Describe how/ when MMS issues a "low open files limit alert" with something like this:

On a host-by-host basis, MMS considers the number of available server connections as reported in the serverStatus.connections object embedded inside Ping documents, for example:

    "serverStatus": {
        "connections": {
               "current": 4,
               "totalCreated": 379514,
               "available": 815
        }
    }

The message "Host has low open files limit" is issued when the sum of current and available is less than minRequiredConnections, where the latter is defined like this:

If
   "maxConns" command line parameter is set for the server
Then
   minRequiredConnections = minimum(maxConns, 20000) - 1
Else 
   minRequiredConnections = 1024

The use of "ulimit" in the warning in the Ping document is potentially confusing. However, the "max open file descriptors" ulimit parameter directly affects the number of available server connections, and hence affects the above metric.

Further, the "open files limit" parameter is related to db.hostInfo()['extra'].maxOpenFiles, which is available in the Ping documents.

I note that while "maxOpenFiles" exists in the extra document in Linux, I do not see it in the extra document on my Mac.