WiredTiger cache limited, cgroup limits correctly recognized. Still Docker image OOMs. Why?

Hi there!

I am quite new to MongoDB. So this may be a very trivial issue. Nevertheless, I do not find any current resources, issues or the like which match my OOM issue.

I have deployed the latest MongoDB Docker image on a dedicated Kubernetes node (6 GB RAM) in standalone mode. I have assigned 4 GB to the MongoDB container. I have limited the WiredTiger cache to 2 GB. My working set is ~15 GB, my indexes ~150 MB.

All in all, without much experience on this topic, I would say this is a decent setup. At least it should work without running out of memory. However, depending on DB traffic, mongod OOMs after an 30-60min.

What am I doing wrong here? I’d highly appreciate any advice on how to get this deployment stable. I have attached db.hostInfo() and db.serverStatus() outputs as a paste.