The OpenCL Memory Model

The OpenCL 1.1 specification https://www.khronos.org/registry/cl/specs/opencl-1.1.pdf available from Khronos defines a memory model in Section 3.3. Please refer to the specification for details on these memory regions and how they relate to work-items, work-groups, and kernels. This document will focus on the mapping of the OpenCL memory model to TI devices. There are four virtual memory regions defined.

Global Memory

This memory region contains global buffers and is the primary conduit for data transfers from the host A15 CPUs to/from the C66 DSPs. This region will also contain OpenCL C program code that will be executed on the C66 DSPs. For this OpenCL implementation, global memory by default maps to the portion of DDR3 partitioned as CMEM contiguous memory.

On K2x devices, MSMC memory is also available as global memory and buffers can be defined to reside in this memory instead of DDR3 through an OpenCL API extension specific to TI. This mechanism will be described in a later section that details handling of the OpenCL buffer creation flags.

Constant Memory

This memory region contains content that remains constant during the execution of a kernel. OpenCL C program code and constant data defined in that code would be placed in this region. For this implementation, constant memory is mapped to the portion of DDR3 partitioned as CMEM contiguous memory.

Local Memory

The local memory region is not defined by the spec to be accessible from the host (ARM A15 cores). This memory is local to a work group. It can be viewed as a core local scratchpad memory and in fact for this implementation it is mapped to L2 that is reserved for this purpose. The use case for local memory is for an OpenCL work-group to migrate a portion of a global buffer to/from a local buffer for performance reasons. This use case is optional for users as access to global buffers in DDR will be cached in both the L2 cache and the L1D cache on the C66 DSPs. However, performance can often be improved by taking the extra step in OpenCL C programs to manage local memory as a scratchpad.

Private Memory

This memory region is for values that are private to a work-item. These values are typically allocated to registers in the C66 DSP core. Sometimes it may be necessary for these values to exist in memory. In these cases the values are stored on the C66 DSP stack, which resides in the reserved portion of the L2 memory.