ivars_19.rb

# From http://eigenclass.org/R2/writings/object-size-ruby-ocaml:
#
# Ruby 1.9 stores up to 3 instance variables in
# the object slot without using an external table,
# so an object with one IV will only take 5 words.
# Beyond 3 instance variables, it reverts to an
# external IV array which is resized exponentially
# (factor 1.25) as new elements are added.
#
# Run with 1.9.
#
require 'benchmark'

class Zero
  
end
class One
  def initialize
    @one = 1
  end
end
class Two
  def initialize
    @one = 1
    @two = 2
  end
end
class Three
  def initialize
    @one   = 1
    @two   = 2
    @three = 3
  end
end
class Four
  def initialize
    @one   = 1
    @two   = 2
    @three = 3
    @four  = 4
  end
end
class Five
  def initialize
    @one   = 1
    @two   = 2
    @three = 3
    @four  = 4
    @five  = 5
  end
end
class Six
  def initialize
    @one   = 1
    @two   = 2
    @three = 3
    @four  = 4
    @five  = 5
    @six   = 6
  end
end
class Seven
  def initialize
    @one   = 1
    @two   = 2
    @three = 3
    @four  = 4
    @five  = 5
    @six   = 6
    @seven = 7
  end
end

puts Benchmark.measure {
  100_000.times do
    Zero.new
  end
}
puts Benchmark.measure {
  100_000.times do
    One.new
  end
}
puts Benchmark.measure {
  100_000.times do
    Two.new
  end
}
puts Benchmark.measure {
  100_000.times do
    Three.new
  end
}
puts Benchmark.measure {
  100_000.times do
    Four.new
  end
}
puts Benchmark.measure {
  100_000.times do
    Five.new
  end
}
puts Benchmark.measure {
  100_000.times do
    Six.new
  end
}
puts Benchmark.measure {
  100_000.times do
    Seven.new
  end
}

  user     system      total        real
Twenty  0.230000   0.000000   0.230000 (  0.233681)
   Ten  0.130000   0.000000   0.130000 (  0.130510)
  Nine  0.130000   0.000000   0.130000 (  0.128657)
 Eight  0.100000   0.000000   0.100000 (  0.103342)
 Seven  0.100000   0.000000   0.100000 (  0.094256)
   Six  0.070000   0.000000   0.070000 (  0.076708)
  Five  0.070000   0.000000   0.070000 (  0.064305)
  Four  0.060000   0.000000   0.060000 (  0.063965)
 Three  0.050000   0.000000   0.050000 (  0.044275)
   Two  0.040000   0.000000   0.040000 (  0.038634)
   One  0.030000   0.000000   0.030000 (  0.035694)
  Zero  0.030000   0.000000   0.030000 (  0.027715) 

ruby 1.9.1p376 (2009-12-07 revision 26041) [i386-darwin10.2.0]

Looks roughly O(n) to me.. perhaps not in memory, but was that your point?

The point is the almost significant increase at 4 ivars. My Ruby though (ruby 1.9.1p243 (2009-07-16 revision 24175) [i386-darwin9.8.0]) is relatively old and wrinkled.

  0.050000   0.000000   0.050000 (  0.053586)
  0.060000   0.000000   0.060000 (  0.062729)
  0.060000   0.000000   0.060000 (  0.069838)
  0.070000   0.000000   0.070000 (  0.072942)
  0.100000   0.000000   0.100000 (  0.107794) <-- 4 ivars
  0.110000   0.000000   0.110000 (  0.112962)
  0.130000   0.000000   0.130000 (  0.129338)
  0.130000   0.000000   0.130000 (  0.135911)

P.S: I was investigating whether the moment where Ruby needs to revert to an external IV array is visible in performance tests. I'd say it is.