Performance of data transfer of OpenCL in Intel HD Graphics?

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Hi, I'm trying to use OpenCL to accelerate the imgproc module. The test results show that the time costed by data transfering is nearly the same as the computing time, and that's a lager number. I use funcs getUMat and copyTo to do the data transfer. The GPU platform in my computer is Intel HD Graphics 4600, and that's the only GPU device.So I want to konw how does the data transfer between the device and host in OpenCV? Is the transfer mode suitable for Intel Graphics?