OpenCL学习笔记(三):OpenCL安装,编程简介与helloworld
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OpenCL安装
安装我不打算花篇幅写,原因是OpenCL实在是可以太多的平台+环境下实现了,包括GPU和FPGA,以及不同的器件支持,在这里我主要把网上可以找到比较不错的经验贴列一下,方便大家,我主要关注了FPGA的,其他GPU的大家网上搜搜吧:
altera opencl sdk下载:
https://www.altera.com.cn/products/design-software/embedded-software-developers/opencl/overview.html
alter的安装指南,《Altera SDK for OpenCL
Getting Started Guide》
理论上看上面两个就够了,你需要做的事情包括:
下载opencl SDK,或者quatuars II软件(含SDK),下载相应开发板的支持(altera上面有一些,但是其他的可能就需要你从相应的供应商那边找了);还需要opencl的license,不然是不能编译的。
中文的一些经验贴可以看:
《Altera OpenCL入门(beta版)》http://wenku.baidu.com/link?url=bkIyo01jXeWfdGsrA_M0J1zomx6f0lYk0NPf-9-MNaC0OkWRmukDwY5yFz0I3Wrctqi5qD3jC8BhQQzjoqw1HXpUgIM68_blz5Cr3vxpaZC
【Altera SoC体验之旅】+ 正式开启OpenCL模式
http://home.eeworld.com.cn/my/space-uid-169743-blogid-247647.html
OpenCL编程简介
下面的图简单说明了OpenCL的编程框架,图是用的GPU,其他类似;
从图中可以看出(参考《OpenCL 编程入门》):
1. 异构计算设备,可以是CPU或GPU。现在也有支持OpenCL的FPGA设备和至强融核协处理设备(MIC)。
2. OpenCL的API通过Context(环境上下文)联系在一起。
3. 运行设备端的程序,经过了编译->设置参数->运行等步骤。
名词的概念:
Platform (平台):主机加上OpenCL框架管理下的若干设备构成了这个平台,通过这个平台,应用程序可以与设备共享资源并在设备上执行kernel。实际使用中基本上一个厂商对应一个Platform,比如Intel, AMD都是这样。
Device(设备):官方的解释是计算单元(Compute Units)的集合。举例来说,GPU是典型的device。Intel和AMD的多核CPU也提供OpenCL接口,所以也可以作为Device。
Context(上下文):OpenCL的Platform上共享和使用资源的环境,包括kernel、device、memory objects、command queue等。使用中一般一个Platform对应一个Context。
Program:OpenCL程序,由kernel函数、其他函数和声明等组成。
Kernel(核函数):可以从主机端调用,运行在设备端的函数。
Memory Object(内存对象):在主机和设备之间传递数据的对象,一般映射到OpenCL程序中的global memory。有两种具体的类型:Buffer Object(缓存对象)和Image Object(图像对象)。
Command Queue(指令队列):在指定设备上管理多个指令(Command)。队列里指令执行可以顺序也可以乱序。一个设备可以对应多个指令队列。
NDRange:主机端运行设备端kernel函数的主要接口。实际上还有其他的,NDRange是非常常见的,用于分组运算,以后具体用到的时候就知道区别了。
Host端来看,OpenCL的组要执行流程是这样的:
其实基本上大部分简单的程序HOST部分都是差不多的,不用改很多,具体下面看一个例子就知道了。
第一个程序
这里贴一个altera官方的vector add的实例code,基本就是helloworld级别了,不过它的host写的很通用(考虑到对多个device统一编程),可以过一遍看看是不是和上面的图对的上。其实看过这个基本其他的也就差不多了。
Host部分:(Kernel在最后)
// Copyright (C) 2013-2014 Altera Corporation, San Jose, California, USA. All rights reserved.
// Permission is hereby granted, free of charge, to any person obtaining a copy of this
// software and associated documentation files (the "Software"), to deal in the Software
// without restriction, including without limitation the rights to use, copy, modify, merge,
// publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to
// whom the Software is furnished to do so, subject to the following conditions:
// The above copyright notice and this permission notice shall be included in all copies or
// substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
// EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES
// OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
// NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT
// HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
// WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
// FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
// OTHER DEALINGS IN THE SOFTWARE.
//
// This agreement shall be governed in all respects by the laws of the State of California and
// by the laws of the United States of America.
///////////////////////////////////////////////////////////////////////////////////
// This host program executes a vector addition kernel to perform:
// C = A + B
// where A, B and C are vectors with N elements.
//
// This host program supports partitioning the problem across multiple OpenCL
// devices if available. If there are M available devices, the problem is
// divided so that each device operates on N/M points. The host program
// assumes that all devices are of the same type (that is, the same binary can
// be used), but the code can be generalized to support different device types
// easily.
//
// Verification is performed against the same computation on the host CPU.
///////////////////////////////////////////////////////////////////////////////////
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include "CL/opencl.h"
#include "AOCL_Utils.h"
using namespace aocl_utils;
// OpenCL runtime configuration
cl_platform_id platform = NULL;
unsigned num_devices = 0;
scoped_array<cl_device_id> device; // num_devices elements
cl_context context = NULL;
scoped_array<cl_command_queue> queue; // num_devices elements
cl_program program = NULL;
scoped_array<cl_kernel> kernel; // num_devices elements
scoped_array<cl_mem> input_a_buf; // num_devices elements
scoped_array<cl_mem> input_b_buf; // num_devices elements
scoped_array<cl_mem> output_buf; // num_devices elements
// Problem data.
const unsigned N = 1000000; // problem size
scoped_array<scoped_aligned_ptr<float> > input_a, input_b; // num_devices elements
scoped_array<scoped_aligned_ptr<float> > output; // num_devices elements
scoped_array<scoped_array<float> > ref_output; // num_devices elements
scoped_array<unsigned> n_per_device; // num_devices elements
// Function prototypes
float rand_float();
bool init_opencl();
void init_problem();
void run();
void cleanup();
// Entry point.
int main() {
// Initialize OpenCL.
if(!init_opencl()) {
return -1;
}
// Initialize the problem data.
// Requires the number of devices to be known.
init_problem();
// Run the kernel.
run();
// Free the resources allocated
cleanup();
return 0;
}
/////// HELPER FUNCTIONS ///////
// Randomly generate a floating-point number between -10 and 10.
float rand_float() {
return float(rand()) / float(RAND_MAX) * 20.0f - 10.0f;
}
// Initializes the OpenCL objects.
bool init_opencl() {
cl_int status;
printf("Initializing OpenCL\n");
if(!setCwdToExeDir()) {
return false;
}
// Get the OpenCL platform.
platform = findPlatform("Altera");
if(platform == NULL) {
printf("ERROR: Unable to find Altera OpenCL platform.\n");
return false;
}
// Query the available OpenCL device.
device.reset(getDevices(platform, CL_DEVICE_TYPE_ALL, &num_devices));
printf("Platform: %s\n", getPlatformName(platform).c_str());
printf("Using %d device(s)\n", num_devices);
for(unsigned i = 0; i < num_devices; ++i) {
printf(" %s\n", getDeviceName(device[i]).c_str());
}
// Create the context.
context = clCreateContext(NULL, num_devices, device, NULL, NULL, &status);
checkError(status, "Failed to create context");
// Create the program for all device. Use the first device as the
// representative device (assuming all device are of the same type).
std::string binary_file = getBoardBinaryFile("vectorAdd", device[0]);
printf("Using AOCX: %s\n", binary_file.c_str());
program = createProgramFromBinary(context, binary_file.c_str(), device, num_devices);
// Build the program that was just created.
status = clBuildProgram(program, 0, NULL, "", NULL, NULL);
checkError(status, "Failed to build program");
// Create per-device objects.
queue.reset(num_devices);
kernel.reset(num_devices);
n_per_device.reset(num_devices);
input_a_buf.reset(num_devices);
input_b_buf.reset(num_devices);
output_buf.reset(num_devices);
for(unsigned i = 0; i < num_devices; ++i) {
// Command queue.
queue[i] = clCreateCommandQueue(context, device[i], CL_QUEUE_PROFILING_ENABLE, &status);
checkError(status, "Failed to create command queue");
// Kernel.
const char *kernel_name = "vectorAdd";
kernel[i] = clCreateKernel(program, kernel_name, &status);
checkError(status, "Failed to create kernel");
// Determine the number of elements processed by this device.
n_per_device[i] = N / num_devices; // number of elements handled by this device
// Spread out the remainder of the elements over the first
// N % num_devices.
if(i < (N % num_devices)) {
n_per_device[i]++;
}
// Input buffers.
input_a_buf[i] = clCreateBuffer(context, CL_MEM_READ_ONLY,
n_per_device[i] * sizeof(float), NULL, &status);
checkError(status, "Failed to create buffer for input A");
input_b_buf[i] = clCreateBuffer(context, CL_MEM_READ_ONLY,
n_per_device[i] * sizeof(float), NULL, &status);
checkError(status, "Failed to create buffer for input B");
// Output buffer.
output_buf[i] = clCreateBuffer(context, CL_MEM_WRITE_ONLY,
n_per_device[i] * sizeof(float), NULL, &status);
checkError(status, "Failed to create buffer for output");
}
return true;
}
// Initialize the data for the problem. Requires num_devices to be known.
void init_problem() {
if(num_devices == 0) {
checkError(-1, "No devices");
}
input_a.reset(num_devices);
input_b.reset(num_devices);
output.reset(num_devices);
ref_output.reset(num_devices);
// Generate input vectors A and B and the reference output consisting
// of a total of N elements.
// We create separate arrays for each device so that each device has an
// aligned buffer.
for(unsigned i = 0; i < num_devices; ++i) {
input_a[i].reset(n_per_device[i]);
input_b[i].reset(n_per_device[i]);
output[i].reset(n_per_device[i]);
ref_output[i].reset(n_per_device[i]);
for(unsigned j = 0; j < n_per_device[i]; ++j) {
input_a[i][j] = rand_float();
input_b[i][j] = rand_float();
ref_output[i][j] = input_a[i][j] + input_b[i][j];
}
}
}
void run() {
cl_int status;
const double start_time = getCurrentTimestamp();
// Launch the problem for each device.
scoped_array<cl_event> kernel_event(num_devices);
scoped_array<cl_event> finish_event(num_devices);
for(unsigned i = 0; i < num_devices; ++i) {
// Transfer inputs to each device. Each of the host buffers supplied to
// clEnqueueWriteBuffer here is already aligned to ensure that DMA is used
// for the host-to-device transfer.
cl_event write_event[2];
status = clEnqueueWriteBuffer(queue[i], input_a_buf[i], CL_FALSE,
0, n_per_device[i] * sizeof(float), input_a[i], 0, NULL, &write_event[0]);
checkError(status, "Failed to transfer input A");
status = clEnqueueWriteBuffer(queue[i], input_b_buf[i], CL_FALSE,
0, n_per_device[i] * sizeof(float), input_b[i], 0, NULL, &write_event[1]);
checkError(status, "Failed to transfer input B");
// Set kernel arguments.
unsigned argi = 0;
status = clSetKernelArg(kernel[i], argi++, sizeof(cl_mem), &input_a_buf[i]);
checkError(status, "Failed to set argument %d", argi - 1);
status = clSetKernelArg(kernel[i], argi++, sizeof(cl_mem), &input_b_buf[i]);
checkError(status, "Failed to set argument %d", argi - 1);
status = clSetKernelArg(kernel[i], argi++, sizeof(cl_mem), &output_buf[i]);
checkError(status, "Failed to set argument %d", argi - 1);
// Enqueue kernel.
// Use a global work size corresponding to the number of elements to add
// for this device.
//
// We don't specify a local work size and let the runtime choose
// (it'll choose to use one work-group with the same size as the global
// work-size).
//
// Events are used to ensure that the kernel is not launched until
// the writes to the input buffers have completed.
const size_t global_work_size = n_per_device[i];
printf("Launching for device %d (%d elements)\n", i, global_work_size);
status = clEnqueueNDRangeKernel(queue[i], kernel[i], 1, NULL,
&global_work_size, NULL, 2, write_event, &kernel_event[i]);
checkError(status, "Failed to launch kernel");
// Read the result. This the final operation.
status = clEnqueueReadBuffer(queue[i], output_buf[i], CL_FALSE,
0, n_per_device[i] * sizeof(float), output[i], 1, &kernel_event[i], &finish_event[i]);
// Release local events.
clReleaseEvent(write_event[0]);
clReleaseEvent(write_event[1]);
}
// Wait for all devices to finish.
clWaitForEvents(num_devices, finish_event);
const double end_time = getCurrentTimestamp();
// Wall-clock time taken.
printf("\nTime: %0.3f ms\n", (end_time - start_time) * 1e3);
// Get kernel times using the OpenCL event profiling API.
for(unsigned i = 0; i < num_devices; ++i) {
cl_ulong time_ns = getStartEndTime(kernel_event[i]);
printf("Kernel time (device %d): %0.3f ms\n", i, double(time_ns) * 1e-6);
}
// Release all events.
for(unsigned i = 0; i < num_devices; ++i) {
clReleaseEvent(kernel_event[i]);
clReleaseEvent(finish_event[i]);
}
// Verify results.
bool pass = true;
for(unsigned i = 0; i < num_devices && pass; ++i) {
for(unsigned j = 0; j < n_per_device[i] && pass; ++j) {
if(fabsf(output[i][j] - ref_output[i][j]) > 1.0e-5f) {
printf("Failed verification @ device %d, index %d\nOutput: %f\nReference: %f\n",
i, j, output[i][j], ref_output[i][j]);
pass = false;
}
}
}
printf("\nVerification: %s\n", pass ? "PASS" : "FAIL");
}
// Free the resources allocated during initialization
void cleanup() {
for(unsigned i = 0; i < num_devices; ++i) {
if(kernel && kernel[i]) {
clReleaseKernel(kernel[i]);
}
if(queue && queue[i]) {
clReleaseCommandQueue(queue[i]);
}
if(input_a_buf && input_a_buf[i]) {
clReleaseMemObject(input_a_buf[i]);
}
if(input_b_buf && input_b_buf[i]) {
clReleaseMemObject(input_b_buf[i]);
}
if(output_buf && output_buf[i]) {
clReleaseMemObject(output_buf[i]);
}
}
if(program) {
clReleaseProgram(program);
}
if(context) {
clReleaseContext(context);
}
}
Kernel部分:
// ACL kernel for adding two input vectors
__kernel void vectorAdd(__global const float *x,
__global const float *y,
__global float *restrict z)
{
// get index of the work item
int index = get_global_id(0);
// add the vector elements
z[index] = x[index] + y[index];
}
kernel部分代码就这几行,__global是一个限定符,表示用外部存储(比如DDR)来存储,其他语法和标准C语言是一样的,就不多说了。
代码中最重要的就是get_global_id,这个是在多work-item工作模式下的常用手段,通过id确定work-item然后进行操作,所有的item都是一样的,因此就add的函数里面就没有习惯的for()的写法了。可以对kernel的设置进行定制,包括compute unit,SIMD模式等,这样来控制程序的并行性,更大的并行往往性能高,但是更耗资源。
具体的Kernel函数的内容可以参考OpenCL的《The OpenCL Specification 1.0》以及altera的opencl编程指南,后面的笔记我会具体写一下。
内存模型
最后写一下Opencl的内存模型,看下面的示意图:
用核函数中的内存变量来简单地解释:用clCreateBuffer 创建、用clSetKernelArg 传递的数据在global memory 和constant memory中;核函数中的寄存器变量在private memory 中;核函数的内部变量、缓存等,在local memory 中。图例中可以看到Device 并不直接访问global memory,而是通过Cache 来访问。可以想象当同时运行的work-item,使用的内存都在同一块cache 中,则内存吞吐的效率最高。对应到work group 中,就是在程序设计上尽量使同一个work group 中的work item 操作连续的内存,以提高访存效率。
本篇就到这里。
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