Amir Kamil

Making Sequential Consistency Practical in Titanium

The memory consistency model in shared memory parallel programming controls the order in which memory operations performed by one thread may be observed by another. The most natural model for programmers is to have memory accesses appear to take effect in the order specified in the original program. Language designers have been reluctant to use this strong semantics, called sequential consistency, due to concerns over the performance of memory fence instructions and related mechanisms that guarantee order. In this paper, we provide evidence for the practicality of sequential consistency by showing that advanced compiler analysis techniques are sufficient to eliminate the need for most memory fences and enable high-level optimizations. Our analyses eliminated over 97% of the memory fences that were needed by a na¨ýve implementation, accounting for 87 to 100% of the dynamically encountered fences in all but one benchmark. The impact of the memory model and analysis on runtime performance depends on the quality of the optimizations: more aggressive optimizations are likely to be invalidated by a strong memory consistency semantics. We consider two specific optimizations pipelining of bulk memory copies and communication aggregation and scheduling for irregular accesses and show that our most aggressive analysis is able to obtain the same performance as the relaxed model when applied to two linear algebra kernels. While additional work on parallel optimizations and analyses is needed, we believe these results provide important evidence on the viability of using a simple memory consistency model without sacrificing performance.

Parallel Languages and Compilers: Perspective From the Titanium Experience

We describe the rationale behind the design of key features of Titanium—an explicitly parallel dialect of Java for high-performance scientific programming—and our experiences in building applications with the language. Specifically, we address Titaniums partitioned global address space model, single program multiple data parallelism support, multi-dimensional arrays and array-index calculus, memory management, immutable classes (class-like types that are value types rather than reference types), operator overloading, and generic programming. We provide an overview of the Titanium compiler implementation, covering various parallel analyses and optimizations, Titanium runtime technology and the GASNet network communication layer. We summarize results and lessons learned from implementing the NAS parallel benchmarks, elliptic and hyperbolic solvers using adaptive mesh refinement, and several applications of the immersed boundary method.

Titanium Language Reference Manual, Version 2.20

The Titanium language is a Java dialect for high-performance parallel scientific computing. Titanium’s differences from Java include multi-dimensional arrays, an explicitly parallel SPMD model of computation with a global address space, a form of value class, and zone-based memory management. This reference manual describes the differences between Titanium and Java.

Programming Abstractions for Data Locality

The goal of the workshop and this report is to identify common themes and standardize concepts for locality-preserving abstractions for exascale programming models. Current software tools are built on the premise that computing is the most expensive component, we are rapidly moving to an era that computing is cheap and massively parallel while data movement dominates energy and performance costs. In order to respond to exascale systems (the next generation of high performance computing systems), the scientific computing community needs to refactor their applications to align with the emerging data-centric paradigm. Our applications must be evolved to express information about data locality. Unfortunately current programming environments offer few ways to do so. They ignore the incurred cost of communication and simply rely on the hardware cache coherency to virtualize data movement. With the increasing importance of task-level parallelism on future systems, task models have to support constructs that express data locality and affinity. At the system level, communication libraries implicitly assume all the processing elements are equidistant to each other. In order to take advantage of emerging technologies, application developers need a set of programming abstractions to describe data locality for the new computing ecosystem. The new programming paradigm should be more data centric and allow to describe how to decompose and how to layout data in the memory.Fortunately, there are many emerging concepts such as constructs for tiling, data layout, array views, task and thread affinity, and topology aware communication libraries for managing data locality. There is an opportunity to identify commonalities in strategy to enable us to combine the best of these concepts to develop a comprehensive approach to expressing and managing data locality on exascale programming systems. These programming model abstractions can expose crucial information about data locality to the compiler and runtime system to enable performance-portable code. The research question is to identify the right level of abstraction, which includes techniques that range from template libraries all the way to completely new languages to achieve this goal.

Hierarchical Computation in the SPMD Programming Model

Large-scale parallel machines are programmed mainly with the single program, multiple data (SPMD) model of parallelism. While this model has advantages of scalability and simplicity, it does not fit well with divide-and-conquer parallelism or hierarchical machines that mix shared and distributed memory. In this paper, we define the recursive single program, multiple data model (RSPMD) that extends SPMD with a hierarchical team mechanism to support hierarchical algorithms and machines. We implement this model in the Titanium language and describe how to eliminate a class of deadlocks by ensuring alignment of collective operations. We present application case studies evaluating the RSPMD model, showing that it enables divide-and-conquer algorithms such as sorting to be elegantly expressed and that team collective operations increase performance of conjugate gradient by up to a factor of two. The model also facilitates optimizations for hierarchical machines, improving scalability of particle in cell by 8x and performance of sorting and a stencil code by up to 40 % and 14 %, respectively.

Enforcing textual alignment of collectives using dynamic checks

Many parallel programs are written in a single-program, multiple-data (SPMD) style, in which synchronization is provided using collective operations that all threads execute simultaneously. If these operations are not properly aligned on all threads, deadlock can occur, and many compiler analyses and optimizations that depend on proper alignment fail. In this paper, we discuss the flaws in the Titanium languages type system for enforcing textual alignment of collectives. We then present a system that uses runtime checks to ensure alignment for two definitions of textual alignment. The system instruments the code to keep track of alignment in each thread and then checks that alignment matches prior to performing a collective operation. We have implemented the system in the Titanium compiler, verifying that it catches alignment errors. We tested its performance on multiple application programs, demonstrating that the checks have no appreciable impact on execution time.

A Local-View Array Library for Partitioned Global Address Space C++ Programs

Multidimensional arrays are an important data structure in many scientific applications. Unfortunately, built-in support for such arrays is inadequate in C++, particularly in the distributed setting where bulk communication operations are required for good performance. In this paper, we present a multidimensional library for partitioned global address space (PGAS) programs, supporting the one-sided remote access and bulk operations of the PGAS model. The library is based on Titanium arrays, which have proven to provide good productivity and performance. These arrays provide a local view of data, where each rank constructs its own portion of a global data structure, matching the local view of execution common to PGAS programs and providing maximum flexibility in structuring global data. Unlike Titanium, which has its own compiler with array-specific analyses, optimizations, and code generation, we implement multidimensional arrays solely through a C++ library. The main goal of this effort is to provide a library-based implementation that can match the productivity and performance of a compiler-based approach. We implement the array library as an extension to UPC++, a C++ library for PGAS programs, and we extend Titanium arrays with specializations to improve performance. We evaluate the array library by porting four Titanium benchmarks to UPC++, demonstrating that it can achieve up to 25% better performance than Titanium without a significant increase in programmer effort.

Trends in Data Locality Abstractions for HPC Systems

The cost of data movement has always been an important concern in high performance computing (HPC) systems. It has now become the dominant factor in terms of both energy consumption and performance. Support for expression of data locality has been explored in the past, but those efforts have had only modest success in being adopted in HPC applications for various reasons. them However, with the increasing complexity of the memory hierarchy and higher parallelism in emerging HPC systems, locality management has acquired a new urgency. Developers can no longer limit themselves to low-level solutions and ignore the potential for productivity and performance portability obtained by using locality abstractions. Fortunately, the trend emerging in recent literature on the topic alleviates many of the concerns that got in the way of their adoption by application developers. Data locality abstractions are available in the forms of libraries, data structures, languages and runtime systems; a common theme is increasing productivity without sacrificing performance. This paper examines these trends and identifies commonalities that can combine various locality concepts to develop a comprehensive approach to expressing and managing data locality on future large-scale high-performance computing systems.

Hierarchical pointer analysis for distributed programs

We present a new pointer analysis for use in shared memory programs running on hierarchical parallel machines. The analysis is motivated by the partitioned global address space languages, in which programmers have control over data layout and threads and can directly read and write to memory associated with other threads. Titanium, UPC, Co-Array Fortran, X10, Chapel, and Fortress are all examples of such languages. The novelty of our analysis comes from the hierarchical machine model used, which captures the increasingly hierarchical nature of modern parallel machines. For example, the analysis can distinguish between pointers that can reference values within a thread, within a shared memory multiprocessor, or within a network of processors. The analysis is presented with a formal type system and operational semantics, articulating the various ways in which pointers can be used within a hierarchical machine model. The hierarchical analysis has several applications, including race detection, sequential consistency enforcement, and software caching. We present results of an implementation of the analysis, applying it to data race detection, and show that the hierarchical analysis is very effective at reducing the number of false races detected.

Implementing High-Performance Geometric Multigrid Solver with Naturally Grained Messages

Structured grid linear solvers often require manually packing and unpacking of communication data to achieve high performance.Orchestrating this process efficiently is challenging, labor-intensive, and potentially error-prone.In this paper, we explore an alternative approach that communicates the data with naturally grained message sizes without manual packing and unpacking. This approach is the distributed analogue of shared-memory programming, taking advantage of the global address space in PGAS languages to provide substantial programming ease. However, its performance may suffer from the large number of small messages. We investigate the runtime support required in the UPC++ library for this naturally grained version to close the performance gap between the two approaches and attain comparable performance at scale using the High-Performance Geometric Multgrid (HPGMG-FV) benchmark as a driver.