From: Mike Gold <mike.gold.4433@...>
Date: 2009-01-23T23:27:59+09:00
Subject: Re: String doesnt auto dup on modification

Brian Candler wrote:
> 
> You could try deferring the dup until the first time you call a 
> destructive method on the string, but the complexity overhead is 
> unlikely to be worth it.

Careful.  Intuition is worse than useless here.  The only way to know is 
to measure the particular case in question.

class Person
  def initialize(name)
    @name = name
  end
  def name_cow
    CopyOnWriteString.new(@name)
  end
  def name_dup
    @name.dup
  end
end

require 'benchmark'

n = 10_000
sizes = [100, 1000, 10_000, 100_000]
objects = sizes.inject(Hash.new) { |acc, size|
  acc.merge!(size => Person.new("x"*size))
}

sizes.each { |size|
  object = objects[size]
  puts "-"*40
  puts "iterations: #{n} size: #{size}"
  Benchmark.bm { |x|
    x.report("cow w/o change") {
      n.times { object.name_cow }
    }
    x.report("dup w/o change") {
      n.times { object.name_dup }
    }
    x.report("cow w/  change") {
      n.times { object.name_cow << "y" }
    }
    x.report("dup w/  change") {
      n.times { object.name_dup << "y" }
    }
  }
}

ruby 1.8.7 (2008-08-11 patchlevel 72) [i386-cygwin]
----------------------------------------
iterations: 10000 size: 100
      user     system      total        real
cow w/o change  0.031000   0.000000   0.031000 (  0.031000)
dup w/o change  0.032000   0.000000   0.032000 (  0.031000)
cow w/  change  0.171000   0.000000   0.171000 (  0.172000)
dup w/  change  0.047000   0.000000   0.047000 (  0.047000)
----------------------------------------
iterations: 10000 size: 1000
      user     system      total        real
cow w/o change  0.032000   0.000000   0.032000 (  0.031000)
dup w/o change  0.046000   0.000000   0.046000 (  0.047000)
cow w/  change  0.172000   0.000000   0.172000 (  0.172000)
dup w/  change  0.063000   0.000000   0.063000 (  0.062000)
----------------------------------------
iterations: 10000 size: 10000
      user     system      total        real
cow w/o change  0.031000   0.000000   0.031000 (  0.032000)
dup w/o change  0.109000   0.000000   0.109000 (  0.109000)
cow w/  change  0.282000   0.000000   0.282000 (  0.281000)
dup w/  change  0.156000   0.000000   0.156000 (  0.156000)
----------------------------------------
iterations: 10000 size: 100000
      user     system      total        real
cow w/o change  0.031000   0.000000   0.031000 (  0.032000)
dup w/o change  0.672000   0.000000   0.672000 (  0.672000)
cow w/  change  1.406000   0.000000   1.406000 (  1.406000)
dup w/  change  1.219000   0.000000   1.219000 (  1.219000)

Destructive methods are less common in real code, and especially so when 
the string comes from a attr_reader method.  It is likely that the case 
to optimize is the non-destructive call (the first of each quadruplet 
above).  But we have to profile the specific situation.
-- 
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