[Intel-gfx] Why the memcpy from a mapped GPU memory is so slow on Intel Bay Trail?

[三月！]

Hello!
      I'm developing some openCL application with Beignet in Ubuntu 14.04 x64 Desktop upon Bay Trail E3825.
      And I found that reading data from GPU memory through whatever drm_intel gem_bo_map or drm_intel_gem_bo_get subdata cost about 0.002 ~ 0.003 second to fetch a 7MiB array, which is not quite satisfing.
      Could anybody help solve this problem?
      Thanks.
 
 Wenhua Shi
 Tongji University
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