Memory usage for MongoMapper' where query

gistfile1.rb

# load_test_books.rb

require 'rubygems'
require 'mongo_mapper'

MongoMapper.connection = Mongo::Connection.new('127.0.0.1', 27017)
MongoMapper.database   = 'dpu-test'

class Book
  include MongoMapper::Document
  key  :name,     String
  key  :price,    Integer
  key  :category, String
end

def load_test_books(nums = 100 * 100 * 10)
  nums.times do |num|
    Book.create(:price => rand(50) + 20, :name => "Ruby##{num+1}", :category => 'ruby')
  end
end

# load 100,000 books
load_test_books


# query_where_all.rb

p Book.count
before_memory_usage = `ps -o rss= -p #{$$}`.to_i
Book.where(:category => 'ruby').all
after_memory_usage = `ps -o rss= -p #{$$}`.to_i

p 'memory usage before sum book price:'
p before_memory_usage

p 'memory usage after sum book price:'
p after_memory_usage

p after_memory_usage - before_memory_usage       #in my computer printed 234.8m


# query_where.rb

p Book.count
before_memory_usage = `ps -o rss= -p #{$$}`.to_i
Book.where(:category => 'ruby')
after_memory_usage = `ps -o rss= -p #{$$}`.to_i

p 'memory usage before sum book price:'
p before_memory_usage

p 'memory usage after sum book price:'
p after_memory_usage

p after_memory_usage - before_memory_usage    #in my computer only print 4kb

# paginate_where.rb
p Book.count
before_memory_usage = `ps -o rss= -p #{$$}`.to_i
total_price = 0
books = Book.where(:category => 'ruby')
pages = books.count / 100 + 1
1.upto pages do |page|
  Book.where(:category => 'ruby').paginate(:page => page, :per_page => 100).each { |book| total_price += book.price }
end

after_memory_usage = `ps -o rss= -p #{$$}`.to_i

p 'memory usage before sum book price:'
p before_memory_usage

p 'memory usage after sum book price:'
p after_memory_usage

p after_memory_usage - before_memory_usage   # print 28kb in my computer