Linux system RAM cannot be freed

Hello everyone, I have a problem, UE4.16.1 , when I call OpenLevel function to switch LEVEL, when switch from a large level to a small level, In Windows system, the RAM will be released in normal, such as from 600MB to 200MB, but in the Ubuntu system, RAM no change, even added 20MB, because I need to frequently switching levels, or load some other resource, etc., I’m worried about, at a certain moment, RAM will be fully filled, cause the program crash, etc., Is there any way to free up memory like Windows? Thank you for your advice