We run a db.m4.10xlarge instance on Amazon's RDS. It runs 3 fairly busy databases and has a replica instance connected to it.
Over the last few weeks I noticed the swap usage has been steadily rising from 0 to about 80MB now. I don't know why it's using the swap space at all when there's plenty of free (freeable?) RAM. I've included charts from the last 2 weeks below:
The same thing is happening on the replica also, which is another concern of mine as our replica lag is continuing to grow in a similar manner.