Understanding memory usage in MMAPv1 is relatively straightforward. MMAPv1 is using the mmap() system call under the hood to pass on the responsibility of the memory page to the underlying operating system. This is why when we use MMAPv1 memory usage will grow unbounded as the operating system is trying to fit as much of our dataset into memory.

With WiredTiger on the other hand, we define internal cache memory usage on startup. By default, the internal cache will be, at the maximum, between half of our RAM in GB—1 GB or 256 MB.

On top of the internal cache, there is also memory that MongoDB can allocate for other operations like maintaining connections and data processing (in-memory sort, MapReduce, aggregation, and others).

MongoDB processes will also use the underlying operating system's filesystem cache just like in MMAPv1. Data in the filesystem cache is compressed.

We can view the settings for the WiredTiger cache via the mongo shell as follows:

> db.serverStatus().wiredTiger.cache

We can adjust its size using the storage.wiredTiger.engineConfig.cacheSizeGB parameter.

The generic recommendation is to leave WiredTiger internal cache size at its default. If our data has a high compression ratio, it may be worth reducing the internal cache size by 10-20% to free more memory for the filesystem cache.