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Before Start

The first thing to do will be get the information of out system:

• System Information: NMIS and OMK support tool will give us all the information needed.
• Monitor services: NMIS can monitor the involved processes - apache2, nmis9d, omkd and mongod - and provide useful information about CPU and memory - among others. 

Number of processes

NMIS runs a daemon to obtain periodically the nodes information.

The number of workers is set in the parameter:

nmisd_max_workers

By default 10. 

Some aprox. configurations:

OMK has the equivalent parameter:

omkd_workers

Setting also omkd_max_requests, will help to have the threads restart gracefully before they get too big.

omkd_max_requests

MongoDB memory usage

MongoDB, in its default configuration, will use will use the larger of either 256 MB or ½ of (ram – 1 GB) for its cache size.

MongoDB cache size can be changed by adding the cacheSizeGB argument to the /etc/mongod.conf configuration file, as shown below.

storage:
  dbPath: /var/lib/mongodb
  journal:
    enabled: true
  wiredTiger:
      engineConfig:
         cacheSizeGB: 1

Here is an interesting information regarding how MongoDB reserves memory for internal cache and WiredTiger, the underneath technology. Also some adjustment that can be done: https://dba.stackexchange.com/questions/148395/mongodb-using-too-much-memory