I have a system with 2 shard where each shard is a replica set

in my 3 server run:

1 mongod for replicaset 1
1 mongod for replicaset 2
1 mongod --cfg
1 mongos

after few hours the mongod instance are killed by oom-killer, on dmesg I have:

[12179.894393] Out of memory: kill process 4747 (mongod) score 378148 or a child
[12179.896914] Killed process 4747 (mongod)