HI

we ran mongodb sharded cluster for several weeks, it works well.
but we found mongos would creates lots of connections to primary nods in a shard set.
when this occured, the respone time of this cluster will increase sharply
and bigger than the number we can accepted.

we found some typical statement from logs, as following:

logs from mongos log:

2018-01-25T22:10:00.006+0800 I ASIO [NetworkInterfaceASIO-TaskExecutorPool-8-0] Connecting to 10.136.142.35:28000
2018-01-25T22:10:00.008+0800 I ACCESS [conn2318094] Successfully authenticated as principal useeeeeeeer on admin
2018-01-25T22:10:00.013+0800 I ASIO [NetworkInterfaceASIO-TaskExecutorPool-1-0] Successfully connected to 10.136.180.52:28000, took 10111ms (10 connections now open to 10.136.180.52:
28000)
2018-01-25T22:10:00.013+0800 I ASIO [NetworkInterfaceASIO-TaskExecutorPool-12-0] Failed to connect to 10.136.179.52:28000 - HostUnreachable: End of file
2018-01-25T22:10:00.013+0800 I ASIO [NetworkInterfaceASIO-TaskExecutorPool-12-0] Failed to close stream: Transport endpoint is not connected
2018-01-25T22:10:00.014+0800 I ASIO [NetworkInterfaceASIO-TaskExecutorPool-8-0] Connecting to 10.136.5.44:28000

logs from mongod:
2018-01-25T22:10:00.002+0800 I NETWORK [thread2] connection accepted from 10.136.180.33:30285 #7564018 (10233 connections now open)
2018-01-25T22:10:00.006+0800 I NETWORK [thread2] connection refused because too many open connections: 10240

notes: 1. we sets the maxincomming conections of mongod is 10240.
2. when the cluster goes into usual status, the conections of mongod is about several hunderds.

we could offer the detail logs and ftdc logs so as to find the root cause.

Thanks.