Is private memory slower than local memory?

Question

I was working on a kernel which had much global memory access per thread so I copied them to local memory which gave a speed up of 40%.

I wanted still more speed up so copied from local to private which degraded the performance

So is it correct that I think we must not use to much private memory which may degrade the performance?

Answer 1

Ashwin's answer is in the right direction but a little misleading.

OpenCL abstracts the address space of variables away from their physical storage, and there is not necessarily a 1:1 mapping between the two.

Consider OpenCL variables declared in the __private address space, which includes automatic non-pointer variables inside functions by default. The NVidia GPU implementation will physically allocate these in registers as far as possible, only spilling over to physical off-chip memory when there is insufficient register capacity. This particular off-chip memory is called "CUDA local" memory, and has similar performance characteristics to memory allocated for __global variables, which explains the performance penalty due to register spill-over. There is no such physical thing as "private memory" in this implementation, only a "private address space", which may be allocated on- or off-chip.

The performance hit is not a direct consequence of using the private address space (or "private memory"), which is typically allocated in high performance memory. It is because, under this implementation, the variable was too large to be allocated on high performance registers, and was therefore "spilled over" to off-chip memory.