Rails track boot time

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.

initializers_boot_time.rb

# config/initializers/boot_time.rb
# frozen_string_literal: true

Rails.application.config.after_initialize do
  boot_time = BootTime.instance.boot_time_elapsed_milliseconds
  Rails.logger.info("Boot time: #{boot_time}ms")
end

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