Use https://github.com/grosser/parallel

gistfile1.rb

jobs_per_process = 100
process_count = 10

User.find_in_batches(:batch_size => jobs_per_process * process_count) do |group|
  batches = group.in_groups(process_count)
  Parallel.each(batches, :in_processes => process_count) do |batch| 
    ActiveRecord::Base.establish_connection
    # Do the actual work
    batch.each {|user| .. }
  end
end