I recently upgraded to mongoid5 and experienced since then a high frequency of MongoDB server dying out of memory or being OOM Killed.
I digged into the charts at MMS and saw a huge number of open cursors, slowly timeouting, causing a 10 times increase in memory usage:

I haven't investigate more yet to find which query is causing this, and as it's not possible to list cusors yet (SERVER-3090) but I'm pretty sure this comes from the mongo driver. The left side of the chart was Mongoid 4 with Moped.

I'll be happy to dig in to this more to help you but I wanted to post first so maybe you can tell me you're already aware or how I can help you with this.

Thanks ! and keep up the good work ☺