How to reduce buffers/cache

A monitoring system keeps alerting that my machine is reaching/breaking through its RAM utilization threshold which is **15 GBs**. I've done some reading and understood that the apparent RAM utilization is not actual and that the extra RAM is used for caching/buffering of disk I/O operation to improve the performance of the server. I'm running MySQL on that server, that's the only notable service running. - So how can I reduce the disk I/O caching/buffering RAM as not to break through the threshold? Could this be a MySQL issue and not Linux's? That's the output of free -gt [root@ipk ~]# free -gt total used free shared buffers cached Mem: 15 15 0 0 0 9 -/+ buffers/cache: 5 10 Swap: 5 0 5 Total: 21 15 6 Linux version is: [root@ipk ~]# uname -rmo 2.6.32-220.el6.x86_64 x86_64 GNU/Linux