MySQL Server > 90% memory consumption
Well, having 10% free of big RAM is still a lot of space, but MySQL memory usage should be fairly predictable. You can check your config with http://www.mysqlcalculator.com/ or similar tool.

You don't want it to get to swap or OOM if swap is disabled, so tune it to use less than total RAM, maybe keep it under 90% for mysql and normal system usage. Then if it gets over 90, you actually know something is off (maybe some OS service memleak or similar stuff), but you still have few GB to delay real trouble.