Tuning Parallel Applications in Parallel

Auto-tuning has recently received significant attention from the High Performance Computing community. Most auto-tuning approaches are specialized to work either on specific domains such as dense linear algebra and stencil computations, or only at certain stages of program execution such as compile time and runtime. Real scientific applications, however, demand a cohesive environment that can efficiently provide auto-tuning solutions at all stages of application development and deployment. Towards that end, we describe a unified end-to-end approach to auto-tuning scientific applications. Our system, Active Harmony, takes a search-based collaborative approach to auto-tuning. Application programmers, library writers and compilers collaborate to describe and export a set of performance related tunable parameters to the Active Harmony system. These parameters define a tuning search-space. The auto-tuner monitors the program performance and suggests adaptation decisions. The decisions are made by a central controller using a parallel search algorithm. The algorithm leverages parallel architectures to search across a set of optimization parameter values. Different nodes of a parallel system evaluate different configurations at each timestep. Active Harmony supports runtime adaptive code-generation and tuning for parameters that require new code (e.g. unroll factors). Effectively, we merge traditional feedback directed optimization and just-in-time compilation. This feature also enables application developers to write applications once and have the auto-tuner adjust the application behavior automatically when run on new systems. We evaluated our system on multiple large-scale parallel applications and showed that our system can improve the execution time by up to 46% compared to the original version of the program. Finally, we believe that the success of any auto-tuning research depends on how effectively application developers, domain-experts and auto-tuners communicate and work together. To that end, we have developed and released a simple and extensible language that standardizes the parameter space representation. Using this language, developers and researchers can collaborate to export tunable parameters to the tuning frameworks. Relationships (e.g. ordering, dependencies, constraints, ranking) between tunable parameters and search-hints can also be expressed

Low-Impact Profiling of Streaming, Heterogeneous Applications

Computer engineers are continually faced with the task of translating improvements in fabrication process technology: i.e., Moore\u27s Law) into architectures that allow computer scientists to accelerate application performance. As feature-size continues to shrink, architects of commodity processors are designing increasingly more cores on a chip. While additional cores can operate independently with some tasks: e.g. the OS and user tasks), many applications see little to no improvement from adding more processor cores alone. For many applications, heterogeneous systems offer a path toward higher performance. Significant performance and power gains have been realized by combining specialized processors: e.g., Field-Programmable Gate Arrays, Graphics Processing Units) with general purpose multi-core processors. Heterogeneous applications need to be programmed differently than traditional software. One approach, stream processing, fits these systems particularly well because of the segmented memories and explicit expression of parallelism. Unfortunately, debugging and performance tools that support streaming, heterogeneous applications do not exist. This dissertation presents TimeTrial, a performance measurement system that enables performance optimization of streaming applications by profiling the application deployed on a heterogeneous system. TimeTrial performs low-impact measurements by dedicating computing resources to monitoring and by aggressively compressing performance traces into statistical summaries guided by user specification of the performance queries of interest

Doctor of Philosophy

dissertationA modern software system is a composition of parts that are themselves highly complex: operating systems, middleware, libraries, servers, and so on. In principle, compositionality of interfaces means that we can understand any given module independently of the internal workings of other parts. In practice, however, abstractions are leaky, and with every generation, modern software systems grow in complexity. Traditional ways of understanding failures, explaining anomalous executions, and analyzing performance are reaching their limits in the face of emergent behavior, unrepeatability, cross-component execution, software aging, and adversarial changes to the system at run time. Deterministic systems analysis has a potential to change the way we analyze and debug software systems. Recorded once, the execution of the system becomes an independent artifact, which can be analyzed offline. The availability of the complete system state, the guaranteed behavior of re-execution, and the absence of limitations on the run-time complexity of analysis collectively enable the deep, iterative, and automatic exploration of the dynamic properties of the system. This work creates a foundation for making deterministic replay a ubiquitous system analysis tool. It defines design and engineering principles for building fast and practical replay machines capable of capturing complete execution of the entire operating system with an overhead of several percents, on a realistic workload, and with minimal installation costs. To enable an intuitive interface of constructing replay analysis tools, this work implements a powerful virtual machine introspection layer that enables an analysis algorithm to be programmed against the state of the recorded system through familiar terms of source-level variable and type names. To support performance analysis, the replay engine provides a faithful performance model of the original execution during replay

Analytical techniques for debugging pervasive computing environments

Thesis (M. Eng.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2004.Includes bibliographical references (p. 63-65).User level debugging of pervasive environments is important as it provides the ability to observe changes that occur in a pervasive environment and fix problems that result from these changes, especially since pervasive environments may from time to time exhibit unexpected behavior. Simple keepalive messages can not always uncover the source of this behavior because systems can be in an incorrect state while continuing to output information or respond to basic queries. The traditional approach to debugging distributed systems is to instrument the entire environment. This does not work when the environments are cobbled together from systems built around different operating systems, programming languages or platforms. With systems from such disparate backgrounds, it is hard to create a stable pervasive environment. We propose to solve this problem by requiring each system and component to provide a health metric that gives an indication of its current status. Our work has shown that, when monitored at a reasonable rate, simple and cheap metrics can reveal the cause of many problems within pervasive environments. The two metrics that will be focused on in this thesis are transmission rate and transmission data analysis. Algorithms for implementing these metrics, within the stated assumptions of pervasive environments, will be explored along with an analysis of these implementations and the results they provided. Furthermore, a system design will be described in which the tools used to analyze the metrics compose an out of bound monitoring system that retains a level of autonomy from the pervasive environment. The described system provides many advantages and additionally operates under the given assumptions regarding the resources available(cont.) within a pervasive environment.by Atish Nigam.M.Eng

Spectral analysis of executions of computer programs and its applications on performance analysis

This work is motivated by the growing intricacy of high performance computing infrastructures. For example, supercomputer MareNostrum (installed in 2005 at BSC) has 10240 processors and currently there are machines with more than 100.000 processors. The complexity of this systems increases the complexity of the manual performance analysis of parallel applications. For this reason, it is mandatory to use automatic tools and methodologies.The performance analysis group of BSC and UPC has a large experience in analyzing parallel applications. The approach of this group consists mainly in the analysis of tracefiles (obtained from parallel applications executions) using performance analysis and visualization tools, such as Paraver. Taking into account the general characteristics of the current systems, this method can sometimes be very expensive in terms of time and inefficient. To overcome these problems, this thesis makes several contributions.The first one is an automatic system able to detect the internal structure of executions of high performance computing applications. This automatic system is able to rule out nonsignificant regions of executions, to detect redundancies and, finally, to select small but significant execution regions. This automatic detection process is based on spectral analysis (wavelet transform, fourier transform, etc..) and works detecting the most important frequencies of the application's execution. These main frequencies are strongly related to the internal loops of the application' source code. Finally, it is important to state that an automatic detection of small but significant execution regions reduces remarkably the complexity of the performance analysis process.The second contribution is an automatic methodology able to show general but nontrivial performance trends. They can be very useful for the analyst in order to carry out a performance analysis of the application. The automatic methodology is based on an analytical model. This model consists in several performance factors. Such factors modify the value of the linear speedup in order to fit the real speedup. That is, if this real speedup is far from the linear one, we will detect immediately which one of the performance factors is undermining the scalability of the application. The second main characteristic of the analytical model is that it can be used to predict the performance of high performance computing applications. From several execution on a few of processors, we extract model's performance factors and we extrapolate these values to executions on higher number of processors. Finally, we obtain a speedup prediction using the analytical model.The third contribution is the automatic detection of the optimal sampling frequency of applications. We show that it is possible to extract this frequency using spectral analysis. In case of sequential applications, we show that to use this frequency improves existing results of recognized techniques focused on the reduction of serial application's instruction execution stream (SimPoint, Smarts, etc..). In case of parallel benchmarks, we show that the optimal frequency is very useful to extract significant performance information very efficiently and accurately.In summary, this thesis proposes a set of techniques based on signal processing. The main focus of these techniques is to perform an automatic analysis of the applications, reporting and initial diagnostic of their performance and showing their internal iterative structure. Finally, these methods also provide a reduced tracefile from which it is easy to start manual finegrain performance analysis. The contributions of the thesis are not reduced to proposals and publications. The research carried out these last years has provided a tool for analyzing applications' structure. Even more, the methodology is general and it can be adapted to many performance analysis methods, improving remarkably their efficiency, flexibility and generality

Scalability Engineering for Parallel Programs Using Empirical Performance Models

Performance engineering is a fundamental task in high-performance computing (HPC). By definition, HPC applications should strive for maximum performance. As HPC systems grow larger and more complex, the scalability of an application has become of primary concern. Scalability is the ability of an application to show satisfactory performance even when the number of processors or the problems size is increased. Although various analysis techniques for scalability were suggested in past, engineering applications for extreme-scale systems still occurs ad hoc. The challenge is to provide techniques that explicitly target scalability throughout the whole development cycle, thereby allowing developers to uncover bottlenecks earlier in the development process. In this work, we develop a number of fundamental approaches in which we use empirical performance models to gain insights into the code behavior at higher scales. In the first contribution, we propose a new software engineering approach for extreme-scale systems. Specifically, we develop a framework that validates asymptotic scalability expectations of programs against their actual behavior. The most important applications of this method, which is especially well suited for libraries encapsulating well-studied algorithms, include initial validation, regression testing, and benchmarking to compare implementation and platform alternatives. We supply a tool-chain that automates large parts of the framework, thus allowing it to be continuously applied throughout the development cycle with very little effort. We evaluate the framework with MPI collective operations, a data-mining code, and various OpenMP constructs. In addition to revealing unexpected scalability bottlenecks, the results also show that it is a viable approach for systematic validation of performance expectations. As the second contribution, we show how the isoefficiency function of a task-based program can be determined empirically and used in practice to control the efficiency. Isoefficiency, a concept borrowed from theoretical algorithm analysis, binds efficiency, core count, and the input size in one analytical expression, thereby allowing the latter two to be adjusted according to given (realistic) efficiency objectives. Moreover, we analyze resource contention by modeling the efficiency of contention-free execution. This allows poor scaling to be attributed either to excessive resource contention overhead or structural conflicts related to task dependencies or scheduling. Our results, obtained with applications from two benchmark suites, demonstrate that our approach provides insights into fundamental scalability limitations or excessive resource overhead and can help answer critical co-design questions. Our contributions for better scalability engineering can be used not only in the traditional software development cycle, but also in other, related fields, such as algorithm engineering. It is a field that uses the software engineering cycle to produce algorithms that can be utilized in applications more easily. Using our contributions, algorithm engineers can make informed design decisions, get better insights, and save experimentation time