Modify mongog Default Maximum Connections

  mongodb, question

1. The default maximum number of connections for 1.mongodb sharding cluster is 819. I want to increase the number, but the adjustment has not been successful.

[root@ctl1m ~]# mongo mongodb://10.0.1.121:30000
 MongoDB shell version v3.4.9
 connecting to: mongodb://10.0.1.121:30000
 MongoDB server version: 3.4.9
 2018-01-18T18:19:13.270+0800 I CONTROL  [main]
 mongos> db.serverStatus().connections
 { "current" : 715, "available" : 104, "totalCreated" : 719 }

From the above results, it can be seen that the maximum number of connections is 819. When the available is 0, an error will be reported when logging in again:

[root@ctl1m ~]# mongo mongodb://10.0.1.121:30000
 MongoDB shell version v3.4.9
 connecting to: mongodb://10.0.1.121:30000
 2018-01-19T10:52:41.277+0800 E QUERY    [thread1] Error: network error while attempting  to run command 'isMaster' on host '10.0.1.121:30000'  :
 connect@src/mongo/shell/mongo.js:237:13
 @(connect):1:6
 exception: connect failed

2. My revision process:
Modify /etc/secruity/limits.conf, restart the system to view ulimit -n is 16384. configure the following in mongos.conf file:
net:
port: 30000
bindIp: ctl1m # Listen to local interface only, comment to listen on all interfaces.
maxIncomingConnections: 8000
Then restart mongos service, check the connection number again, db.serverStatus().connections, found 819. What is the problem with my amendment? Do you have any amendment methods? Thank you

I also tried to start mongos service with the following method, but it still had no effect:
/usr/bin/mongos –config /etc/mongod/mongos.conf –maxConns=3000

Ulimit -n has been modified and has come into effect. Most of all, when I modified the service file, the total number of connections was normal:

[unit]
 Description=High-performance, schema-free document-oriented database
 After=network.target
 Documentation=https://docs.mongodb.org/manual
 
 [Service]
 User=root
 Group=root
 Environment="OPTIONS=-f /etc/mongod/mongos.conf"
 ExecStart=/usr/bin/mongos $OPTIONS --maxConns 10000
 ExecStartPre=/usr/bin/mkdir -p /var/run/mongodb
 ExecStartPre=/usr/bin/chown mongod:mongod /var/run/mongodb
 ExecStartPre=/usr/bin/chmod 0755 /var/run/mongodb
 PermissionsStartOnly=true
 PIDFile=/var/run/mongodb/mongos/mongod.pid
 
 # file size
 LimitFSIZE=infinity
 # cpu time
 LimitCPU=infinity
 # virtual memory size
 LimitAS=infinity
 # open files
 LimitNOFILE=64000
 # processes/threads
 LimitNPROC=64000
 # total threads (user+kernel)
 TasksMax=infinity
 TasksAccounting=false
 # Recommended limits for for mongod as specified in
 # http://docs.mongodb.org/manual/reference/ulimit/#recommended-settings
 
 [Install]
 WantedBy=multi-user.target