Log on to the specific needs of master / query from a node, mongos can not find
db.serverStatus()
Which this
"storageEngine" : { "name" : "wiredTiger", "supportsCommittedReads" : true, "readOnly" : false, "persistent" : true },
WiredTiger internal cache in the end to see how much memory the way that line in the following command in the mongo shell
db.runCommand( { serverStatus: 1 } ).wiredTiger.cache["bytes currently in the cache"]
If you do not want to restart mongoDB, you can modify the line as follows
db.adminCommand({setParameter: 1, wiredTigerEngineRuntimeConfig: "cache_size=8G"})