How to profile a Decidim app

The development_app includes a bunch of gems that profile the application. Run the following command in the decidim root’s folder:

bundle exec rake development_app

and then move into it and boot the server

cd development_app
bundle exec rails s


Bullet detects N+1 queries and suggests how to fix them, although it does not catch them all. It is currently configured in config/initializers/bullet.rb to log in the regular rails log and also in its own log/bullet.log. You will now see entries like the following:

user: xxx
USE eager loading detected
  Decidim::Comments::Comment => [:author]
  Add to your query: .includes([:author])
Call stack

It also warns you when there is an unnecessary eager load.


This gem can analyze memory, database, and call stack with flamegraphs. It will show up in development on the top left corner and it gives you all sorts of profiling insights about that page. It will tell you where the response time was spend on in the call stack.

This gem is further enhanced with the flamegraph, stackprof and memory_profiler gems which provide more detailed analysis. Try out by appending ?pp=flamegraph, ?pp=profile-gc or ?pp=analyze-memory to the URL. You can read more about these options at

Profiling best practices

You need to take the insights of these gems with a grain of salt though, if you run this in development. Rails' development settings have nothing to do with a production set up where classes are not reloaded and assets are precompiled and served from a web server. Therefore, you should mimic these settings as much as possible if you want your findings to be realistic.