32 decoders open failed in one process

When we suppose to open 32 decoders through /dev/nvhost-nvdec node in ONE process, there alwasy failed after 16 have successfully opened, with following trace:
libv4l2: error attempting to open more then 16 video devices
[ERROR] (NvV4l2Element.cpp:63) Could not open device ‘/dev/nvhost-nvdec’: Device or resource busy

While if we open 32 decoders in more than ONE process, there will be OK, so could somebody please tell me why?

Please refer to
https://devtalk.nvidia.com/default/topic/1020755/jetson-tx1/mmapi-can-t-decode-more-than-8-video-streams-at-the-same-time/post/5196704/#5196704
https://devtalk.nvidia.com/default/topic/1025375/jetson-tx1/-mmapi-libv4l2-error-attempting-to-open-more-then-16-video-devices/post/5250938/#5250938