Mastering MongoDB 4.x - Second Edition

Monitoring memory usage in WiredTiger

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

With WiredTiger, on the other hand, we define the internal cache memory usage on startup. By default, the internal cache will be, at a maximum, between half of our RAM, which is in-between 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 ...