A Ruby app is getting R14 or R15 errors via the logs or Dashboard alerts.
R15 errors occur when an app is using more that the available memory for the current dyno plan. This results in the app using the (slower) hard disk as a fallback which is known as "swapping".
For more information on this issue please see this article: https://devcenter.heroku.com/articles/ruby-memory-use
To track down memory leaks, intensive endpoints or processes in your Ruby application we also recommend using a monitoring addon to gather more detailed information https://elements.heroku.com/addons#monitoring This article https://blog.codeship.com/debugging-a-memory-leak-on-heroku/ (written by one of our Ruby team) is also a useful reference.