How do you tune innodb_read_io_threads?

The default value of innodb_read_io_threads and innodb_write_io_threads is 4. How do you check if your server load needs a higher number of threads?

As I regularly check show engine innodb status \G, there is no traceable activity:

-------- FILE I/O -------- I/O thread 0 state: waiting for completed aio requests (insert buffer thread) I/O thread 1 state: waiting for completed aio requests (log thread) I/O thread 2 state: waiting for completed aio requests (read thread) ... I/O thread 32 state: waiting for completed aio requests (write thread) I/O thread 33 state: waiting for completed aio requests (write thread) Pending normal aio reads: [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0] , aio writes: [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0] , 

Furthermore, what is the downside of increasing it to the maximum value of 64? If not using, they are harmless idle threads.