Ruby on Rails Memory Profiling

Adding a new gem to the Gemfile is a good way to get a feature live sooner, but it also comes with a cost. A big cost is maintenance, which I won’t get into with this post. Another big one is the resource requirements this new gem suddenly adds to your application. Without tracking this, you can easily end up with an app that takes up several hundred MB memory in object allocation.

In order to stay on top of this we have the excellent memory_profiler gem. It allows you to fairly easily inspect which gems or files allocate the most memory when you boot your application. Once installed gem install memory_profiler, you can run the following script in your Rails app to get a detailed report over memory usage.

require 'memory_profiler'

report = MemoryProfiler.report do
  require_relative '../config/environment'
  # Add specific code here if you want to profile deeper
end

report.pretty_print

Run and store the output for easier inspection in your editor ruby memory-profiler.rb > output.txt.

This will give you a lot of information on how much memory is being used, which gems are responsible and even the lines on which the objects are being allocated.

Only require necessary gems

This process made me notice that I was requiring rubocop even though it was only rarely used. Changing it to gem 'rubocop', require: false in the Gemfile sorted it right out.

Before:
Total allocated: 196866115 bytes (1543730 objects)
Total retained:  61546346 bytes (259608 objects)

After:
total allocated: 172762095 bytes (1343193 objects)
total retained:  54188357 bytes (246362 objects)

Difference:
Less allocated 24104020 bytes ~ 24MB
Less retained 7357989 bytes ~ 7MB

Conclusion

I implore you to try this out on your Rails application, you might be surprised by what you find. I also warmly recommend this talk by Sam Saffron RedDotRuby 2015 - Keynote: Off the Rails by Sam Saffron.