What's the difference between DtoD and PtoP memory copies?

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While profiling application with nvprof I found both PtoP and DtoD memcpy. I am not sure about the difference between these two.

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Jonathan Olson On

Device to device (DtoD) is a copy within a single GPU.
Peer to Peer (PtoP) is from one GPU to another.

The latter only happens in multi-GPU systems.

And for other transfers host refers to CPU, device refers to GPU.

The naming device to device transfer predates a lot of the work on multiple GPUs, otherwise the naming might have been a little different.