martinciu's dev blog

tl;dr;

Rails 3.1 Asset Pipeline is not slow! It is your app! If your app became slow after upgrading to Rails 3.1 and you use Mongoid – turn preloading models off. If this won’t help profile your app with rack-perftools_profiler.

Rails 3.1

Rails 3.1 is out for a while. It has a bunch of new features and enhancements and a bit of controversy. One of the new features that I wanted to play with was the asset pipeline. So after Release Candidate 4 was out I decided to upgrade zubibu.com to the newest version.

Upgrade

It is pretty easy to upgrade. You can find detailed instructions on David Rice’s blog post. Some additional fixes are in comments below his post so read them as well. If you want to fully use asset pipeline you also have to modify all urls in your stylesheets from:

background: #ffffff url(images/backend/bg.png) repeat-x 0 0

to:

background: #ffffff url(<%= asset_path "backend/bg.png"%>) repeat-x 0 0

and add .erb extension to all modified CSS files. One good regular expression should be enough for updating all your files.

Horror

I restarted application and then the horror begun. It was impossible to load whole page with all the assts. Some assets returned 500 status code, some load in 30+ seconds. I tried some tricks with linking app/assets/images/ folder to public/images/. It helped a lot, as images was not served by rails, but still CSS and JavaScript files was very slow. If I don’t reload pages by CMD+R I was able to work, but styling pages was almost impossible.

Why is it so slow?

I couldn’t believe that asset pipeline is so slow. Yes, it should be slower that serving static files from disk but it should not be THAT slow. I decided to check what is wrong. I created new rails 3.1 application, added one model with scaffold, one stylesheet and one javascript. I started the application and everything was alright. Assets was loading under 50ms. Then I’ve added more images, stylesheets, and javascripts. About twenty files together. And still everything was running smoothly. We use mongoid as an ORM in zubibu, so I migrated the only model to mongoid. But it still was damn fast! It was fast until I added more models to my test application. After adding 30 models (with relations between them) assets started to load in about 3 seconds each. The models was very simple without any logic – just a few fields and references each. It means that it could be models them self but the fact that there are many models.

perftools.rb

I didn’t want to dig in mongoid source code for ever, so decided to use great profiling tool: perftools.rb and because I need to profile web app I used rack-perftools_profiler middleware. I ran profiler two times: first with 1 model and than with 30 models loaded. Reading profile data is quite hard and, I am not the best in this, but I noticed that when app is running with many models loaded it spends much more time in Mongoid::load_models method which looks like this:

14 def load_models(app)
15   return unless ::Mongoid.preload_models
16   app.config.paths["app/models"].each do |path|
17     Dir.glob("#{path}/**/*.rb").sort.each do |file|
18       load_model(file.gsub("#{path}/" , "").gsub(".rb", ""))
19     end
20   end
21 end

And what is ::Mongoid.preload_models? It is config option turn on by default that reloads all models on each request. According to mongoid documentation you can turn it off if don’t use single collection inheritance in your app. To turn it off simply add following line to your congif.applicaton file:

config.mongoid.preload_models = false

I did it in my application and it works! It is usable again!