First of all, I need Memcache to be installed in the server.
I don’t want say too much about installation of Memcache. The topic is MAKE MEMCACHE WORK ON PHPBB3.
First get acm_memcache.php from code center of phpbb.
Put this file under /root/includes/acm/
There is acm_file.php already.
Then modify config.php
Replace $acm_type = ‘file’
$acm_type = ‘memcache’
Of course, if you do not use the default memcache installation, you need to config acm_memcache.php to let it know your IP and port.