user_with_thread_pool_example.rb

#
# ThreadPool cribbed from: http://burgestrand.se/articles/quick-and-simple-ruby-thread-pool.html
#                          http://burgestrand.se/code/ruby-thread-pool/  
#                          http://burgestrand.se/code/ruby-thread-pool/thread-pool.rb
#
require 'thread'
class ThreadPool
  def initialize(size)
    @size = size
    @jobs = Queue.new
    @pool = Array.new(@size) do |i|
      Thread.new do
        Thread.current[:id] = i
        catch(:exit) do
          loop do
            job, args = @jobs.pop
            job.call(*args)
          end
        end
      end
    end
  end
  def schedule(*args, &block)
    @jobs << [block, args]
  end
  def shutdown
    @size.times do
      schedule { throw :exit }
    end
    @pool.map(&:join)
  end
end

#
# Your Rake Task Body
#
#
namespace :users do
  task :check_all_the_users do
    #
    # Thread Pool will never exceed 150 threads
    # play with this number to find a good balance
    #
    tp = ThreadPool.new(150) 
    Users.find_each(batch_size: 500) do |batch|
      batch.map do |user|
        tp.schedule(user) do |user|
          # Do something with the user here
        end
      end
    end
  end
end