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Cross-device bandwidth for discrete GPU (HD 5870)

Question asked by cadorino on Feb 9, 2012
Latest reply on Mar 5, 2012 by cadorino

Hi,

I'm testing a system equipped with a Fusion A8-3850 and an HD 5870 gpu. I was planning to test the memory access bandwidth in the following cases:

 

1) The discrete GPU (HD 5870) reads from a buffer allocated in the host memory (CL_MEM_ALLOC_HOST_PTR | CL_MEM_READ_ONLY)

2) The integrated GPU (6550D) reads from a buffer allocated in the host memory (CL_MEM_ALLOC_HOST_PTR | CL_MEM_READ_ONLY)

 

I was assuming that the result of the first test (discrete gpu) would never be higher than the PCI-express bandwidth (approx 8GB/s), but I'm getting a bandwidth that is around 40 GB/s.

I'm checking the bandwidth by using both the GlobalMemoryTest sample shipped with the AMD SDK and a program written by myself. The results are very similar.

 

Can you explain me if it is (and why it is) possible to get a cross-domain (gpu->cpu) read bandwidth higher than the PCI one from a discrete GPU?.

 

Thank you very much!

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