Rails track boot time

config_application.rb

# config/application.rb

module MyApp
  class Application < Rails::Application
    # ....
    # NOTE: This is a workaround to measure boot time after all initializers are loaded
    # `Rails.application.config.after_initialize` included
    # https://guides.rubyonrails.org/configuring.html#initializers
    initializer :boot_time, after: :finisher_hook do
      boot_time = BootTime.instance.boot_time_elapsed_milliseconds
      Rails.logger.info("Boot time: #{boot_time}ms")
    end
  end
end

config_boot.rb

# config/boot.rb
# frozen_string_literal: true

# Measure boot time
require_relative "../lib/boot_time"
BootTime.instance.store_start_timestamp

ENV["BUNDLE_GEMFILE"] ||= File.expand_path("../Gemfile", __dir__)

require "bundler/setup" # Set up gems listed in the Gemfile.
require "bootsnap/setup" # Speed up boot time by caching expensive operations.

lib_boot_time.rb

# lib/boot_time.rb
# frozen_string_literal: true

require "singleton"

class BootTime
  include Singleton

  attr_reader :start_timestamp

  def store_start_timestamp
    return if @start_timestamp

    @start_timestamp = Process.clock_gettime(Process::CLOCK_MONOTONIC)
  end

  def boot_time_elapsed_milliseconds
    return 0.0 unless @start_timestamp

    @boot_time_elapsed_milliseconds ||= (time_since_start * 1000).round(2)
  end

  def reset!
    @start_timestamp = nil
    @boot_time_elapsed_milliseconds = nil
  end

  private

  def time_since_start
    Process.clock_gettime(Process::CLOCK_MONOTONIC) - @start_timestamp
  end
end