A high memory limit is not a problem if you use PHP-FPM and can control how many threads PHP can open at any given time. A good rule is to use "ondemand" mode in PHP-FPM with a thread count of 80%-100% of your available CPUs. So if your server has 8 CPUs, use 8 for pm.max_children (the thread count for PHP-FPM). That means your server will never require more than 8 GBs of RAM assuming you have a 1024M memory limit.