0 running time in GPU methods

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Hi,

I am doing GPU performance test and measuring processing time of some general methods. Here's the measured processing time and performed task:

CUDA dev count: 1
D:\gpu test\images\5000x2750\01.bmp
16 load src1 in CPU
796 upload src1 CPU -> GPU
0 Resize
0 BGR2GRAY
16 BGR2HSV
47 BGR2YUV
0 Threshold

D:\gpu test\images\5000x2750\02.bmp
31 load src1 in CPU
0 upload src1 CPU -> GPU
16 Resize
0 BGR2GRAY
0 BGR2HSV
0 BGR2YUV
31 Threshold

My first question is that, In many times, I get 0 processing time. Why is that?

Second, 47ms BGR2YUV is slower than doing it on CPU. Why is it so slow?

Thanks in advance!