NVIDIA NVMM buffer size and nvv4l2h265enc

Hi all:
I know that (memory:NVMM) means processing with on board hardware DMA buffers. But how can I get the performance parameters of this kind of on board source? (Like the total size or read/write speed).
Futhermore, I see the encode and decode ability is great different between jetson NANO and jetson AGX XAVIER. So, what impect the encode/decode performance when I use nvv4l2h265enc can nvv4l2decoder?