Week 5


This week we are going to take a look at OpenCL (Open Computing Language)


Machines are connected into clusters through protocols such as MPI allowing large tasks to be spread across multiple machines; CPUs now have multiple cores able to run several threads simultaneously, and GPUs can now run hundreds and thousands of lightweight threads simultaneously.

Right now each of these levels are programmed independently and there are various projects looking at trying to integrate them together where the compiler and the code can adapt to the execution environment.

OpenCL is one example that has been gaining support from various vendors to integrate work on the CPU and the GPU so its easier for software to take advantage of any available GPUs. It may not be the eventual "winner" but something similar will be a more common way of programming GPUs for general work in the future.

There is a short overview from AMD here:

and one from nvidia giving the relationship to CUDA

and if you prefer watching a video instead:

This is a very general (and somewhat long) introduction:

and here is another series of short videos from AMD which we are going to go through in class today

The next step is getting the CPUs and the GPUs more physically integrated - eg Intel's Sandy Bridge,  AMDs HSA (Heterogeneous Systems Architecture) which will allow both kinds of processors to access the same memory eliminating the major slowdown in moving data to and from the GPU

andy galaxy example code based off of the nvidia 'OpenCL Simple OpenGL Interop' code available at http://developer.nvidia.com/opencl-sdk-code-samples

here is another version of the C code that will run on either the CPU or the GPU (depending on how you set CL_DEVICE_TYPE_CPU or CL_DEVICE_TYPE_GPU) but with less checking for an optimum device. On my macbook pro w/ an nVidia 330M I get 13ms per frame, while the 2.66 Ghz Intel Core I7 takes 50ms per frame


here is another set of example code from apple: https://developer.apple.com/library/mac/navigation/#section=Frameworks&topic=OpenCL

and here is a version of the galaxy code compiled with xcode that should be happy with an AMD or nVidia GPU

Here is a Hello World example from apple

// File:       hello.c
// Abstract:   A simple "Hello World" compute example showing basic usage of OpenCL which
//             calculates the mathematical square (X[i] = pow(X[i],2)) for a buffer of
//             floating point values.
// Version:    <1.0>
// Copyright ( C ) 2008 Apple Inc. All Rights Reserved.
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <math.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <OpenCL/opencl.h>
// Use a static data size for simplicity
#define DATA_SIZE (1024)
// Simple compute kernel which computes the square of an input array

const char *KernelSource = "\n" \
"__kernel void square(                                                       \n" \
"   __global float* input,                                              \n" \
"   __global float* output,                                             \n" \
"   const unsigned int count)                                           \n" \
"{                                                                      \n" \
"   int i = get_global_id(0);                                           \n" \
"   if(i < count)                                                       \n" \
"       output[i] = input[i] * input[i];                                \n" \
"}                                                                      \n" \
int main(int argc, char** argv)
    int err;                            // error code returned from api calls
    float data[DATA_SIZE];              // original data set given to device
    float results[DATA_SIZE];           // results returned from device
    unsigned int correct;               // number of correct results returned
    size_t global;                      // global domain size for our calculation
    size_t local;                       // local domain size for our calculation
    cl_device_id device_id;             // compute device id
    cl_context context;                 // compute context
    cl_command_queue commands;          // compute command queue
    cl_program program;                 // compute program
    cl_kernel kernel;                   // compute kernel
    cl_mem input;                       // device memory used for the input array
    cl_mem output;                      // device memory used for the output array
    // Fill our data set with random float values

    int i = 0;
    unsigned int count = DATA_SIZE;
    for(i = 0; i < count; i++)
        data[i] = rand() / (float)RAND_MAX;
    // Connect to a compute device
    // cl_int clGetDeviceIDs(    cl_platform_id     platform,
    //                           cl_device_type     device_type,
    //                           cl_uint            num_entries,
    //                           cl_device_id *     devices,
    //                           cl_uint *          num_devices)
    // cl_device_type                 Description
    // CL_DEVICE_TYPE_CPU             - An OpenCL device that is the host processor. The host processor runs the
    //                                  OpenCL implementations and is a single or multi-core CPU.
    // CL_DEVICE_TYPE_GPU             - An OpenCL device that is a GPU. By this we mean that the device can also
    //                                  be used to accelerate a 3D API such as OpenGL or DirectX.
    // CL_DEVICE_TYPE_ACCELERATOR     - Dedicated OpenCL accelerators (for example the IBM CELL Blade). These devices
    //                                  communicate with the host processor using a peripheral interconnect such as PCIe.
    // CL_DEVICE_TYPE_DEFAULT         - The default OpenCL device in the system.
    // CL_DEVICE_TYPE_ALL             - All OpenCL devices available in the system.

    int gpu = 1;
    err = clGetDeviceIDs(NULL, gpu ? CL_DEVICE_TYPE_GPU : CL_DEVICE_TYPE_CPU, 1, &device_id, NULL);
    if (err != CL_SUCCESS)
        printf("Error: Failed to create a device group!\n");
        return EXIT_FAILURE;
    // Create a compute context
    // cl_context clCreateContext(    const cl_context_properties *    properties,
    //                                cl_uint                          num_devices,
    //                                const cl_device_id *             devices,
    //                                (void CL_CALLBACK  *   pfn_notify) (
    //                                    const char *       errinfo,
    //                                    const void *       private_info, size_t cb,
    //                                    void *             user_data
    //                                 ),
    //                                void *                           user_data,
    //                                cl_int *                         errcode_ret)

    context = clCreateContext(0, 1, &device_id, NULL, NULL, &err);
    if (!context)
        printf("Error: Failed to create a compute context!\n");
        return EXIT_FAILURE;
    // Create a command commands
    // cl_command_queue clCreateCommandQueue(  cl_context                     context,
    //                                         cl_device_id                   device,
    //                                         cl_command_queue_properties    properties,
    //                                         cl_int *                       errcode_ret)

    commands = clCreateCommandQueue(context, device_id, 0, &err);
    if (!commands)
        printf("Error: Failed to create a command commands!\n");
        return EXIT_FAILURE;
    // Create the compute program from the source buffer

    program = clCreateProgramWithSource(context, 1, (const char **) & KernelSource, NULL, &err);
    if (!program)
        printf("Error: Failed to create compute program!\n");
        return EXIT_FAILURE;
    // Build the program executable
    // cl_int clBuildProgram (    cl_program              program,
    //                            cl_uint                 num_devices,
    //                            const cl_device_id *    device_list,
    //                            const char *            options,
    //                            void (CL_CALLBACK *pfn_notify)(cl_program program, void *user_data),
    //                            void *                  user_data)

    err = clBuildProgram(program, 0, NULL, NULL, NULL, NULL);
    if (err != CL_SUCCESS)
        size_t len;
        char buffer[2048];
        printf("Error: Failed to build program executable!\n");
        clGetProgramBuildInfo(program, device_id, CL_PROGRAM_BUILD_LOG, sizeof(buffer), buffer, &len);
        printf("%s\n", buffer);
    // Create the compute kernel in the program we wish to run
    // cl_kernel clCreateKernel (    cl_program      program,
    //                               const char *    kernel_name,
    //                               cl_int *        errcode_ret)

    kernel = clCreateKernel(program, "square", &err);
    if (!kernel || err != CL_SUCCESS)
        printf("Error: Failed to create compute kernel!\n");
    // Create the input and output arrays in device memory for our calculation
    // cl_mem clCreateBuffer (     cl_context     context,
    //                             cl_mem_flags   flags,
    //                             size_t         size,
    //                             void *         host_ptr,
    //                             cl_int *       errcode_ret)

    input = clCreateBuffer(context,  CL_MEM_READ_ONLY,  sizeof(float) * count, NULL, NULL);
    output = clCreateBuffer(context, CL_MEM_WRITE_ONLY, sizeof(float) * count, NULL, NULL);
    if (!input || !output)
        printf("Error: Failed to allocate device memory!\n");
    // Write our data set into the input array in device memory
    // cl_int clEnqueueWriteBuffer (   cl_command_queue     command_queue,
    //                                 cl_mem               buffer,
    //                                 cl_bool              blocking_write,
    //                                 size_t               offset,
    //                                 size_t               cb,
    //                                 const void *         ptr,
    //                                 cl_uint              num_events_in_wait_list,
    //                                 const cl_event *     event_wait_list,
    //                                 cl_event *           event)

    err = clEnqueueWriteBuffer(commands, input, CL_TRUE, 0, sizeof(float) * count, data, 0, NULL, NULL);
    if (err != CL_SUCCESS)
        printf("Error: Failed to write to source array!\n");
    // Set the arguments to our compute kernel
    // cl_int clSetKernelArg (     cl_kernel       kernel,
    //                             cl_uint         arg_index,
    //                             size_t          arg_size,
    //                             const void *    arg_value)

    err = 0;
    err  = clSetKernelArg(kernel, 0, sizeof(cl_mem), &input);
    err |= clSetKernelArg(kernel, 1, sizeof(cl_mem), &output);
    err |= clSetKernelArg(kernel, 2, sizeof(unsigned int), &count);
    if (err != CL_SUCCESS)
        printf("Error: Failed to set kernel arguments! %d\n", err);
    // Get the maximum work group size for executing the kernel on the device
    // cl_int clGetKernelWorkGroupInfo (   cl_kernel                   kernel,
    //                                     cl_device_id                device,
    //                                     cl_kernel_work_group_info   param_name,
    //                                     size_t                      param_value_size,
    //                                     void *                      param_value,
    //                                     size_t *                    param_value_size_ret)

    err = clGetKernelWorkGroupInfo(kernel, device_id, CL_KERNEL_WORK_GROUP_SIZE, sizeof(local), &local, NULL);
    if (err != CL_SUCCESS)
        printf("Error: Failed to retrieve kernel work group info! %d\n", err);
    // Execute the kernel over the entire range of our 1d input data set
    // using the maximum number of work group items for this device
    // cl_int clEnqueueNDRangeKernel (     cl_command_queue      command_queue,
    //                                     cl_kernel             kernel,
    //                                     cl_uint               work_dim,
    //                                     const size_t *        global_work_offset,
    //                                     const size_t *        global_work_size,
    //                                     const size_t *        local_work_size,
    //                                     cl_uint               num_events_in_wait_list,
    //                                     const cl_event *      event_wait_list,
    //                                     cl_event *            event)

    global = count;
    err = clEnqueueNDRangeKernel(commands, kernel, 1, NULL, &global, &local, 0, NULL, NULL);
    if (err)
        printf("Error: Failed to execute kernel!\n");
        return EXIT_FAILURE;
    // Wait for the command commands to get serviced before reading back results
    // cl_int clFinish (    cl_command_queue command_queue)

    // Read back the results from the device to verify the output
    // cl_int clEnqueueReadBuffer (    cl_command_queue    command_queue,
    //                                 cl_mem              buffer,
    //                                 cl_bool             blocking_read,
    //                                 size_t              offset,
    //                                 size_t              cb,
    //                                 void *              ptr,
    //                                 cl_uint             num_events_in_wait_list,
    //                                 const cl_event *    event_wait_list,
    //                                 cl_event *          event)

    err = clEnqueueReadBuffer( commands, output, CL_TRUE, 0, sizeof(float) * count, results, 0, NULL, NULL ); 
    if (err != CL_SUCCESS)
        printf("Error: Failed to read output array! %d\n", err);
    // Validate our results

    correct = 0;
    for(i = 0; i < count; i++)
        if(results[i] == data[i] * data[i])
    // Print a brief summary detailing the results

    printf("Computed '%d/%d' correct values!\n", correct, count);
    // Shutdown and cleanup

    return 0;


Coming Next Time


last revision 2/21/12 - added in xcode version that should run on amd or nVidia GPUs on the mac