirmed bugs. Our latest test generator prototypes are even able to automatically learn the input language of a given program, which allows to generate highly effective tests for arbitrary programs without any particular setup.

Answering natural-language questions is a remarkable ability of search engines. However, brittleness in the state-of-the-art is exposed when handling complexity in question formulation. Such complexity has two distinct dimensions: (i) diversity of expressions which convey the same information need, and, (ii) complexity in the information need itself. We propose initial solutions to these challenges: (i) syntactic differences in question formulation can be tackled with a continuous-learning framework that extends template-based answering with semantic similarity and user feedback; (ii) complexity in information needs can be addressed by stitching pieces of evidence from multiple documents to build a noisy graph, within which answers can be detected using optimal interconnections. The talk will discuss results for these proposals, and conclude with promising open directions in question answering.

Energy consumption is an important concern for cloud datacenters. Its cost represents about 80% of the total cost of ownership and it is estimated that in 2020, the US datacenters alone will spend about $13 billion on energy bills. Generally, the servers are manufactured in such a way that they achieve high energy efficiency at high utilizations. Thereby for a low cost per computation all datacenter servers should push the utilization as high as possible. In order to fight the historically low utilization, cloud computing adopted server virtualization. This technology enables a cloud provider to pack (consolidate) the entire set of virtual machines (VMs) on a small set of physical servers and thereby, reduce the number of active servers. Even so, the datacenter servers rarely reach utilizations higher than 50% which means that they operate with a set of long-term unused resources (called 'holes'). Our first contribution is a cloud management system that dynamically splits/fusions VMs such that they can better fill the holes. However the datacenter resource fragmentation has a more fundamental problem. Over time, cloud applications demand more and more memory but the physical servers provide more an more CPU. In nowadays datacenters, the two resources are strongly coupled since they are bounded to a physical sever. Our second contribution is a practical way to decouple the CPU-memory tuple that can simply be applied to a commodity server. The underutilization observed on physical servers is also true for virtual machines. It has been shown that VMs consume only a small fraction of the allocated resources because the cloud customers are not able to correctly estimate the resource amount necessary for their applications. Our third contribution is a system that estimates the memory consumption (i.e. the working set size) of a VM, with low overhead and high accuracy. Thereby, we can now consolidate the VMs on based on their working set size (not the booked memory). However, the drawback of this approach is the risk of memory starvation. If one or multiple VMs have an sharp increase in memory demand, the physical server may run out of memory. This event is undesirable because the cloud platform is unable to provide the client with the memory he paid for. Our fourth contribution is a system that allows a VM to use remote memory provided by a different rack server. Thereby, in the case of a peak memory demand, our system allows the VM to allocate memory on a remote physical server.

Embedding a domain-specific language (DSL) in a general purpose host language is an efficient way to develop a new DSL. Various kinds of languages and paradigms can be used as host languages, including object-oriented, functional, statically typed, and dynamically typed variants, all having their pros and cons. For deep embedding, statically typed languages enable early checking and potentially good DSL error messages, instead of reporting runtime errors. Dynamically typed languages, on the other hand, enable flexible transformations, thus avoiding extensive boilerplate code. In this talk, I will discuss the concept of gradually typed symbolic expressions that mix static and dynamic typing for symbolic data. The key idea is to combine the strengths of dynamic and static typing in the context of deep embedding of DSLs. Moreover, I will briefly outline an ongoing research effort of developing a new framework for developing heterogenous domain-specific languages.