Compilers and More: A GPU and Accelerator Programming Model

时间:2009-02-26   来源:The Portland Group, Inc   网友评论:0   人气: 196 作者: Michael Wolfe Compiler Engine

Okay, maybe the title should be "Languages and More," but I promise to talk about compilers further on below.

In recent columns, I've discussed parallel programming and programming GPUs and accelerators in particular. In May, I predicted that accelerator-based systems would dominate high performance computing, and suggested that an evolutionary approach to migrating applications from CPUs to accelerators was possible and appropriate. In September, I discussed in more detail the specifics of GPU hardware architecture, pointing out its strengths for high performance computing (lots of parallelism), as well as its weaknesses (limited to specific parallelism domains). In October, I showed what it takes to start porting a CPU program to a GPU, exposing some of the complexities of the interactions between the host and the GPU. The specific examples use NVIDIA's very popular CUDA language, but I discuss OpenCL briefly as well (which should be about ready for public discussion about the time you read this). In my most recent column, I showed the details of optimizing a simple matmul kernel for a GPU, including testing various ways to organize it and vary the parallelism parameters.

If you read these, or are familiar with current approaches to programming accelerators, you are either discomforted by the complexities, or excited at the levels of control you can get. The low-level programming model in CUDA and OpenCL certainly has its place, though it's not for the faint of heart. So, to go back to the first of those columns, can we come up with a different model of GPU and accelerator programming, one that retains most of the advantages of CUDA or OpenCL, but without requiring complete program rewrites, that can be applied to different target accelerators, and that retains the potential to develop and test in a more accessible environment? In short, a model that allows HPC programmers to focus on domain science instead of on computer science?

Architectural Model

Let's start by looking at the features of the architecture that we want to use to advantage. Current GPUs are specific implementations of a programming model that works well for graphics problems. They support two levels of parallelism: an outer fully-parallel doall loop level, and an inner synchronous (SIMD or vector) loop level. Each level can be multidimensional (2 or 3 dimensions), but the domain must be strictly rectangular. The synchronous level may not be fully implemented with SIMD or vector operations, so explicit synchronization is supported (and required) across this level. No synchronization is supported between parallel threads across the doall level.

For those familiar with memory models, current GPUs implement a particularly weak model. In particular, they don't support memory coherence between threads, unless those threads are parallel only at the synchronous level and the memory operations are separated by an explicit barrier. Otherwise, if one thread updates a memory location and another reads the same location, or two threads store a value to the same location, the hardware does not guarantee the results. You can't say it gets the wrong answers, because such programs are defined as being in error. There is a software-managed cache on a GPU, and there are some hardware caches that can be used as well, but only in certain situations (and limited to read-only data).