Is the access performance of __constant memory as same as __global memory on OpenCL

Question

As I know. Constant memory on CUDA is a specific memory. And it is faster than global memory. But in OpenCL's Spec. I get the following words.

The __constant or constant address space name is used to describe variables allocated in global memory and which are accessed inside a kernel(s) as read-only variables

So the __constant memory is from the __global memory. Does that mean it have the same accessing performance with the __global memory?

Answer 1

It depends on the hardware and software architecture of the OpenCL platform you are using. For example, one can envision an architecture with read-only caches that don't need to participate in cache coherency. These caches could be used for constant memory but not global memory. So you might see faster accesses to constant memory.

That being said, none of the architectures I'm familiar with operate this way. So that's just hypothetical.

Answer 2

The OpenCL standard does not specify how constant memory should be implemented, but in NVIDIA GPUs constant memory is cached. I don't know what AMD does.