We have a Mongo Replica Set of 1 Primary, 3 secondary & 1 Arbiter server. During the recent times, we could see high memory utilization (95 to 99%) in all members of replica set (except Arbiter). On analysis, all we could see is an increase in number of mongo queries which are taking more than 100ms.
At the moment, we see a minimum of 100,000 queries taking more than 100ms every day. Among those queries, the below insert query counts to 80% of the total,

insert <db name>.system.indexes keyUpdates:0 locks(micros) w:55 XXXms

Could you please check and let us know what could be the trigger behind this particular query ? We did check with our application developers and they did confirm that it isn't from the application.

Thanks.