问题
If I transfer a single byte from a CUDA kernel to PCI-E to the host (zero-copy memory), how much is it slow compared to transferring something like 200 Megabytes?
What I would like to know, since I know that transferring over PCI-E is slow for a CUDA kernel, is: does it change anything if I transfer just a single byte or a huge amount of data? Or perhaps since memory transfers are performed in "bulks", transferring a single byte is extremely expensive and useless with respect to transferring 200 MBs?
回答1:
Hope this pic explain everything. The data is generated by bandwidthTest in CUDA samples. The hardware environment is PCI-E v2.0, Tesla M2090 and 2x Xeon E5-2609. Please note both axises are in log scale.
Given this figure, we can see that the overhead of launching a transfer request takes a constant time. Regression analysis on the data gives an estimated overhead time of 4.9us for H2D, 3.3us for D2H and 3.0us for D2D.
来源：https://stackoverflow.com/questions/17729351/cuda-how-much-slower-is-transferring-over-pci-e