We're running OpenLDAP 2.4.40 (the latest available release) with just one replica server (connected via TLS) and have the following picture - http://i.imgur.com/om0lMiy.png

On the graph you can see memory consumption of the slapd process on the host: in the beginngin it started without replica, then replica server was connected (memory consumption became around 4 Gigs) and then OOM (out-of-memory) killer on linux machine just killed the process.

There are ~400 000 users in our ldap database.

OpenLDAP was compiled from sources using "./configure --prefix=/ldap2440 --with-tls --enable-slapd".

Are there any ways to understand what's is going wrong and how to fix it?

This server is really important for us, please share any ideas how to make it stable!