Learn from http://hooopo.iteye.com/blog/1186120

find_each_parallel.rb

jobs_per_process = 100
process_count = 10

User.find_in_batches(:batch_size => jobs_per_process * process_count, :conditions => "name like '%00'") do |group|
  batches = group.in_groups(process_count)
  batches.each do |batch|
    Process.fork do
      ActiveRecord::Base.establish_connection
      batch.reject { |user| user.nil? }.
        each {|user| puts user.name }
    end
  end
  Process.waitall
end