HI

when lots  inserts,  such as  20k per second ,

mongod  was slow , and logs print:
2018-09-05T14:41:34.987+0800 I COMMAND  [conn2021516] serverStatus was very slow:

{ after basic: 0, after asserts: 0, after backgroundFlushing: 0, a fter connections: 0, after dur: 0, after extra_info: 603, after globalLock: 603, after locks: 603, after network: 603, after opLatencies: 603, after  opcounters: 603, after opcountersRepl: 603, after repl: 603, after security: 603, after sharding: 603, after storageEngine: 603, after tcmalloc: 60 3, after wiredTiger: 603, at end: 1025 }

 
As a dba, we counldn't  find  the  bottleneck is.