I have an issue where when I start up modded minecraft, PC uses little ram and system monitor reports that java never exceeds the max of 8gb ram I set. When I start flying around and mapping out the server using a minimap and world map, RAM usage keeps rising till PC starts lagging showing 90%+ of ram being used despite system monitor reporting 8gb for java and mb (or less) of usage on everything else. I even tried using free and htop in terminal but same results. Closing minecraft makes ram drop, but never all the way as shown in the images.
I am on Linux Mint 21.2, running minecraft through prism launcher, modpack was ATM8, but I noticed it happens on all modpacks. Not sure if this is a minecraft or mint issue, so I hope someone can explain or point me in the right direction
If I understand your reply correctly (the output of free would be clearer), having 80% of RAM used as cache would indicate that it is used for caching files such as map and chunk data. This is normal in Linux, and the game would lag more without this feature. The link I shared explains it better than I can.
If you feel this is still not the case, you can share info such as free output, list of open files, etc, while the issue occurs.