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Seminar Schedule

September 1, 2016

Speaker: Leo Osvald


Abstract: First-class functions dramatically increase expressiveness, at the expense of static guarantees. In ALGOL or PASCAL, functions could be passed as arguments but never escape their defining scope. Thus, function arguments could serve as temporary access tokens or capabilities, enabling callees to perform some action, but only for the duration of the call. In modern languages, such programming patterns are no longer available.

The central thrust of this paper is to re-introduce second-class functions and other values alongside first-class entities in modern languages. We formalize second-class values with stack-bounded lifetimes as an extension to simply-typed lambda calculus, and for richer type systems such as F<: and systems with path-dependent types. We generalize the binary first vs second-class distinction to arbitrary privilege lattices, with the underlying type lattice as a special case. In this setting, abstract types naturally enable privilege parametricity. We prove type soundness and lifetime properties in Coq.

We implement our system as an extension of Scala, and present several case studies. First, we modify the Scala Collections library and add privilege annotations to all higher-order functions. Privilege parametricity is key to retain the high degree of code-reuse between sequential and parallel as well as lazy and eager collections. Second, we use scoped capabilities to introduce a model of checked exceptions in the Scala library, with only few changes to the code. Third, we employ second-class capabilities for memory safety in a region-based off-heap memory library.

September 8, 2016

Speaker: Jad Hbeika

Title: Locality-aware Task-parallel Execution on GPUs

Abstract: GPGPUs deliver high speedup for regular applications while remaining energy efficient. In recent years, there has been much focus on tuning irregular, task-parallel applications and/or the GPU architecture in order to achieve similar benefits for irregular applications running on GPUs. While most of the previous works have focused on minimizing the effect of control and memory divergence, which are prominent in irregular applications and which degrade the performance, there has been less attention paid to decreasing cache pressure and hence improving performance of applications given the small cache sizes on GPUs. In this paper we tackle two problems. First we extract data parallelism from irregular task parallel applications, which we do by subdividing each task into sub tasks at the CPU side and sending these sub tasks to the GPU for execution. By doing so we take advantage of the massive parallelism provided by the GPU. Second, to mitigate the memory demands of many tasks that access irregular data structures, we schedule these subtasks in a way to minimize the memory footprint of each warp running on the GPU. We use our framework with 3 task-parallel algorithms and show that we can achieve significant speedups over optimized GPU code.

September 15, 2016

Speaker: Suyash Gupta

Title: IMSuite: A benchmark suite for simulating distributed algorithms

Abstract: Considering the diverse nature of real-world distributed applications that makes it hard to identify a representative subset of distributed benchmarks, we focus on their underlying distributed algorithms. We present and characterize a new kernel benchmark suite (named IMSuite) that simulates some of the classical distributed algorithms in task parallel languages. We present multiple variations of our kernels, broadly categorized under two heads: (a) varying synchronization primitives (with and without fine grain synchronization primitives); and (b) varying forms of parallelization (data parallel and recursive task parallel). Our characterization covers interesting aspects of distributed applications such as distribution of remote communication requests, number of synchronization, task creation, task termination and atomic operations. We study the behavior (execution time) of our kernels by varying the problem size, the number of compute threads, and the input configurations. We also present an involved set of input generators and output validators.

Paper link:

September 22, 2016

Speakers: Ruby Tahboub and Gegory Essertel

Title: Flare: Scale up Apache Spark with Native Compilation and Set Your Data on Fire!

Abstract: The need for modern data analytics to combine relational, procedural, and map-reduce-style functional processing is widely recognized. State-of-the-art systems like Spark have recognized the need to add SQL front-ends and relational query optimization.

But how good are these relational extensions when scaled-up on modern hardware? We present the first comparative evaluation of Spark SQL on the standard TPC-H benchmark and show that there isa significant performance gap to best-of-breed relational query engines. We present Flare, a new back-end for Spark SQL that brings performance closer to the best SQL engines, without giving up the added expressiveness of Spark. Flare leverages the fact that Spark SQL is entirely implemented in Scala, and maps Spark’s internal query plans to Delite, an existing general-purpose compiler framework for embedded DSLs, also implemented in Scala.

September 29, 2016

Speaker: Yonghwi Kwon

Title: LDX: Causality Inference by Lightweight Dual Execution

Abstract: Causality inference, such as dynamic taint anslysis, has many applications (e.g., information leak detection). It determines whether an event e is causally dependent on a preceding event c during execution. We develop a new causality inference engine LDX. Given an execution, it spawns a slave execution, in which it mutates c and observes whether any change is induced at e. To preclude nondeterminism, LDX couples the executions by sharing syscall outcomes. To handle path differences induced by the perturbation, we develop a novel on-the-fly execution alignment scheme that maintains a counter to reflect the progress of execution. The scheme relies on program analysis and compiler transformation. LDX can effectively detect information leak and security attacks with an average overhead of 6.08% while running the master and the slave concurrently on seperate CPUs, much lower than existing systems that require instruction level monitoring. Furthermore, it has much better accuracy in causality inference.

October 6, 2016

Speaker: Xiao Wang

Title: High Performance Model Based Image Reconstruction

Abstract: Computed Tomography (CT) Image Reconstruction is an important technique used in a wide range of applications, ranging from explosive detection, medical imaging to scientific imaging. Among available reconstruction methods, Model Based Iterative Reconstruction (MBIR) produces higher quality images and allows for the use of more general CT scanner geometries than is possible with more commonly used methods. However, the high computational cost of MBIR often makes it impractical in applications for which it would otherwise be ideal.

This talk describes how our understanding in physical geometry, numerical algorithm and computer architecture can transform the highly irregular and intrinsically sequential tomographic application into a highly parallel one.

This talk also describes a novel organization of the scanner data into super-voxels (SV) that, combined with a super-voxel buffer (SVB), dramatically increase locality and prefetching, enable parallelism across SVs and lead to an average speedup of 187X on 20 cores.

This application is an example of algorithm that appears to be irregular, but has sufficient regularity that with careful tuning it can achieve large speedups on both a single-core and multiple cores. Currently, this tuning is almost entirely manual and expressed in OpenMP and MPI. Having programming language and compiler support to aid in the tuning of this and many other applications would be of significant benefit.

October 13, 2016

Speaker: Terry Ching-Hsiang Hsu

Title: Enforcing Least Privilege Memory Views for Multithreaded Applications

Abstract: Failing to properly isolate components in the same address space has resulted in a substantial amount of vulnerabilities. Enforcing the least privilege principle for memory accesses can selectively isolate software components to restrict attack surface and prevent unintended cross-component memory corruption. However, the boundaries and interactions between software components are hard to reason about and existing approaches have failed to stop attackers from exploiting vulnerabilities caused by poor isolation. We present the secure memory views (SMV) model: a practical and efficient model for secure and selective memory isolation in monolithic multithreaded applications. SMV is a third generation privilege separation technique that offers explicit access control of memory and allows concurrent threads within the same process to partially share or fully isolate their memory space in a controlled and parallel manner following application requirements. An evaluation of our prototype in the Linux kernel (TCB less than 1,800 LOC) shows negligible runtime performance overhead in real-world applications including Cherokee web server (less than 0.69%), Apache httpd web server (less than 0.93%), and Mozilla Firefox web browser (less than 1.89%) with at most 12 LOC changes. Link to paper:

October 20, 2016

Speaker: Yuseok Jeon

Title: TypeSan: Practical Type Confusion Detection

Abstract: Typecasting is a fundamental concept in C++ to convert a pointer from one object type into another. However, downcasting has critical security implications due to potentially different object memory layouts. Due to missing type safety in C++, a downcasted pointer can violate a programmer’s intended pointer semantics, allowing an attacker to corrupt the underlying memory in a type-unsafe fashion. This vulnerability class is receiving increasing attention and is known as type confusion. Several existing approaches detect different forms of type confusion, but these solutions are limited due to both high run-time performance overhead and low detection coverage. This paper presents TypeSan, a practical type-confusion detector which provides both low run-time overhead and high detection coverage. TypeSan relies on an efficient per-object metadata storage service based on a compact memory shadowing scheme. Our experimental results confirm that TypeSan is practical, even when explicitly checking almost all the relevant typecasts in a given C++ program. Compared to the state of the art, TypeSan yields orders of magnitude higher coverage at 4–10 times lower performance overhead on SPEC and 2 times on Firefox.

October 27, 2016

Speaker: James Decker

Title: TrumPy: A Proposal for Higher Performance Computing in Python through Ignorance

Abstract: TBA

November 3, 2016

Speaker: Roopsha Samanta

Title: Prepping for the Job Hunt

Abstract: Dr. Samanta will speak about good practices before and after graduation for entering professional academia.

November 10, 2016

Speaker: Gregory Essertel

Title: SPLASH 2016: A Synopsis

Abstract: Greg will recap the keynotes which were presented at SPLASH 2016.

November 17, 2016

Speaker: Xilun Wu

Title: EmptyHeaded: A Relational Engine for Graph Processing

Abstract: This Thursday I will introduce you to a recent research in data management. The group led by Christopher Ré invents a high-performance graph processing engine (named EmptyHeaded) offering high-level programming interface while being faster than low-level engines on graph pattern queries and competitive on other common graph benchmarks. The core of this engine is, a novel query compiler based on GHD (Generalized Hypertree Decomposition) instead of Relational Algebra, a code generator converting GHD into optimized C++ code, and the EmptyHeaded execution engine which selects appropriate the data layout and parallel intersection algorithms for generated code based on the characteristics of data such as density skew and cardinality skew. I will go into details of these three parts. The novelty of this paper comes from the emergence of a worst-case optimal join algorithm and the corresponding query plan optimization (data management), the application of code generation technique in query compilation (compiler), and the progress of parallel set intersection algorithm (parallel computing). I hope this case can bring some inspirations for our PL or DB research.

November 24, 2016

Speaker: N/A - Thanksgiving

Title: N/A

Abstract: N/A

December 1, 2016

Speaker: Nathan Burrow

Title: TBA

Abstract: TBA

December 8, 2016

Speaker: Fei Wang

Title: TBA

Abstract: TBA