how to release the caching which is used by Mongodb?

Question

Mongodb use the Memory Mapped File ,when I use a long time , I see the free memory has left less by command 'free -m' in ubuntu and the caching use a lot. Then kill the Mongodb the caching still cost a lot ? how can I release the caching ?

Answer 1

MongoDB will (at least seem) to use up a lot of available memory, but it actually leaves it up to the OS's VMM to tell it to release the memory (see Caching in the MongoDB docs.)

You should be able to release any and all memory by restarting MongoDB.

However, to some extent MongoDB isn't really "using" the memory.

For example from the MongoDB docs Checking Server Memory Usage ...

Depending on the platform you may see the mapped files as memory in the process, but this is not strictly correct. Unix top may show way more memory for mongod than is really appropriate. The Operating System (the virtual memory manager specifically, depending on OS) manages the memory where the "Memory Mapped Files" reside. This number is usually shown in a program like "free -lmt".

It is called "cached" memory.

MongoDB uses the LRU (Least Recently Used) cache algorithm to determine which "pages" to release, you will find some more information in these two questions ...

• MongoDB limit memory
• MongoDB index/RAM relationship
• Mongod start with memory limit (You can't.)