High Performance Computing 25 SP OpenCL Programming

Open Computing Language.

OpenCL is Open Computing Language.

• Open, royalty-free standard C-language extension.
• For parallel programming of heterogeneous systems using GPUs, CPUs , CBE, DSP and other processors including embedded mobile devices.
• Managed by Khronos Group.

Anatomy of OpenCL

• Platform Layer APi
• Runtime Api
• Language Specification

Compilation Model

OpenCL uses dynamic/runtime compilation model like OpenGL.

1. The code is compiled to an IR.
2. The IR is compiled to a machine code for execution.

And in dynamic compilation, step 1 is done usually once and the IR is stored. The app loads the IR and performs step 2 during the app runtime.

Execution Model

OpenCL program is divided into

• Kernel: basic unit of executable code.
• Host: collection of compute kernels and internal functions.

The host program invokes a kernel over an index space called an NDRange.

NDRange is N-Dimensional Range, and can be a 1, 2, 3-dimensional space.

A single kernel instance at a point of this index space is called work item. Work items are further grouped into work groups.

OpenCL Memory Model

Multiple distinct address spaces: Address can be collapsed depending on the device's memory subsystem.

Address space:

• Private: private to a work item.
• Local: local to a work group.
• Global: accessible by all work items in all work groups.
• Constant: read only global memory.

Comparison with CUDA:

Memory region for host and kernel:

Programming Model

Data Parallel Programming Model

1. Define N-Dimensional computation domain
2. Work-items can be grouped together as work group.
3. Execute multiple work-groups in parallel.

Task Parallel Programming Model

Data parallel execution model must be implemented by all OpenCL computing devices, but task parallel programming is a choice for vendor.

Some computing devices such as CPUs can also execute task-parallel computing kernels.

• Executes as s single work item.
• A computing kernel written in OpenCL.
• A native function.

OpenCL Framework

The basic OpenCL program structure:

Contexts are used to contain the manage the state of the world.

Command-queue coordinates execution of the kernels.