the second parameter of function rte_mempool_create mustn't be 2^q - 1

in DPDK header file lib/librte_mempool/rte_mempool.h, function rte_mempool_create document says:

The optimum size (in terms of memory usage) for a mempool is when n is a power of two minus one: n = (2^q - 1).

But actually it isn't true, in the rest of this function, it says again:

It is advised to choose cache_size to have "n modulo cache_size == 0", if this is not the case, some elements will always stay in the pool and will never be used.

Obviously, these statement is self-contradictory. If n is (2^q -1), it is impossible to have "n modulo cache_size == 0".

Per our experiment, "n modulo cache_size == 0" is better than "n = (2^q -1)". In all the rte_mempool_create calls in DPDK source code, we don't find any case which used 2^q - 1 for the second parameter. So we're inclined to use "n modulo cache_size == 0".