A Determinizing Compiler

The advent of multicores mandates parallel programming. While parallelism presents a panoply of problems, few are as pernicious and prevalent as nondeterminism, in which the output of a program is affected by more than just its inputs, e.g., uncontrollable scheduling choices made by the operating system. A few parallel languages do guarantee determinism, but do so through draconian restrictions. It is time for a new era of bug-free parallel programming that will enable programmers to shift easily from sequential to parallel worlds.We propose a determinizing compiler: starting from a non-deterministic program, our compiler inserts just enough additional synchronization to guarantee deterministic behavior, even in the presence of nondeterministic scheduling choices. A brute-force solution would simply generate sequential code, but our compiler will strive to preserve parallelism to impose a minimal loss of performance

Determinism Should Ensure Deadlock-Freedom

The advent of multicore processors has made concurrent programming models mandatory. However, most concurrent programming models come with two major pitfalls: non-determinism and deadlocks. By determinism, we mean the output behavior of the program is independent of the scheduling choices (e.g., the operating system) and depends only on the input behavior. A few concurrent models provide deterministic behavior by providing constructs that impose additional synchronization, but improper or out-of-order use of these constructs leads to problems like deadlocks. In this paper, we argue for both determinism and deadlock-freedom. We propose a design of a deterministic, deadlock-free model. Any program that uses this model is guaranteed to produce the same output for a given input. Additionally, the program will never deadlock: the program will either terminate or run forever

Buffer Sharing in CSP-like Programs

Most compilers focus on optimizing performance, often at the expense of memory, but efficient memory use can be just as important in constrained environments such as embedded systems. In this paper, we present a memory reduction technique for the deterministic concurrent programming language SHIM. We focus on reducing memory consumption by sharing buffers among the tasks, which use them to communicate using CSP-style rendezvous. We determine pairs of buffers that can never be in use simultaneously and use a shared region of memory for each pair. Our technique produces a static abstraction of a SHIM program’s dynamic behavior, which we then analyze to find buffers that can share memory. Experimentally, we find our technique runs quickly on modest-sized programs and often reduces memory requirements by half

Static Deadlock Detection for the SHIM Concurrent Language

Concurrent programming languages are becoming mandatory with the advent of multi-core processors. Two major concerns in any concurrent program are data races and deadlocks. Each are potentially subtle bugs that can be caused by non-deterministic scheduling choices in most concurrent formalisms. As an alternative, the SHIM concurrent language guarantees the absence of data races by eschewing shared memory, but a SHIM program may still deadlock if a program violates a communication protocol. We present a model-checking-based static deadlock detection technique for the SHIM language. Although SHIM is asynchronous, its semantics allow us to model it synchronously without losing precision, greatly reducing the state space that must be explored. This plus the obvious division between control and data in SHIM programs makes it easy to construct concise abstractions. Experimentally, we find our procedure runs in only a few seconds for modest-sized programs, making it practical to use as part of a compilation chain

Buffer Sharing in Rendezvous Programs

Most compilers focus on optimizing performance, often at the expense of memory, but efficient memory use can be just as important in constrained environments such as embedded systems. This paper presents a memory reduction technique for rendezvous communication, which is applied to the deterministic concurrent programming language SHIM. It focuses on reducing memory consumption by sharing communication buffers among tasks. It determines pairs of buffers that can never be in use simultaneously and use a shared region of memory for each pair. The technique produces a static abstraction of a SHIM program's dynamic behavior, which is then analyzed to find buffers that are never occupied simultaneously. Experiments show the technique runs quickly on modest-sized programs and can sometimes reduce memory requirements by half

Ensuring Deterministic Concurrency through Compilation

Multicore shared-memory architectures are becoming prevalent but bring many programming challenges. Among the biggest is non-determinism: the output of the program does not depend merely on the input, but also on scheduling choices taken by the operating system. In this paper, we discuss and propose additional tools that provide determinism guarantees-compilers that generate deterministic code, libraries that provide deterministic constructs, and analyzers that check for determinism. Additionally, we discuss techniques to check for problems like deadlock that can result from the use of these deterministic constructs

A JPEG Decoder in SHIM

Image compression plays an important role in multimedia systems, digital systems, handheld systems and various other devices. Efficient image processing techniques are needed to make images suitable for use in embedded systems. This paper describes an implementation of a JPEG decoder in the SHIM programming language. SHIM is a software/hardware integration language whose aim is to provide communication between hardware and software while providing deterministic concurrency. The paper shows that a JPEG decoder is a good application and reasonable test case for the SHIM language and illustrates the ease with which conventional sequential decoders can be modified to achieve concurrency

Placing Conditional Disclosure of Secrets in the Communication Complexity Universe

In the conditional disclosure of secrets (CDS) problem (Gertner et al., J. Comput. Syst. Sci., 2000) Alice and Bob, who hold n-bit inputs x and y respectively, wish to release a common secret z to Carol (who knows both x and y) if and only if the input (x,y) satisfies some predefined predicate f. Alice and Bob are allowed to send a single message to Carol which may depend on their inputs and some shared randomness, and the goal is to minimize the communication complexity while providing information-theoretic security. Despite the growing interest in this model, very few lower-bounds are known. In this paper, we relate the CDS complexity of a predicate f to its communication complexity under various communication games. For several basic predicates our results yield tight, or almost tight, lower-bounds of Omega(n) or Omega(n^{1-epsilon}), providing an exponential improvement over previous logarithmic lower-bounds. We also define new communication complexity classes that correspond to different variants of the CDS model and study the relations between them and their complements. Notably, we show that allowing for imperfect correctness can significantly reduce communication - a seemingly new phenomenon in the context of information-theoretic cryptography. Finally, our results show that proving explicit super-logarithmic lower-bounds for imperfect CDS protocols is a necessary step towards proving explicit lower-bounds against the class AM, or even AM cap coAM - a well known open problem in the theory of communication complexity. Thus imperfect CDS forms a new minimal class which is placed just beyond the boundaries of the "civilized" part of the communication complexity world for which explicit lower-bounds are known

Programming Shared Memory Multiprocessors with Deterministic Message-Passing Concurrency: Compiling SHIM to Pthreads

Multicore shared-memory architectures are becoming prevalent and bring many programming challenges. Among the biggest are data races: accesses to shared resources that make a program's behavior depend on scheduling decisions beyond its control. To eliminate such races, the SHIM concurrent programming language adopts deterministic message passing as it sole communication mechanism. We demonstrate such language restrictions are practical by presenting a SHIM to C-plus-Pthreads compiler that can produce efficient code for shared-memory multiprocessors. We present a parallel JPEG decoder and FFT exhibiting 3.05 and 3.3times speedups on a four-core processor

A Deterministic Multi-Way Rendezvous Library for Haskell

The advent of multicore processors requires mainstream concurrent programming languages with high level concurrency constructs and effective debugging techniques. Unfortunately, many concurrent programming languages are non-deterministic and allow data races. We present a deterministic concurrent communication library for an existing multi-threaded language. We implemented the SHIM communication model in the Haskell functional language, which supports asynchronous communication and transactional memory. The SHIM model uses multi-way rendezvous to guarantee determinism. We describe two implementations of the model in Haskell, demonstrating the ease of writing such a library. We illustrate our library with examples and experimentally compare two implementations. We also compare our new model with equivalent sequential programs and parallel versions using Haskell's existing concurrency mechanisms