An example of deriving performance properties from a visualrepresentation of program execution

Through geometry, program visualization can yield performance properties. We derive all possible synchronization sequences and durations of blocking and concurrent execution for two process programs from a visualization mapping processes, synchronization, and program execution to Cartesian graph axes, line segments, and paths, respectively. Relationships to Petri nets are drawn     

Symbolic synthesis of masking fault-tolerant distributed programs

We focus on automated addition of masking fault-tolerance to existing fault-intolerant distributed programs. Intuitively, a program is masking fault-tolerant, if it satisfies its safety and liveness specifications in the absence and presence of faults. Masking fault-tolerance is highly desirable in distributed programs, as the structure of such programs are fairly complex and they are often subject to various types of faults. However, the problem of synthesizing masking fault-tolerant distributed programs from their fault-intolerant version is NP-complete in the size of the program’s state space, setting the practicality of the synthesis problem in doubt. In this paper, we show that in spite of the high worst-case complexity, synthesizing moderate-sized masking distributed programs is feasible in practice. In particular, we present and implement a BDD-based synthesis heuristic for adding masking fault-tolerance to existing fault-intolerant distributed programs automatically. Our experiments validate the efficiency and effectiveness of our algorithm in the sense that synthesis is possible in reasonable amount of time and memory. We also identify several bottlenecks in synthesis of distributed programs depending upon the structure of the program at hand. We conclude that unlike verification, in program synthesis, the most challenging barrier is not the state explosion problem by itself, but the time complexity of the decision procedures.     

Detecting Race Conditions in Parallel Programs that Use Semaphores

We address the problem of detecting race conditions in programs that use semaphores for synchronization. Netzer and Miller showed that it is NP-complete to detect race conditions in programs that use many semaphores. We show in this paper that it remains NP-complete even if only two semaphores are used in the parallel programs. For the tractable case, i.e., using only one semaphore, we give two algorithms for detecting race conditions from the trace of executing a parallel program on p processors, where n semaphore operations are executed. The first algorithm determines in O(n) time whether a race condition exists between any two given operations. The second algorithm runs in O( np log n) time and outputs a compact representation from which one can determine in O(1) time whether a race condition exists between any two given operations. The second algorithm is near-optimal in that the running time is only O( log n) times the time required simply to write down the output.     

Synchronization under a commercial operating system

Mutual exclusion and general synchronization of concurrent processes (‘tasks’) are well-known principles for constructing reliable real-time systems. This paper shows how to implement these principles under a typical commercial operating system which provides incomplete synchronization operations. The problem of synchronizing erroneous tasks is discussed briefly.     

A sequential cooperative game theoretic approach to scheduling multiple large-scale applications in grids

Scheduling large-scale applications in heterogeneous distributed computing systems is a fundamental NP-complete problem that is critical to obtaining good performance and execution cost. In this paper, we address the scheduling problem of an important class of large-scale Grid applications inspired by the real world, characterized by a huge number of homogeneous, concurrent, and computationally intensive tasks that are the main sources of performance, cost, and storage bottlenecks. We propose a new formulation of this problem based on a cooperative distributed game-theory-based method applied using three algorithms with low time complexity for optimizing three important metrics in scientific computing: execution time, economic cost, and storage requirements. We present comprehensive experiments using simulation and real-world applications that demonstrate the effectiveness of our approach in terms of time and fairness compared to other related algorithms.     

A practical algorithm for static analysis of parallel programs

One approach to analyzing the behavior of a concurrent program requires determining the reachable program states. A program state consists of a set of task states, the values of shared variables used for synchronization, and local variables that derive the values directly from synchronization operations. However, the number of reachable states rises exponentially with the number of tasks and becomes intractable for many concurrent programs. A variation of this approach merges a set of related states into a single virtual state. Using this approach, the analysis of concurrent programs becomes feasible as the number of virtual states is often orders of magnitude less than the number of reachable states. This paper presents a method for determining the virtual states that describe the reachable program states, and the reduction in the number of states is analyzed. The algorithms given have been implemented in a static program analyzer for multitasking Fortran, and the results obtained are discussed.     

Dynamic mapping of cooperating tasks to nodes in a distributed system

Networks of workstations (NOWs) are a low-cost and widespread platform for parallel computing. This paper focuses on the dynamic task-scheduling problem in NOW environments. The aim is to minimize the completion time of parallel programs by distributing cooperating concurrent tasks to homogeneous networked nodes. Cooperation dependencies as well as creation and termination dependencies between tasks are taken into account. An event lattice model is introduced to describe past, actual and future behavior of a parallel program in execution. Based on this model an algorithm is presented to dynamically assign tasks to the nodes of a dedicated distributed system. Crucial for the efficiency of this approach is a top-down construction of all operating system entities involved in distributed resource management, particularly the close cooperation of the compiler and runtime system, which allows the creation of event lattice clippings at runtime.     

基于抢占阈值调度的周期任务最小响应时间分析*

针对采用抢占阈值调度策略的系统,在假设任务对释放偏移没有特定要求的情况下,给出使得任务响应时间最小时应满足的条件,并根据该条件得到了任务最小响应时间的计算公式。该公式的一个重要应用是在分布式系统中分析后继子任务的最大释放抖动,有效降低端到端任务的最大响应时间的计算结果,从而有利于提高系统的可调度性。     

Detecting Race Conditions in Parallel Programs that Use Semaphores

Abstract. We address the problem of detecting race conditions in programs that use semaphores for synchronization. Netzer and Miller showed that it is NP-complete to detect race conditions in programs that use many semaphores. We show in this paper that it remains NP-complete even if only two semaphores are used in the parallel programs. For the tractable case, i.e., using only one semaphore, we give two algorithms for detecting race conditions from the trace of executing a parallel program on p processors, where n semaphore operations are executed. The first algorithm determines in O(n) time whether a race condition exists between any two given operations. The second algorithm runs in O( np log n) time and outputs a compact representation from which one can determine in O(1) time whether a race condition exists between any two given operations. The second algorithm is near-optimal in that the running time is only O( log n) times the time required simply to write down the output.     

Reproducible execution of SR programs

Reproducing the execution of a concurrent program is important in debugging and testing. It requires that, regardless of the actual order in which processes may execute, the reproduced execution is identical, with respect to the order in which certain activities occur, to a previously recorded execution. This paper presents a solution to the reproducibility problem for programs written in the SR concurrent programming language. Our solution transforms an arbitrary SR program into one for recording an event sequence and one for replaying from an event sequence. SR provides a rich collection of synchronization mechanisms, including rendezvous, asynchronous message passing, remote procedure call, and dynamic process creation. SR language features allow: flexible invocation servicing (e.g. use of invocation parameters in selecting an invocation to service in message passing or rendezvous); dynamically created processes and resource (module) instances; dynamic communication paths between processes; and dynamic distribution of programs across multiple machines. Because of these features, adaptations of previous solutions to the reproducibility problem for other languages and notations do not work for SR. Our solution handles all the above features. It results in a naturally distributed control algorithm for programs that are distributed. This paper also describes the implementations of our transformation tools. Copyright © 1999 John Wiley & Sons, Ltd.     

Complexity issues in automated synthesis of failsafe fault-tolerance

We focus on the problem of synthesizing failsafe fault-tolerance where fault-tolerance is added to an existing (fault-intolerant) program. A failsafe fault-tolerant program satisfies its specification (including safety and liveness) in the absence of faults. However, in the presence of faults, it satisfies its safety specification. We present a somewhat unexpected result that, in general, the problem of synthesizing failsafe fault-tolerant distributed programs from their fault-intolerant version is NP-complete in the state space of the program. We also identify a class of specifications, monotonic specifications, and a class of programs, monotonic programs, for which the synthesis of failsafe fault-tolerance can be done in polynomial time (in program state space). As an illustration, we show that the monotonicity restrictions are met for commonly encountered problems, such as Byzantine agreement, distributed consensus, and atomic commitment. Furthermore, we evaluate the role of these restrictions in the complexity of synthesizing failsafe fault-tolerance. Specifically, we prove that if only one of these conditions is satisfied, the synthesis of failsafe fault-tolerance is still NP-complete. Finally, we demonstrate the application of monotonicity property in enhancing the fault-tolerance of (distributed) nonmasking fault-tolerant programs to masking.     

A Threshold Scheduling Strategy for Sisal on Distributed-Memory Machines

The problem of scheduling tasks on distributed memory machines is known to be NP-complete in the strong sense, ruling out the possibility of a pseudo-polynomial algorithm. This paper introduces a new heuristic algorithm for scheduling Sisal (Streams and Iterations In a Single Assignment Language) programs on a distributed memory machine, Intel Touchstone i860. Our compile time scheduling method works on IF-2, an intermediate form based on the dataflow parallelism in the program. We initially carry out a dependence analysis, to bind the implicit dependencies across IF-2 graph boundaries, followed by a cost assignment based on Intel Touchstone i860 timings. The scheduler works in two phases. The first phase of the scheduler finds the earliest and latest completion times of each task given by the shortest and longest paths from root task to the given task respectively. A threshold defined as the difference between the latest and the earliest start times of the task, is found. The scheduler varies the value of the allowable threshold, and determines the best value for minimal schedule length. In the second phase of the scheduler, we merge the processors to generate schedule to match the available number of processors. Schedule results for several benchmark programs have been included to demonstrate the effectiveness of our approach.     

Criticality: static profiling for real-time programs

With the increasing performance demand in real-time systems it becomes more and more important to provide feedback to programmers and software development tools on the performance-relevant code parts of a real-time program. So far, this information was limited to an estimation of the worst-case execution time (WCET) and its associated worst-case execution path (WCEP) only. However, both, the WCET and the WCEP, only provide partial information. Only code parts that are on one of the WCEPs are indicated to the programmer. No information is provided for all other code parts. To give a comprehensive view covering the entire code base, tools in the spirit of program profiling are required. This work proposes an efficient approach to compute worst-case timing information for all code parts of a program using a complementary metric, called criticality. Every statement of a program is assigned a criticality value, expressing how critical the code is with respect to the global WCET. This gives valuable information how close the worst execution path passing through a specific program part is to the global WCEP. We formally define the criticality metric and investigate some of its properties with respect to dominance in control-flow graphs. Exploiting some of those properties, we propose an algorithm that reduces the overhead of computing the metric to cover complete programs. We also investigate ways to efficiently find only those code parts whose criticality is above a given threshold. Experiments using well-established real-time benchmark programs show an interesting distribution of the criticality values, revealing considerable amounts of highly critical as well as uncritical code. The metric thus provides ideal information to programmers and software development tools to optimize the worst-case execution time of these programs.     

云环境下周期和非周期混合实时任务双容错调度算法

云环境中的处理机故障已成为云计算不可忽视的问题,容错成为设计和发展云计算系统的关键需求。针对一些容错调度算法在任务调度过程中调度效率低下以及任务类型单一的问题,提出一种处理机和任务主副版本分组的容错调度方法;并给出了副版本可重叠执行的判定方法,以及任务最坏响应时间的计算公式。通过实验和分析表明,和以前算法相比,将处理机分成两组分别执行任务主版本和任务副版本,减少了任务调度所需进行可调度测试的时间,增加了副版本重叠执行的机会,减少了所需的处理机个数,对提高系统处理机的利用率和容错调度的效率具有重要的意义。     

安全协议的进程代数规约到逻辑程序的自动转换

安全协议用于实现开放互连网络的安全通讯，它本质上是分布式并发程序，使用进程代数可以将其描述为角色进程的并发合成系统。使用抽象方法，安全协议角色进程并发合成模型可以转化为逻辑程序；通过计算逻辑程序的不动点，能够对安全协议无穷会话的并发交叠运行进行验证。本文基于Objective Caml语言，实现了安全协议进程代数描述述到安全协议逻辑程序的自动转化。     

一种并发程序依赖性分析方法

并发程序的依赖性分析是并发程序分析，理解，调试，测试和维护的重要手段，由于并发程序执行的不确定性，目前，尚有很多难点有待解决，针对Ada任务机制，首先提出了一种简洁，有效的并发程序表示方法－并发程序流图，然后讨论了由任务间同步引起的同步依赖和由访问共享变量引起的任务数据依赖，建立了并发程序依赖图，并在此基础上给出了一种有效的并发程序依赖性分析算法，得到一个比较精确的依赖性，较好地解决了并发程序依赖关系不可传递性问题。     

An Integrated Path and Timing Analysis Method based on Cycle-Level Symbolic Execution

Previously published methods for estimation of the worst-case execution time on high-performance processors with complex pipelines and multi-level memory hierarchies result in overestimations owing to insufficient path and/or timing analysis. This does not only give rise to poor utilization of processing resources but also reduces the schedulability in real-time systems. This paper presents a method that integrates path and timing analysis to accurately predict the worst-case execution time for real-time programs on high-performance processors. The unique feature of the method is that it extends cycle-level architectural simulation techniques to enable symbolic execution with unknown input data values; it uses alternative instruction semantics to handle unknown operands. We show that the method can exclude many infeasible (or non-executable) program paths and can calculate path information, such as bounds on number of loop iterations, without the need for manual annotations of programs. Moreover, the method is shown to accurately analyze timing properties of complex features in high-performance processors using multiple-issue pipelines and instruction and data caches. The combined path and timing analysis capability is shown to derive exact estimates of the worst-case execution time for six out of seven programs in our benchmark suite.     

一种快速程序最坏执行时间分析方法研究

给出一种带有路径冲突检测的程序最坏情况执行时间估计方法,这种方法首先检测程序中存在的分支约束,然后将程序中存在的分支约束信息转化为程序流程控制图（CFG图）中结点之间的语义冲突,并按照结点对的形式保存在相应的冲突数组里,在接下来的WCET计算阶段通过边搜索程序执行路径边检测冲突数组里保存的已有的冲突关系以便在搜索路径的同时排除非可行执行路径,最终在可行执行路径集中选择具有最大执行时间的执行路径。与以往的方法相比,在保持估计精度的前提下,本文的方法避免了穷举所有执行路径带来的复杂度,提高了搜索的效率。实验结果表明本文方法对于语句间语义依赖关系比较强的实时程序能够快速且有效地给出估计结果。     

Task assignment using a problem-space genetic algorithm

The task assignment problem is one of assigning tasks of a parallel program among the processors of a distributed computing system in order to reduce the job turnaround time and to increase the throughput of the system. Since the task assignment problem is known to be NP-complete except in a few special situations, satisfactory suboptimal solutions obtainable in a reasonable amount of computation time are generally sought. In the paper we introduce a technique based on the problem-space genetic algorithm (PSGA) for the static task assignment problem in both homogeneous and heterogeneous distributed computing systems to reduce the task turnaround time and to increase the throughput of the system by properly balancing the load and reducing the interprocessor communication time among processors. The PSGA based approach combines the power of genetic algorithms, a global search method, with a simple and fast problem-specific heuristic to search a large solution space efficiently and effectively to find the best possible solution in an acceptable CPU time. Experimental results on test examples from the literature show considerable improvements in both the assignment cost and the CPU times over the previous work. The proposed scheme is also applied to a digital signal processing (DSP) system consisting of 119 tasks to illustrate its balancing properties and computational advantage on a large system. The proposed scheme offers 12–30% improvement in the assignment cost as compared to the previous best known results for the DSP example.     

一种基于文件传输路径的分布式程序实时性评估方法

首先根据分布式系统实时性评估的基本思想，结合分布式程序执行的特点，对分布式程序的实时性评估进行理论分析．然后．根据分布式程序在系统中所处的位置、程序执行时所需的数据文件和数据文件的传输路径，确定每个数据文件的最小传输时间和最大传输时间．在此基础上，计算出程序的最早响应时间和最迟响应时间，从而形成一种基于文件传输路径的分布式程序实时性评估方法、通过实例评估和模拟分析，说明了这种评估方法是有效性的．