Memory not entirely used, switch on swap

Summary

On a production 3 nodes mongodb replicaset we're observing that the db is using only 40% of the ram, then it uses the swap memory.

Motivation

• Does this affect any team outside of WT?  no
• How likely is it that this use case or problem will occur? It has been a month so far.

• If the problem does occur, what are the consequences and how severe are they? We're observing some severe slowness at application level.

• Is this issue urgent? Yes, this is a production environment.

Acceptance Criteria (Definition of Done)
(When will this ticket be considered done? What is the acceptance criteria for this ticket to be closed?)

• Testing
  We'd like to know if we need to set the cacheSize parameter and how to set it, actually it's not not set. How much should we set the value?

• Documentation update. No

[Optional] Suggested Solution

no