I've recently just set up my own VPS and installed APC but concrete5 doesn't seem to be using it for its cache. Its enabled in phpinfo() and I've added
define('CACHE_LIBRARY', 'apc'); to my site.php
However it doesnt seem to have made a different and I checked on the apc.php report
and it only shows 1 file being cached and that the apc.php page. On others screenshots of using APC with concrete5 shows at least 50+ files cached.
Any help would be greatly appriecated as everything is set up as far as I can tell.
Both cores are running C5 v5.6.1.
Though I really doubt this will fix it for you, I used a capital "A" in Apc in the config/site.php file: