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Introduction
Coding for the Intel® Processor Graphics
Platform-Level Considerations
Application-Level Optimizations
Optimizing OpenCL™ Usage with Intel® Processor Graphics
Check-list for OpenCL™ Optimizations
Performance Debugging
Using Multiple OpenCL™ Devices
Coding for the Intel® CPU OpenCL™ Device
OpenCL™ Kernel Development for Intel® CPU OpenCL™ device
Mapping Memory Objects
Using Buffers and Images Appropriately
Using Floating Point for Calculations
Using Compiler Options for Optimizations
Using Built-In Functions
Loading and Storing Data in Greatest Chunks
Applying Shared Local Memory
Using Specialization in Branching
Considering native_ and half_ Versions of Math Built-Ins
Using the Restrict Qualifier for Kernel Arguments
Avoiding Handling Edge Conditions in Kernels
Using Shared Context for Multiple OpenCL™ Devices
Sharing Resources Efficiently
Synchronization Caveats
Writing to a Shared Resource
See Also
Partitioning the Work
Keeping Kernel Sources the Same
Basic Frequency Considerations
Eliminating Device Starvation
Limitations of Shared Context with Respect to Extensions
Why Optimizing Kernel Code Is Important?
Avoid Spurious Operations in Kernel Code
Perform Initialization in a Separate Task
Use Preprocessor for Constants
Use Signed Integer Data Types
Use Row-Wise Data Accesses
Tips for Auto-Vectorization
Local Memory Usage
Avoid Extracting Vector Components
Task-Parallel Programming Model Hints
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Writing to a Shared Resource
According to the OpenCL™ specification, you need to ensure that the commands that change the content of a shared memory object, complete in the previous command queue before the memory object is used by commands, executed in another command-queue. One way to achieve this is using events:
cl_event eventGuard; cl_buffer bufferShared=clCreateBuffer(shared_context,CL_MEM_READ_WRITE…); //Populating the buffer from the host, queue is regular in-order clEnqueueWriteBuffer(cpu_queue, bufferShared,…); //Setting the arguments and processing buffer with a kernel SetKernelArg(kernel, 0, sizeof(cl_mem), (void*)&bufferShared); … clEnqueueNDRangeKernel(cpu_queue, kernel, … &eventGuard); //make sure the first device is done clWaitForEvents(1, &eventGuard); //alternatively you can use clFinish(cpu_queue) if in the same thread … //Now using buffer by second device clEnqueueWriteBuffer(gpu_queue, bufferShared,…); clEnqueueNDRangeKernel(gpu_queue, kernel, … &eventGuard); …
If you want to write data (or output kernel results) to the same buffer simultaneously on two devices, use properly aligned, non-overlapping sub-buffers.
cl_buffer bufferShared = clCreateBuffer(shared_context, CL_MEM_ WRITE …);
//make sure alignment for the resp devices
cl_int gpu_align;
clGetDeviceInfo(gpuDeviceId, CL_DEVICE_MEM_BASE_ADDR_ALIGN,…&gpu_align);
gpu_align /= 8; //in bytes
//make sure that cpuPortion is properly aligned first!
cl_buffer_region cpuBufferRegion = { 0, cpuPortion};
cl_buffer_region gpuBufferRegion = { cpuPortion, theRest};
cl_buffer subbufferCPU = clCreateSubBuffer(bufferShared, 0,
CL_BUFFER_CREATE_TYPE_REGION, &cpuBufferRegion, &err);
cl_buffer subbufferGPU = clCreateSubBuffer(bufferShared, 0,
CL_BUFFER_CREATE_TYPE_REGION, &gpuBufferRegion, &err);
//now work with 2 sub-buffers on 2 devices simultaneously - (refer to the //prev. section)
..
//the sub-resources should be released properly
clReleaseMemObject(subbufferCPU);
clReleaseMemObject(subbufferGPU);
clReleaseMemObject(bufferShared);
See Also
The OpenCL™ 1.2 Specification at http://www.khronos.org/registry/cl/specs/opencl-1.2.pdf