OpenCL map buffer to cpu then umap, it will take a long wating time to call kernel

another question.   I used opencl1.1 on Mali628(Exynos5422)。   1. first I create a buffer buffer = clCreateBuffer(context, CL_MEM_READ_WRITE | CL_MEM_ALLOC_HOST_PTR, 1280*720*4, NULL, &errorNumber);   2. then map to cpu and umap