title: Why the data transfer is slow from GPU to CPU?
body: <p>Today I have figured out something that really made me wondering. I have the Samsung Exynos 4412 ARM9 CPU which has a GPU400(QuadCore). I tried to get a texture from the GPU to CPU by all known methods and its really slow. The same scenario and slow speed happens also in modern CPUs and GPUs in the PC Platform. My wondering is how that happens and the Samsung Exynos is an SoC and both of them has the same memory and I should not care about the bus.
  Why that happens ?</p>
  
  <p>The data from the GPU to the CPU is transferred by many methods which I have tried  glReadpixels, gltexSubImage2D, gltexImage2d, FBO. 
  The frame rate drops from 40FPS to 7FPs or 7FPS while using any of those methods, on a texture 1024*1024 24bits.</p>