Constant BITSTREAM_BUFFERS 

const BITSTREAM_BUFFERS: usize = 1;

Output bitstream buffers per session: one, because submit_frame locks, copies, and unlocks each frame’s bitstream before it returns, so no second buffer is ever outstanding (the lock blocks; a doNotWait lock on Linux answers an unfinished encode with an empty bitstream rather than NV_ENC_ERR_LOCK_BUSY). The ring stays, so a pipelined depth is one constant away.