A comparison of software and hardware synchronization mechanisms for distributed shared memory multiprocessors

technical reportEfficient synchronization is an essential component of parallel computing. The designers of traditional multiprocessors have included hardware support only for simple operations such as compare-and-swap and load-linked/store-conditional, while high level synchronization primitives such as locks, barriers, and condition variables have been implemented in software [9,14,15]. With the advent of directory-based distributed shared memory (DSM) multiprocessors with significant flexibility in their cache controllers [7,12,17], it is worthwhile considering whether this flexibility should be used to support higher level synchronization primitives in hardware. In particular, as part of maintaining data consistency, these architectures maintain lists of processors with a copy of a given cache line, which is most of the hardware needed to implement distributed locks. We studied two software and four hardware implementations of locks and found that hardware implementation can reduce lock acquire and release times by 25-94% compared to well tuned software locks. In terms of macrobenchmark performance, hardware locks reduce application running times by up to 75% on a synthetic benchmark with heavy lock contention and by 3%-6% on a suite of SPLASH-2 benchmarks. In addition, emerging cache coherence protocols promise to increase the time spent synchronizing relative to the time spent accessing shared data, and our study shows that hardware locks can reduce SPLASH-2 execution times by up to 10-13% if the time spent accessing shared data is small. Although the overall performance impact of hardware lock mechanisms varies tremendously depending on the application, the added hardware complexity on a flexible architecture like FLASH [12] or Avalanche [7] is negligible, and thus hardware support for high level synchronization operations should be provided

Using consistent subcuts for detecting stable properties

We present a general protocol for detecting whether a property holds in a distributed system, where the property is a member of a subclass of stable properties we call the locally stable properties. Our protocol is based on a decentralized method for constructing a maximal subset of the local states that are mutually consistent, which in turn is based on a weakened version of vectored time stamps. The structure of our protocol lends itself to refinement, and we demonstrate its utility by deriving some specialized property-detection protocols, including two previously known protocols that are known to be effective

Optimal and Error-Free Multi-Valued Byzantine Consensus Through Parallel Execution

Multi-valued Byzantine Consensus (BC), in which $n$ processes must reach agreement on a single $L$-bit value, is an essential primitive in the design of distributed cryptographic protocols and fault-tolerant distributed systems. One of the most desirable traits for a multi-valued BC protocol is to be error-free. In other words, have zero probability of producing incorrect results. The most efficient error-free multi-valued BC protocols are built as extension protocols, which reduce agreement on large values to agreement on small sequences of bits whose lengths are independent of $L$. The best extension protocols achieve $\mathcal{O}(Ln)$ communication complexity, which is optimal, when $L$ is large relative to $n$. Unfortunately, all known error-free and communication-optimal BC extension protocols require each process to broadcast at least $n$ bits with a binary Byzantine Broadcast (BB) protocol. This design limits the scalability of these protocols to many processes, since when $n$ is large, the binary broadcasts significantly inflate the overall number of bits communicated by the extension protocol. In this paper, we present Byzantine Consensus with Parallel Execution (BCPE), the first error-free and communication-optimal BC extension protocol in which each process only broadcasts a single bit with a binary BB protocol. BCPE is a synchronous and deterministic protocol, and tolerates $f < n/3$ faulty processes (the best resilience possible). Our evaluation shows that BCPE\u27s design makes it significantly more scalable than the best existing protocol by Ganesh and Patra. For 1,000 processes to agree on 2 MB of data, BCPE communicates $10.92\times$ fewer bits. For agreement on 10 MB of data, BCPE communicates $6.97\times$ fewer bits. BCPE also matches the best existing protocol in all other standard efficiency metrics

Multiagent cooperation for solving global optimization problems: an extendible framework with example cooperation strategies

This paper proposes the use of multiagent cooperation for solving global optimization problems through the introduction of a new multiagent environment, MANGO. The strength of the environment lays in itsflexible structure based on communicating software agents that attempt to solve a problem cooperatively. This structure allows the execution of a wide range of global optimization algorithms described as a set of interacting operations. At one extreme, MANGO welcomes an individual non-cooperating agent, which is basically the traditional way of solving a global optimization problem. At the other extreme, autonomous agents existing in the environment cooperate as they see fit during run time. We explain the development and communication tools provided in the environment as well as examples of agent realizations and cooperation scenarios. We also show how the multiagent structure is more effective than having a single nonlinear optimization algorithm with randomly selected initial points

A Concurrency Control Method Based on Commitment Ordering in Mobile Databases

Disconnection of mobile clients from server, in an unclear time and for an unknown duration, due to mobility of mobile clients, is the most important challenges for concurrency control in mobile database with client-server model. Applying pessimistic common classic methods of concurrency control (like 2pl) in mobile database leads to long duration blocking and increasing waiting time of transactions. Because of high rate of aborting transactions, optimistic methods aren`t appropriate in mobile database. In this article, OPCOT concurrency control algorithm is introduced based on optimistic concurrency control method. Reducing communications between mobile client and server, decreasing blocking rate and deadlock of transactions, and increasing concurrency degree are the most important motivation of using optimistic method as the basis method of OPCOT algorithm. To reduce abortion rate of transactions, in execution time of transactions` operators a timestamp is assigned to them. In other to checking commitment ordering property of scheduler, the assigned timestamp is used in server on time of commitment. In this article, serializability of OPCOT algorithm scheduler has been proved by using serializability graph. Results of evaluating simulation show that OPCOT algorithm decreases abortion rate and waiting time of transactions in compare to 2pl and optimistic algorithms.Comment: 15 pages, 13 figures, Journal: International Journal of Database Management Systems (IJDMS

Timed Runtime Monitoring for Multiparty Conversations

We propose a dynamic verification framework for protocols in real-time distributed systems. The framework is based on Scribble, a tool-chain for design and verification of choreographies based on multiparty session types, developed with our industrial partners. Drawing from recent work on multiparty session types for real-time interactions, we extend Scribble with clocks, resets, and clock predicates constraining the times in which interactions should occur. We present a timed API for Python to program distributed implementations of Scribble specifications. A dynamic verification framework ensures the safe execution of applications written with our timed API: we have implemented dedicated runtime monitors that check that each interaction occurs at a correct timing with respect to the corresponding Scribble specification. The performance of our implementation and its practicability are analysed via benchmarking

Detection of global state predicates

The problem addressed here arises in the context of Meta: how can a set of processes monitor the state of a distributed application in a consistent manner? For example, consider the simple distributed application as shown here. Each of the three processes in the application has a light, and the control processes would each like to take an action when some specified subset of the lights are on. The application processes are instrumented with stubs that determine when the process turns its lights on or off. This information is disseminated to the control processes, each of which then determines when its condition of interest is met. Meta is built on top of the ISIS toolkit, and so we first built the sensor dissemination mechanism using atomic broadcast. Atomic broadcast guarantees that all recipients receive the messages in the same order and that this order is consistent with causality. Unfortunately, the control processes are somewhat limited in what they can deduce when they find that their condition of interest holds