程序代写代做代考 graph GPU algorithm kernel compiler C c/c++ html cuda Introduction to OpenCL

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Introduction to OpenCL
Cliff Woolley, NVIDIA Developer Technology Group

Welcome to the OpenCL Tutorial!
OpenCL Platform Model
OpenCL Execution Model
Mapping the Execution Model onto the Platform Model Introduction to OpenCL Programming
Additional Information and Resources
OpenCL is a trademark of Apple, Inc.

Design Goals of OpenCL
 Use all computational resources in the system — CPUs, GPUs and other processors as peers
 Efficient parallel programming model — Based on C99
— Data- and task- parallel computational model
— Abstract the specifics of underlying hardware
— Specify accuracy of floating-point computations
Desktop and Handheld Profiles
© Copyright Khronos Group, 2010

OPENCL PLATFORM MODEL

It’s a Heterogeneous World
 A modern platform includes:
– One or more CPUs
– One or more GPUs
– Optional accelerators (e.g., DSPs)
CPU
CPU
GPU
GMCH ICH
DRAM
GMCH = graphics memory control hub ICH = Input/output control hub
© Copyright Khronos Group, 2010

OpenCL Platform Model
Computational Resources
Host
… …
…
… …
… …
…
… …
… …
Compute Device
Processing Element
Compute Unit

OpenCL Platform Model
Computational Resources
Host
… …
…
… …
… …
…
… …
… …
Compute Device
Processing Element
Compute Unit

OpenCL Platform Model
on CUDA Compute Architecture
CPU
CUDA Streaming Processor
Host
… …
…
… …
… …
…
… …
… …
Compute Device
Processing Element
Compute Unit
CUDA-Enabled GPU
CUDA Streaming Multiprocessor

Anatomy of an OpenCL Application
OpenCL Application
Host Code Device Code
• Written in C/C++ • Written in OpenCL C
• Executes on the host • Executes on the device
Host code sends commands to the Devices:
… to transfer data between host memory and device memories … to execute device code
Host
Compute Devices
… …
…
… …
… …
…
… …
… …

Anatomy of an OpenCL Application
 Serial code executes in a Host (CPU) thread
 Parallel code executes in many Device (GPU) threads
across multiple processing elements
OpenCL Application
Serial code
Parallel code
Serial code
Parallel code
Host = CPU
Device = GPU
Host = CPU
Device = GPU
…
…

OPENCL EXECUTION MODEL

Decompose task into work-items Define N-dimensional computation domain
Execute a kernel at each point in computation domain
Traditional loop as a function in C
OpenCL C kernel
void trad_mul(int n,
const float *a,
const float *b,
float *c)
{
int i;
for (i=0; i global -> local … and back
Private Memory
Work-Item
Local Memory Workgroup
Private Memory
Work-Item
Local Memory Workgroup
Private Memory
Private Memory
Work-Item
Work-Item
Compute Device
Global/Constant Memory
Host Memory
Host
© Copyright Khronos Group, 2010

INTRODUCTION TO OPENCL PROGRAMMING

OpenCL Framework
Platform layer
— Platform query and context creation
Compiler for OpenCL C
 Runtime
— Memory management and command execution within a context

OpenCL Framework
Context
CPU
GPU
Programs Kernels
Memory Objects
Command Queues
dp_mul
arg [0]
arg [0]
value argv[0a]lvualeue
arg [1]
arg [1]
value argv[1a]lvualeue
arg [2]
arg [2]
value argv[2a]lvualeue
In
In
Order
Order
Queue
Queue
GPU
Compute Device
Out of
Out of
Order
Order
Queue
Queue
__kernel void
dp_mul(global const float *a,
global const float *b,
global float *c) {
int id = get_global_id(0);
c[id] = a[id] * b[id]; }
dp_mul
CPU program binary
dp_mul
GPU program binary
Third party names are the property of their owners.
© Copyright Khronos Group, 2010
Images
Buffers

OpenCL Framework: Platform Layer
Programs
CPU GPU
Context
Kernels
Memory Objects
Command Queues
dp_mul
arg [0]
arg [0]
value argv[0a]lvualeue
arg [1]
arg [1]
value argv[1a]lvualeue
arg [2]
arg [2]
value argv[2a]lvualeue
In
In
Order
Order
Queue
Queue
GPU
Compute Device
Out of
Out of
Order
Order
Queue
Queue
__kernel void
dp_mul(global const float *a,
global const float *b,
global float *c) {
int id = get_global_id(0);
c[id] = a[id] * b[id]; }
dp_mul
CPU program binary
dp_mul
GPU program binary
Third party names are the property of their owners.
© Copyright Khronos Group, 2010
Images
Buffers

OpenCL Framework: Platform Layer
Query platform information
— clGetPlatformInfo(): profile, version, vendor, extensions
— clGetDeviceIDs(): list of devices
— clGetDeviceInfo(): type, capabilities
Create an OpenCL context for one or more devices One or more devices
Context =
cl_context
cl_device_id
Memory and device code shared by these devices
cl_mem cl_program
Command queues to send commands to these devices
cl_command_queue

Platform Layer:
Context Creation (simplified)
// Get the platform ID
cl_platform_id platform; clGetPlatformIDs(1, &platform, NULL);
Number returned
// Get the first GPU device associated with the platform
cl_device_id device;
clGetDeviceIDs(platform, CL_DEVICE_TYPE_GPU, 1, &device, NULL);
// Create an OpenCL context for the GPU device
cl_context context;
context = clCreateContext(NULL, 1, &device, NULL, NULL, NULL);
Context Error User Error properties callback data code

Platform Layer:
Error Handling, Resource Deallocation
Error handling:
— All host functions return an error code — Context error callback
Resource deallocation
— Reference counting API: clRetain*(), clRelease*()
Both are removed from code samples for clarity — Please see SDK samples for complete code

OpenCL Framework: OpenCL C
Programs
CPU GPU
Context
Kernels
Memory Objects
Command Queues
dp_mul
arg [0]
arg [0]
value argv[0a]lvualeue
arg [1]
arg [1]
value argv[1a]lvualeue
arg [2]
arg [2]
value argv[2a]lvualeue
In
In
Order
Order
Queue
Queue
GPU
Compute Device
Out of
Out of
Order
Order
Queue
Queue
__kernel void
dp_mul(global const float *a,
global const float *b,
global float *c) {
int id = get_global_id(0);
c[id] = a[id] * b[id]; }
dp_mul
CPU program binary
dp_mul
GPU program binary
Third party names are the property of their owners.
© Copyright Khronos Group, 2010
Images
Buffers

OpenCL C
 Derived from ISO C99 (with some restrictions)
 Language Features Added — Work-items and work-groups — Vector types
— Synchronization
— Address space qualifiers
 Also includes a large set of built-in functions — Image manipulation
— Work-item manipulation
— Math functions
© Copyright Khronos Group, 2010

OpenCL C Language Restrictions
 Pointers to functions are not allowed
 Pointers to pointers allowed within a kernel, but not as an argument  Bit-fields are not supported
 Variable-length arrays and structures are not supported
 Recursion is not supported
Writes to a pointer to a type less than 32 bits are not supported*  Double types are not supported, but reserved
3D Image writes are not supported
Some restrictions are addressed through extensions

OpenCL C Optional Extensions
 Extensions are optional features exposed through OpenCL  The OpenCL working group has already approved many
extensions to the OpenCL specification:
— Double precision floating-point types (Section 9.3)
— Built-in functions to support doubles
— Atomic functions (Section 9.5, 9.6, 9.7)
— Byte-addressable stores (write to pointers to types < 32-bits) (Section 9.9) — 3D Image writes (Section 9.8) — Built-in functions to support half types (Section 9.10) Now core features in OpenCL 1.1 Work-items and work-groups  A kernel is a function executed for each work-item __kernel void square(__global float* input, __global float* output) { int i = get_global_id(0); output[i] = input[i] * input[i]; } Built-in function Address space qualifier get_global_id(0) = 7 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 input output Function qualifier 6 1 1 0 9 2 4 1 1 9 7 6 8 2 2 5 36 1 1 0 81 4 16 1 1 81 49 36 64 4 4 25 © Copyright Khronos Group, 2010 Work-items and work-groups 6 1 1 0 9 2 4 1 1 9 7 6 8 2 2 5 work-group work-item get_group_id(0) = 0 get_local_size(0) = 8 get_group_id(0) = 1 get_local_size(0) = 8 get_local_id(0) = 3 get_global_id(0) = 11 input get_work_dim() = 1 get_global_size(0) = 16 get_num_groups(0) = 2 © Copyright Khronos Group, 2010 OpenCL C Data Types  Scalar data types — char, uchar, short, ushort, int, uint, long, ulong, float — bool, intptr_t, ptrdiff_t, size_t, uintptr_t, void, half (storage)  Image types — image2d_t, image3d_t, sampler_t  Vector data types — Vector lengths 2, 3, 4, 8, 16 (char2, ushort4, int8, float16, double2, ...) — Endian safe — Aligned at vector length — Vector operations 3-vectors new in OpenCL 1.1 double is an optional type in OpenCL © Copyright Khronos Group, 2010 OpenCL C Synchronization Primitives  Built-in functions to order memory operations and synchronize execution: — mem_fence(CLK_LOCAL_MEM_FENCE and/or CLK_GLOBAL_MEM_FENCE)  waits until all reads/writes to local and/or global memory made by the calling work- item prior to mem_fence() are visible to all threads in the work-group — barrier(CLK_LOCAL_MEM_FENCE and/or CLK_GLOBAL_MEM_FENCE)  waits until all work-items in the work-group have reached this point and calls mem_fence(CLK_LOCAL_MEM_FENCE and/or CLK_GLOBAL_MEM_FENCE)  Used to coordinate accesses to local or global memory shared among work-items OpenCL C Kernel Example __kernel void dp_mul(__global const float *a, __global const float *b, __global float *c, int N) { int id = get_global_id (0); if (id < N) } c[id] = a[id] * b[id]; OpenCL Framework: Runtime Programs CPU GPU Context Kernels Memory Objects Command Queues dp_mul arg [0] arg [0] value argv[0a]lvualeue arg [1] arg [1] value argv[1a]lvualeue arg [2] arg [2] value argv[2a]lvualeue In In Order Order Queue Queue GPU Compute Device Out of Out of Order Order Queue Queue __kernel void dp_mul(global const float *a, global const float *b, global float *c) { int id = get_global_id(0); c[id] = a[id] * b[id]; } dp_mul CPU program binary dp_mul GPU program binary Third party names are the property of their owners. © Copyright Khronos Group, 2010 Images Buffers OpenCL Framework: Runtime  Command queues creation and management  Device memory allocation and management  Device code compilation and execution  Event creation and management (synchronization, profiling) OpenCL Runtime: Kernel Compilation Context CPU GPU Programs Kernels Memory Objects Command Queues dp_mul arg [0] arg [0] value argv[0a]lvualeue arg [1] arg [1] value argv[1a]lvualeue arg [2] arg [2] value argv[2a]lvualeue In In Order Order Queue Queue GPU Compute Device Out of Out of Order Order Queue Queue __kernel void dp_mul(global const float *a, global const float *b, global float *c) { int id = get_global_id(0); c[id] = a[id] * b[id]; } dp_mul CPU program binary dp_mul GPU program binary Third party names are the property of their owners. © Copyright Khronos Group, 2010 Images Buffers Kernel Compilation  A cl_program object encapsulates some source code (with potentially several kernel functions) and its last successful build — clCreateProgramWithSource() // Create program from source — clBuildProgram() // Compile program  A cl_kernel object encapsulates the values of the kernel’s arguments used when the kernel is executed — clCreateKernel() // Create kernel from successfully compiled program — clSetKernelArg() // Set values of kernel’s arguments Kernel Compilation // Build program object and set up kernel arguments const char* source = "__kernel void dp_mul(__global const float *a, \n" " " " "{ \n" " " " "} \n"; __global const float *b, \n" __global float *c, \n" int N) \n" int id = get_global_id (0); \n" if (id < N) \n" c[id] = a[id] * b[id]; \n" cl_program program = clCreateProgramWithSource(context, 1, &source, NULL, NULL); clBuildProgram(program, 0, NULL, NULL, NULL, NULL); cl_kernel kernel = clCreateKernel(program, ―dp_mul", NULL); clSetKernelArg(kernel, 0, sizeof(cl_mem), (void*)&d_buffer); clSetKernelArg(kernel, 1, sizeof(int), (void*)&N); OpenCL Runtime: Memory Objects Programs CPU GPU Context Kernels Memory Objects Command Queues dp_mul arg [0] arg [0] value argv[0a]lvualeue arg [1] arg [1] value argv[1a]lvualeue arg [2] arg [2] value argv[2a]lvualeue In In Order Order Queue Queue GPU Compute Device Out of Out of Order Order Queue Queue __kernel void dp_mul(global const float *a, global const float *b, global float *c) { int id = get_global_id(0); c[id] = a[id] * b[id]; } dp_mul CPU program binary dp_mul GPU program binary Third party names are the property of their owners. © Copyright Khronos Group, 2010 Images Buffers Memory Objects  Two types of memory objects (cl_mem): — Buffer objects — Image objects Memory objects can be copied to host memory, from host memory, or to other memory objects  Regions of a memory object can be accessed from host by mapping them into the host address space Buffer Object  One-dimensional array  Elements are scalars, vectors, or any user-defined structures  Accessed within device code through pointers Image Object  Two- or three-dimensional array  Elements are 4-component vectors from a list of predefined formats  Accessed within device code via built-in functions (storage format not exposed to application) — Sampler objects are used to configure how built-in functions sample images (addressing modes, filtering modes)  Can be created from OpenGL texture or renderbuffer OpenCL Runtime: Command Queues Programs Kernels CPU GPU Context Memory Objects Command Queues dp_mul arg [0] arg [0] value argv[0a]lvualeue arg [1] arg [1] value argv[1a]lvualeue arg [2] arg [2] value argv[2a]lvualeue In In Order Order Queue Queue GPU Compute Device Out of Out of Order Order Queue Queue __kernel void dp_mul(global const float *a, global const float *b, global float *c) { int id = get_global_id(0); c[id] = a[id] * b[id]; } dp_mul CPU program binary dp_mul GPU program binary Third party names are the property of their owners. © Copyright Khronos Group, 2010 Images Buffers Commands  Memory copy or mapping  Device code execution  Synchronization point Command Queue  Sequence of commands scheduled for execution on a specific device — Enqueuing functions: clEnqueue*() — Multiple queues can execute on the same device Two modes of execution: — In-order: Each command in the queue executes only when the preceding command has completed (including memory writes) — Out-of-order: No guaranteed order of completion for commands // Create a command-queue for a specific device Error code cl_command_queue cmd_queue = clCreateCommandQueue(context, device_id, 0, NULL); Properties Data Transfer between Host and Device // Create buffers on host and device size_t size = 100000 * sizeof(int); int* h_buffer = (int*)malloc(size); cl_mem d_buffer = clCreateBuffer(context, CL_MEM_READ_WRITE, size, NULL, NULL); ... // Write to buffer object from host memory clEnqueueWriteBuffer(cmd_queue, d_buffer, CL_FALSE, 0, size, h_buffer, 0, NULL, NULL); ... // Read from buffer object to host memory clEnqueueReadBuffer(cmd_queue, d_buffer, CL_TRUE, 0, size, h_buffer, 0, NULL, NULL); Blocking? Offset Event synch Kernel Execution: NDRange  Host code invokes a kernel over an index space called an NDRange — NDRange = ―N-Dimensional Range‖ of work-items — NDRange can be a 1-, 2-, or 3-dimensional space — Work-group dimensionality matches work-item dimensionality Kernel Invocation // Set number of work-items in a work-group size_t localWorkSize = 256; int numWorkGroups = (N + localWorkSize – 1) / localWorkSize; // round up size_t globalWorkSize = numWorkGroups * localWorkSize; // must be evenly divisible by localWorkSize clEnqueueNDRangeKernel(cmd_queue, kernel, 1, NULL, &globalWorkSize, &localWorkSize, 0, NULL, NULL); NDRange Command Synchronization  Queue barrier command: clEnqueueBarrier() — Commands after the barrier start executing only after all commands before the barrier have completed  Events: a cl_event object can be associated with each command — Commands return events and obey event waitlists  clEnqueue*(..., num_events_in_waitlist, *event_waitlist, *event); — Any commands (or clWaitForEvents()) can wait on events before executing — Event object can be queried to track execution status of associated command and get profiling information  Some clEnqueue*() calls can be optionally blocking — clEnqueueReadBuffer(..., CL_TRUE, ...); Synchronization: Queues & Events  You must explicitly synchronize between queues — Multiple devices each have their own queue — Possibly multiple queues per device — Use events to synchronize © Copyright Khronos Group, 2010 ADDITIONAL INFORMATION AND RESOURCES Next Steps  Begin hands-on development with our publicly available OpenCL driver and GPU Computing SDK  Read the OpenCL Specification and the extensive documentation provided with the SDK  Read and contribute to OpenCL forums at Khronos and NVIDIA NVIDIA OpenCL Resources  NVIDIA OpenCL Web Page: — http://www.nvidia.com/object/cuda_opencl.html  NVIDIA OpenCL Forum: — http://forums.nvidia.com/index.php?showforum=134  NVIDIA driver, profiler, code samples for Windows and Linux: — http://developer.nvidia.com/object/opencl.html Khronos OpenCL Resources  OpenCL Specification — http://www.khronos.org/registry/cl/specs/opencl-1.1.pdf  OpenCL Registry — http://www.khronos.org/registry/cl/  OpenCL Developer Forums — http://www.khronos.org/message_boards/  OpenCL Quick Reference Card — http://www.khronos.org/files/opencl-1-1-quick-reference-card.pdf  OpenCL Online Man pages — http://www.khronos.org/registry/cl/sdk/1.1/docs/man/xhtml/ © Copyright Khronos Group, 2010 OpenCL Books • The OpenCL Programming Book – Available now: search for OpenCL on Amazon • OpenCL Programming Guide - The ―Red Book‖ of OpenCL – Coming in July 2011; rough cut available on Safaribooks – http://my.safaribooksonline.com/9780132488006 Questions?

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