Question

I’m confused by some comments I’ve seen about blocking and cudaMemcpy. It is my understanding that the Fermi HW can simultaneously execute kernels and do a cudaMemcpy.

I read that Lib func cudaMemcpy() is a blocking function. Does this mean the func will block further execution until the copy has has fully completed? OR Does this mean the copy won’t start until the previous kernels have finished?

e.g. Does this code provide the same blocking operation?

SomeCudaCall<<<25,34>>>(someData);
cudaThreadSynchronize();

vs

SomeCudaCall<<<25,34>>>(someParam);
cudaMemcpy(toHere, fromHere, sizeof(int), cudaMemcpyHostToDevice);

Answer 1

Your examples are equivalent. If you want asynchronous execution you can use streams or contexts and cudaMemcpyAsync, so that you can overlap execution with copy.