OpenCL Platform Model

The OpenCL platform model defines the logical representation of all hardware capable of executing an OpenCL program. OpenCL platforms are defined by the grouping of a host processor and one or more OpenCL compute devices. The host processor, which runs the OS for the system, is also responsible for the general bookkeeping and task launch duties associated with the execution of OpenCL applications. The device is the hardware element in the system on which the compute kernels of an OpenCL application are executed. Each device is further divided into a set of compute units. The number of compute units depends on the target hardware. A compute unit is further subdivided into processing elements. A processing element is the fundamental computation engine in the compute unit, which is responsible for executing the operations of one work item.

The preceding figure shows a conceptual view of the OpenCL platform model. An OpenCL platform always starts with a host processor. For platforms created with Xilinx® devices, the host processor is an x86 based processor communicating to the devices using PCIe®. The host processor has the following responsibilities:

Manage the operating system and enable drivers for all devices.
Execute the application host program.
Set up all global memory buffers and manage data transfer between the host and the device.
Monitor the status of all compute units in the system.

In all OpenCL platforms, the host processor tasks are executed using a common set of OpenCL API. The implementation of the OpenCL API functions is provided by the hardware vendor and is referred to as the OpenCL runtime library. The OpenCL runtime library is responsible for translating user commands described by the OpenCL API into hardware specific commands for a given device. For example, when the application programmer allocates a memory buffer using the clCreateBuffer API, it is the responsibility of the runtime library to keep track of where the allocated buffer physically resides in the system, and of the mechanism required for buffer access. It is important for the application programmer to keep in mind that the OpenCL API is portable across vendors, but the runtime library provided by a vendor is not. Therefore, OpenCL applications have to be linked at compile time with the runtime library that is paired with the target execution device.

The other component of a platform is the device. A device in the context of OpenCL is the physical collection of hardware resources onto which the application kernels are executed. A platform must have at least one device available for the execution of kernels. Also, per the OpenCL platform model, all devices in a platform do not have to be of identical type.