http://m.onkey.org/active-record-batch-processing-in-parallel-processes

gistfile1.rb

if GC.respond_to?(:copy_on_write_friendly=)
  GC.copy_on_write_friendly = true
end

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)

  batches.each do |batch|
    Process.fork do
      ActiveRecord::Base.establish_connection

      # Do the actual work
      batch.each {|user| .. }
    end
  end

  Process.waitall
end