too slow to unset big array (after mem fragment)

i have a big array with 20k elements, i have no problem building it, because the elements is recv from socket, i can socket_select() on 1k clients, read from them. i plan to unset the whole array every 1hour this is a not big problem when for a few times, it takes me 0.02 seconds but after many hours of running, unset will sometime takes 10 seconds. this is big problem because it's 1 unset() statement, and all incoming connection is blocked, the client may get connection time out summary: 1. one process, no fork (just has to be) 2. socket_select(), process each client's data simultaneously. some data will be put into array 3. each element value is /true/, and key is numeric(ip2long) $array[ip2long($ip)] = true; 4. fast to unset, but slow to unset after a few hours, with similar amount of elements