Locks are used to ensure exclusive access to shared memory locations. Unfortunately, lock operations are expensive, so much work has been done on optimizing their performance for common access patterns. One such pattern is found in networking applications, where there is a single thread dominating lock accesses. An important special case arises when a single-threaded program calls a thread-safe library that uses locks.An effective way to optimize the dominant-thread pattern is to "bias" the lock implementation so that accesses by the dominant thread have negligible overhead. We take this approach in this work: we simplify and generalize existing techniques for biased locks, producing a large design space with many trade-offs. For example, if we assume the dominant process acquires the lock infinitely often (a reasonable assumption for packet processing), it is possible to make the dominant process perform a lock operation without expensive fence or compare-and-swap instructions. This gives a very low overhead solution; we confirm its efficacy by experiments. We show how these constructions can be extended for lock reservation, re-reservation, and to reader-writer situations.
Abstract. Clocks are a mechanism for providing synchronization barriers in concurrent programming languages. They are usually implemented using primitive communication mechanisms and thus spare the programmer from reasoning about low-level implementation details such as remote procedure calls and error conditions. Clocks provide flexibility, but programs often use them in specific ways that do not require their full implementation. In this paper, we describe a tool that mitigates the overhead of general-purpose clocks by statically analyzing how programs use them and choosing optimized implementations when available. We tackle the clock implementation in the standard library of the X10 programming language-a parallel, distributed object-oriented language. We report our findings for a small set of analyses and benchmarks. Our tool only adds a few seconds to analysis time, making it practical to use as part of a compilation chain.
Concurrent programming languages are growing in importance with the advent of multi-core systems. However, concurrent programs suffer from problems, such as data races and deadlock, absent from sequential programs. Unfortunately, traditional race and deadlock detection techniques fail on both large programs and small programs with complex behaviors.In this paper, we present a compositional deadlock detection technique for a concurrent language-SHIM-in which tasks run asynchronously and communicate using synchronous CSP-style rendezvous. Although SHIM guarantees the absence of data races, a SHIM program may still deadlock if the communication protocol is violated. Our previous work used NuSMV, a symbolic model checker, to detect deadlock in a SHIM program, but it did not scale well with the size of the problem. In this work, we take an incremental, divide-and-conquer approach to deadlock detection.In practice, we find our procedure is faster and uses less memory than the existing technique, especially on large programs, making our algorithm a practical part of the compilation chain.
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.
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