Currently we default the WiredTiger cache size based on the total amount of memory in the host. However, this doesn't work well when running multiple instances of mongod on the same host. The new option will also make it better possible to size other internal memory structures, such as the TCMalloc thread cache, based on available memory without requiring exposing all such details to the user.

The --memorySize option allows one simple setting to communicate intent to limit total memory usage of mongod to the given number.