CUDA: cudaEventElapsedTime returns device not ready error

It's because cudaEventRecord is asynchronous. It finishes its execution immediately, regardless of the status. Asynchronous functions simply put an order on a "CUDA execution queue". When GPU finishes its current assignment, it pops the next order and executes it. It is all done in a separate thread, handled by the CUDA driver, separate of your program host thread.

cudaEventRecord is an order which says more-or-less something like this: "When you are done all previous work, flag me in this variable".

If your host thread then asks for cudaEventElapsedTime, but the GPU didn't finish its work yet, it gets confused and reports "not ready yet!". cudaEventSynchronize() stalls the current host thread until the GPU reaches the cudaEventRecord order that you placed earlier. After that you are guaranteed that cudaEventElapsedTime will have a meaningful answer for you.

cudaThreadSynchronize() is just a stronger tool: it stalls current thread until GPU finishes all assigned tasks, and not just those until the event.