Secret Revealed: Why Ruby 2.0 Loads Files Faster

Steve Robinson

1 min read

I’ve been working on a codebase which is Rails 4.0 and Ruby 1.9.3 and something that’s annoying about this app is its incredibly large load time. It takes about 26 seconds to boot up.

bundle exec rake environment  22.22s user 1.14s system 91% cpu 25.632 total

Now we’re not doing anything fancy during the bootup so I figured it must the gems that we are loading that’s taking so much time. While googling for ways to debug the boot time I came across this really cool gem called Bumbler which prints out the load times for each and every gem in your app.
These are some of the gems that were taking lots of time to load.
Its amazing that it takes 3 seconds to load Carrierwave. I double checked my initializers to make sure there’s nothing that could slow down the loading and there was none. So is Ruby’s `require` slow?
To find out I did some digging and ended up at this bug report which talks about improvements to Ruby’s require method. It seems when requiring a file Ruby iterates through the $LOAD_PATHS array to see if the file is loaded which is extremely inefficient if there are many files to load.
The patch made uses a hash to maintain the loaded files so that it can be looked up in O(1) time.
This is explained by Xavier Shay here.
In addition to this the string objects in the arrays $LOAD_PATHS and $LOADED_FEATURES are also frozen so these can be cached. Freezing strings makes them immutable. This is well explained in this other article by Masaya Tarui, author of the patch.
All this has made Ruby 2.0 require files much faster. I temporarily switched to Ruby 2.2 and take a look at this –

bundle exec rake environment  3.95s user 0.85s system 99% cpu 4.822 total

Bumbler report:
That’s a whopping 81% improvement over Ruby 1.9.3 in my case. Your mileage may vary. But this is amazing. So as soon as I get the chance I am going to upgrade to Ruby 2.2 🙂

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