Debugging a Stuck or Slow Ruby Program

Last updated October 09, 2025

When a Ruby program doesn’t crash but hangs indefinitely or becomes very slow, it can be difficult to debug. Add a sampling profiler to see where the program hangs or slows down.

You can add a crude sampling profiler to any Ruby application. Spin up a thread in the background and report the backtrace of all executing threads. For example, if your application hangs during deployment, add this code to an initializer in config/initializers/sampling_profiler.rb:

Thread.new do
 loop do
 sleep 10 # seconds
 puts "=" * 80;
 Thread.list.each.with_index { |t, i| puts "== Thread #{i}"; puts t.backtrace }
 end
end

In this example, the location of all executing code prints out every 10 seconds. The location gives you a hint of where the program hangs or slows down.

After isolating that code, you can add additional debugging statements or attempt to remediate the problem directly.