When to Scale Up in RDS: Freeable Memory
Monitoring freeable memory will help you know when it is time to scale your Amazon RDS cluster. Freeable memory is not reported by the database, but rather by the OS. Freeable memory is the combination of unused and temporarily used memory. It is the memory that the system can grant without adverse effects. When an Amazon RDS instance runs out of freeable memory the OS may do up to three things.