Memory usage remains 99 % even after create index is done on MongoDB Collection

Question

While doing indexing on MongoDB. Now we have nearly 350 GBs of data in the database and its deployed as a windows service in AWS EC2.

And we are doing indexing for some experimentation. But every time I run the indexing command the memory usage goes to 99% and even after the indexing is done the memory usage keeps like that until I restart the service.

The instance has 30 GB of RAM and SSD drive. And right now we have the DB setup as stand alone (not sharded till now). And we are using the latest version of MongoDB.

Any feedback related to this will be helpful.

Thanks,

Arpan

Answer 1

That's normal behavior for MongoDB.

MongoDB grabs all the RAM it can get to cache each accessed document as long as possible. When you add an index to a collection, each document needs to be read once to build the index, which causes MongoDB to load each document into RAM. It then keeps them in RAM in case you want to access them later. But MongoDB will not squat the RAM. When another process needs memory, MongoDB will willingly release it.

This is explained in the FAQ:

Does MongoDB require a lot of RAM?

Not necessarily. It’s certainly possible to run MongoDB on a machine with a small amount of free RAM.

MongoDB automatically uses all free memory on the machine as its cache. System resource monitors show that MongoDB uses a lot of memory, but its usage is dynamic. If another process suddenly needs half the server’s RAM, MongoDB will yield cached memory to the other process.

Technically, the operating system’s virtual memory subsystem manages MongoDB’s memory. This means that MongoDB will use as much free memory as it can, swapping to disk as needed. Deployments with enough memory to fit the application’s working data set in RAM will achieve the best performance.

See also: FAQ: MongoDB Diagnostics for answers to additional questions about MongoDB and Memory use.