Cron should be done in MYSql -Hosting

HI, I need RAM memory and cache should be cleared in mysql thru cron every 30 mins once. Every 3 hours once my website is getting slow and RAM memory and cache getting 50% full. By running this query via putty memory getting clear. I set a cron but it is not running. Query to run in cron : echo 3 > /proc/sys/vm/drop_caches Apply for the experienced person only. Apache Server also to be maximum optimized. Thanks, Divya Saravanan.