I am making a call to the cublasSasum function only once from cuBLAS. I do see that the actual kernel it calls (asum_kernel) is called twice as seen from profiling via nsys. I am computing a sum for a total of 4096^2 elements.

 Time (%)  Total Time (ns)  Instances   Avg (ns)     Med (ns)    Min (ns)   Max (ns)   StdDev (ns)                              Name                             
 --------  ---------------  ---------  -----------  -----------  ---------  ---------  -----------  -------------------------------------------------------------
     17.9          415,655          2    207,827.5    207,827.5      3,264    412,391    289,296.5  void asum_kernel<int, float, float>(cublasAsumParams<T2, T3>)

Is the call made twice because the kernel performs block level reductions first, saves the result, and performs the final reduction on these temporary results?

PS : I tried the same with just 4 elements with 4 threads and only one grid block. Still the asum_kernel is called twice though one is sufficient in this case?!?