Strange MongoDB behavior in an OpenVZ container. 100% CPU load. What's the matter?

Question

MongoDB

Hello.
There is the latest Debian 7.6 hosting the latest ProxmoxVE. One of the containers has a MongoDB database deployed. In another container - a small application on nodejs.
The application receives a request from the user, then accesses the database and sees if the same record is in it. If there is - increases the "counter", if not - adds a record.
Problem:
With a minimum number of requests, there is a load under 100% CPU, although before the upgrade (and subsequent downgrade), there was no such problem.
Collection - capped collection, indexes (seemingly) are configured.
What information to provide to understand the issue?

Answer 1

turn it on and see slow log
and just see how your requests are being executed
and how many of them