There's no real analogue to a CPU core (or thread) on a GPU, there are warp schedulers (Nvidia) and functional units with varying throughput rates. The closest analogue to a CPU thread is the GPU warp (Nvidia), which shares a program counter and executes as a 32-wide vector. AMD wavefronts (64-wide) are a little bit different, but not by much. The CUDA "thread" is really an illusion that makes a SIMD lane easier to program to (sometimes...)