When using the Web UI the memory usage chart is really wrong because when you add up the usage and find the total of usage it does not match to what is left when doing the math. Also in the server selection, the memory chat is also wrong. In the game, it says we are using 3GB and in the web UI it says 7GB when we have 6GB dedicated to that specific server. This bug is really annoying and concerns me a lot of the time because I don’t know if my server is maxing out on memory or not. But I am sure this is a bug on the web UI and not the server plugin side.
This question pops up occationally, and has several explanations. The shortest and best is here: Ram problem?
The short of it is that you may in most cases ignore the values reported by the WebUI, since it reports a mix of free mem, and dedicated mem. If you want exact memory values you need to log on to the server with ssh and use the apropriate linux commands: http://www.binarytides.com/linux-command-check-memory-usage/
Theres a rule of thumb I use for my MineOS installation:
The total sum of allocated memory to all my Minecraft servers should not exceed the amount of physical RAM available on my server.