FHPC 2012 Programme, 15 September 2012
9:00-10:30: Compilation technology
Welcome and Workshop Opening
Andrzej Filinski (University of Copenhagen, Denmark); Clemens Grelck (University of Amsterdam, Netherlands)
Keynote: Using Domain-Specific Languages and Access-Execute Descriptors to Expand the Parallel Code Synthesis Design Space
Paul H J Kelly (Imperial College, UK)
This talk is about the following idea: can we simultaneously raise the level at which programmers can reason about code, and also provide the compiler with a model of the computation that enables it to generate faster code than you could reasonably write by hand? We have been working with three large computational fluid dynamics frameworks, and I will present some of our experience in building compiler tools at various levels of abstraction. Our primary goal is to build tools that automatically synthesise the best possible implementation. By getting the abstraction right, we can capture design choices far beyond what a conventional compiler can do.
I will illustrate this with examples involving low-level parallel code generation (eg for GPUs), high-level cross-cutting almost-algorithmic choices (such as whether to actually build a global sparse system matrix), and semantic properties (enabling massive common subexpression elimination in finite-element assembly). What is the right code to generate, for a given hardware platform? How does this change as problem parameters change? The key, we believe, is to start with the right representation of the problem, and to build tools that can automate the combination of code generation alternatives.
Bo Joel Svensson, Mary Sheeran (Chalmers University of Technology, Sweden)
Nowadays, performance in processors is increased by adding more cores or wider vector units, or by combining accelerators like GPUs and traditional cores on a chip. Programming for these diverse architectures is a challenge. We would like to exploit all the resources at hand without putting too much burden on the programmer. Ideally, the programmer should be presented with a machine model abstracted from the specific number of cores, SIMD width or the existence of a GPU or not. Intel’s Array Building Blocks (ArBB) is a system that takes on these challenges. ArBB is a language for data parallel and nested data parallel programming, embedded in C++. By offering a retargetable dynamic compilation framework, it provides vectorisation and threading to programmers without the need to write highly architecture specific code.
We aim to bring the same benefits to the Haskell programmer by implementing a Haskell frontend (embedding) of the ArBB system. We call this embedding EmbArBB. We use standard Haskell embedded language procedures to provide an interface to the ArBB functionality in Haskell. EmbArBB is work in progress and does not currently support all of the ArBB functionality. Some small programming examples illustrate how the Haskell embedding is used to write programs. ArBB code is short and to the point in both C++ and Haskell. Matrix multiplication has been benchmarked in sequential C++, ArBB in C++, EmbArBB and the Repa library. The C++ and the Haskell embeddings have almost identical performance, showing that the Haskell embedding does not impose any large extra overheads. Two image processing algorithms have also been benchmarked against Repa. In these benchmarks at least, EmbArBB performance is much better than that of the Repa library, indicating that building on ArBB may be a cheap and easy approach to exploiting data parallelism in Haskell.
10:30-11:00: Morning Break
11:00-12:30: Dataflow Programming
Jeremiah J. Willcock, Ryan R. Newton, Andrew Lumsdaine (Indiana University, USA)
Flow graph models have recently become increasingly popular as a way to express parallel computations. However, most of these models either require specialized languages and compilers or are library-based solutions requiring coarse-grained applications to achieve acceptable performance. Yet, graph algorithms and other irregular applications are increasingly important to modern high-performance computing, and these applications are not amenable to coarsening without complicating algorithm structure. One effective existing approach for these applications relies on active messages. However, the separation of control flow between the main program and active message handlers introduces programming difficulties.
To ameliorate this problem, we present Avalanche, a flow graph model for fine-grained applications that automatically generates active-message handlers. Avalanche is built as a C++ library on top of our previously-developed Active Pebbles model; a set of combinators builds graphs at compile-time, allowing several optimizations to be applied by the library and a standard C++ compiler. In particular, consecutive flow graph nodes can be fused; experimental results show that flow graphs built from small components can still efficiently operate on fine-grained data.
Jocelyn Sérot (Blaise Pascal University, France); Greg Michaelson (Heriot-Watt University, UK)
We propose to use Hume, a general purpose, functionally inspired, programming language, initially oriented to resource-aware embedded applications, to implement fine-grain parallel applications on FPGAs. We show that the Hume description of programs as a set of asynchronous boxes connected by wires has a very natural interpretation in terms of register-transfer level hardware description, hence leading to efficient implementations on FPGAs. The paper describes the basic compilation process from a subset of Hume to synthetisable RTL VHDL and show preliminary experimental results obtained with a very simple perceptron application.
Stanislav Böhm, Marek Běhálek (VŠB–Technical University of Ostrava, Czech Republic)
Petri nets are a well established graphical and mathematical modelling language for a description of concurrent systems. The main scope of this paper is to present our approach how to use Petri nets for high-performance computing. They are rarely used in this area. As a proof of concept, we are developing a tool Kaira. The modelling language in the tool is based on our extension of Coloured Petri Nets. The basic concept is to use a visual language to model parallel behaviour and communication. Sequential parts of a program are written in C/C++. In contrast to other Petri Nets based tools, Kaira is not intended only for modelling and simulation, but it can also generate standalone parallel applications from models. Generated applications use MPI and threads. This paper also presents new Kaira’s features including modules for computations on structured objects, more controllable semantics of mapping to MPI processes and a support for the hybrid computing
12:30-14:00: Lunch Break
14:00-15:30: Performance vs. Abstraction
Prabhat Totoo, Pantazis Deligiannis, Hans-Wolfgang Loidl (Heriot-Watt University, UK)
This paper provides a performance and programmability comparison of high-level parallel programming support in Haskell, F# and Scala. Developing several parallel versions, we employ skeleton-based, semi-explicit and explicit approaches to parallelism. We focus on advanced language features for separating computational and coordination aspects of the code and tuning performance. We also assess the impact of functional purity and multi-paradigm design of the languages on program development and performance.
Basis for these comparisons are several Barnes-Hut implementations of the n-body problem in all three languages, on both Linux and Windows. Our performance measurements on state-of-the-art multi-cores achieve a speedup up to 5.62 (on 8 cores) with a highly-tuned Haskell version. For comparable implementations in Scala and F# we achieve speedups of 4.51 (on 8 cores) and 2.28 (on 4 cores), respectively. We observe that near best speedups are achieved using the highest level abstraction in these languages.
Cosmin E. Oancea, Christian Andreetta, Jost Berthold, Fritz Henglein (University of Copenhagen, Denmark); Alain Frisch (LexiFi, France)
This paper presents a real-world pricing kernel for financial derivatives and evaluates the language and compiler tool chain that would allow expressive, hardware-neutral algorithm implementation and efficient execution on graphics-processing units (GPU). The language issues refer to preserving algorithmic invariants, e.g., inherent parallelism made explicit by map-reduce-scan functional combinators. Efficient execution is achieved by manually applying a series of generally-applicable compiler transformations that allows the generated-OpenCL code to yield speedups as high as 70x and 540x on a commodity mobile and desktop GPU, respectively.
Apart from the concrete speed-ups attained, our contributions are twofold: First, from a language perspective, we illustrate that even state-of-the-art auto-parallelization techniques are incapable of discovering all the requisite data parallelism when rendering the functional code in Fortran-style imperative array processing form. Second, from a performance perspective, we study which compiler transformations are necessary to map the high-level functional code to hand-optimized OpenCL code for GPU execution. We discover a rich optimization space with nontrivial trade-offs and cost models. Memory reuse in map-reduce patterns, strength reduction, branch divergence optimization, and memory access coalescing, exhibit significant impact individually. When combined, they enable essentially full utilization of all GPU cores.
Functional programming has played a crucial double role in our case study: Capturing the naturally data-parallel structure of the pricing algorithm in a transparent, reusable and entirely hardware-independent fashion; and supporting the correctness of the subsequent compiler transformations to a hardware-oriented target language by a rich class of universally valid equational properties. Given the observed difficulty of automatically parallelizing imperative sequential code and the inherent labor of porting hardware-oriented and -optimized programs, our case study suggests that functional programming technology can facilitate high-level expression of leading-edge performant portable high-performance systems for massively parallel hardware architectures.
James Swaine, Burke Fetscher, Robert Bruce Findler (Northwestern University, USA); Vincent St-Amour (Northeastern University, USA); Matthew Flatt (University of Utah, USA)
This paper presents the latest chapter in our adventures coping with a large, sequentially-tuned, legacy runtime system in today’s parallel world. Specifically, this paper introduces our new graphical visualizer that helps programmers understand how to program in parallel with Racket’s futures and, to some extent, what performs well in sequential Racket.
Overall, our experience with parallelism in Racket is that we can achieve reasonable parallel performance in Racket without sacri- ficing the most important property of functional programming lan- guage implementations, namely safety. That is, Racket programmers are guaranteed that every Racket primitive (and thus all functions built using Racket primitives) will either behave properly, or it will signal an error explaining what went wrong.
That said, however, it is challenging to understand how to best use futures to achieve interesting speedups, and the visualizer is our attempt to more widely disseminate key performance details of the runtime system in order to help Racket programmers maximize performance.
15:30-16:00: Afternoon Break
16:00-17:00: Domain-Specific Middleware
Luca Toscano, Gabriele D’Angelo, Moreno Marzolla (University of Bologna, Italy)
Discrete Event Simulation (DES) is a widely used technique in which the state of the simulator is updated by events happening at discrete points in time (hence the name). DES is used to model and analyze many kinds of systems, including computer architectures, communication networks, street traffic, and others. Parallel and Distributed Simulation (PADS) aims at improving the efficiency of DES by partitioning the simulation model across multiple processing elements, in order to enable larger and/or more detailed studies to be carried out. The interest on PADS is increasing since the widespread availability of multicore processors and affordable high performance computing clusters. However, designing parallel simulation models requires considerable expertise, the result being that PADS techniques are not as widespread as they could be.
In this paper we describe ErlangTW, a parallel simulation middleware based on the Time Warp synchronization protocol. ErlangTW is entirely written in Erlang, a concurrent, functional programming language specifically targeted at building distributed systems. We argue that writing parallel simulation models in Erlang is considerably easier than using conventional programming languages. Moreover, ErlangTW allows simulation models to be executed either on single-core, multicore and distributed computing architectures. We describe the design and prototype implementation of ErlangTW, and report some preliminary performance results on multicore and distributed architectures using the well known PHOLD benchmark.</p>
Michael Flænø Werk, Joakim Ahnfelt-Rønne, Ken Friis Larsen (University of Copenhagen, Denmark)
We present a domain specific language embedded in Haskell for specifying stochastic processes, called SPL. It is designed with the goal of matching the notation used in mathematical finance, where the price of a financial contract is specified using stochastic processes and distributions.
SPL is declarative in the sense that it is agnostic of the choice of discretization and of the computational model. We provide an implementation of SPL that performs Monte Carlo simulation using GPGPU, and we present data indicating that this gives a 100x speedup compared to hand-written sequential C, and that the speedup scales linearly with the number of available cores.