Avoid Using Metaprogramming (Seriously!)

Ruby is sexy, Ruby is cool and its metaprogramming potential offers some really cook features. However you might not realize that your cleverness is slowing down your code.

Today I was working on cleaning up merb_helper a Merb plugin that brings a lot of the stuff Rails developers are used to. In Merb we aim for speed and try to avoid magic.

merb_plugin didn’t receive a lot of love from the main contributors but few features were added by different contributors and the code became hard to maintain.

Looking at the code I quickly found this bad boy:

(Old Merb Time DSL using metaprogramming)

module MetaTimeDSL

    {:second => 1, 
     :minute => 60, 
     :hour => 3600, 
     :day => [24,:hours], 
     :week => [7,:days], 
     :month => [30,:days], 
     :year => [364.25, :days]}.each do |meth, amount|
      define_method "m_#{meth}" do
        amount = amount.is_a?(Array) ? amount[0].send(amount[1]) : amount
        self * amount
      end
      alias_method "m_#{meth}s".intern, "m_#{meth}"
    end

  end
  Numeric.send :include, MetaTimeDSL

The above code looks awful to me and I decided to rewrite it a way I thought would be more efficient:

 module TimeDSL

    def second
      self * 1
    end
    alias_method :seconds, :second

    def minute
      self * 60
    end
    alias_method :minutes, :minute

    def hour
      self * 3600
    end
    alias_method :hours, :hour

    def day
      self * 86400
    end
    alias_method :days, :day

    def week
      self * 604800
    end
    alias_method :weeks, :week

    def month
      self * 2592000
    end
    alias_method :months, :month

    def year
      self * 31471200
    end
    alias_method :years, :year

  end
  Numeric.send :include, TimeDSL

To make sure I was right, I run the following benchmarks:

require 'benchmark'
TIMES = (ARGV[0] || 100_000).to_i

Benchmark.bmbm do |x|

  x.report("metaprogramming 360.seconds") do
    TIMES.times do    
      360.m_seconds
    end
  end

  x.report("no metaprogramming 360.hours") do
    TIMES.times do
      360.seconds
    end
  end

  x.report("metaprogramming 360.minutes") do
    TIMES.times do    
      360.m_minutes
    end
  end

  x.report("no metaprogramming 360.minutes") do
    TIMES.times do
      360.minutes
    end
  end

  x.report("metaprogramming 360.hours") do
    TIMES.times do    
      360.m_hours
    end
  end

  x.report("no metaprogramming 360.hours") do
    TIMES.times do
      360.hours
    end
  end

  x.report("metaprogramming 360.days") do
    TIMES.times do    
      360.m_days
    end
  end

  x.report("no metaprogramming 360.days") do
    TIMES.times do
      360.days
    end
  end

  x.report("metaprogramming 360.weeks") do
    TIMES.times do    
      360.m_weeks
    end
  end

  x.report("no metaprogramming 360.weeks") do
    TIMES.times do
      360.weeks
    end
  end

  x.report("metaprogramming 18.months") do
    TIMES.times do    
      18.m_months
    end
  end

  x.report("no metaprogramming 18.months") do
    TIMES.times do
      18.months
    end
  end

  x.report("metaprogramming 7.years") do
    TIMES.times do    
      7.m_years
    end
  end

  x.report("no metaprogramming 7.years") do
    TIMES.times do
      7.years
    end
  end

end


 Rehearsal ------------------------------------------------------------------
metaprogramming 360.seconds      0.130000   0.000000   0.130000 (  0.133164)
no metaprogramming 360.hours     0.050000   0.000000   0.050000 (  0.042655)
metaprogramming 360.minutes      0.130000   0.000000   0.130000 (  0.133327)
no metaprogramming 360.minutes   0.040000   0.000000   0.040000 (  0.042401)
metaprogramming 360.hours        0.140000   0.000000   0.140000 (  0.134312)
no metaprogramming 360.hours     0.040000   0.000000   0.040000 (  0.043125)
metaprogramming 360.days         0.130000   0.000000   0.130000 (  0.134949)
no metaprogramming 360.days      0.050000   0.000000   0.050000 (  0.043745)
metaprogramming 360.weeks        0.130000   0.000000   0.130000 (  0.135581)
no metaprogramming 360.weeks     0.050000   0.000000   0.050000 (  0.043544)
metaprogramming 18.months        0.130000   0.000000   0.130000 (  0.135234)
no metaprogramming 18.months     0.050000   0.000000   0.050000 (  0.044354)
metaprogramming 7.years          0.140000   0.000000   0.140000 (  0.144062)
no metaprogramming 7.years       0.050000   0.000000   0.050000 (  0.044392)
--------------------------------------------------------- total: 1.260000sec

                                     user     system      total        real
metaprogramming 360.seconds      0.130000   0.000000   0.130000 (  0.132567)
no metaprogramming 360.hours     0.040000   0.000000   0.040000 (  0.042777)
metaprogramming 360.minutes      0.140000   0.000000   0.140000 (  0.132554)
no metaprogramming 360.minutes   0.040000   0.000000   0.040000 (  0.043193)
metaprogramming 360.hours        0.130000   0.000000   0.130000 (  0.133027)
no metaprogramming 360.hours     0.050000   0.000000   0.050000 (  0.042613)
metaprogramming 360.days         0.130000   0.000000   0.130000 (  0.138637)
no metaprogramming 360.days      0.050000   0.000000   0.050000 (  0.043213)
metaprogramming 360.weeks        0.130000   0.000000   0.130000 (  0.134049)
no metaprogramming 360.weeks     0.040000   0.000000   0.040000 (  0.043713)
metaprogramming 18.months        0.140000   0.000000   0.140000 (  0.134941)
no metaprogramming 18.months     0.040000   0.000000   0.040000 (  0.043980)
metaprogramming 7.years          0.150000   0.000000   0.150000 (  0.143389)
no metaprogramming 7.years       0.040000   0.000000   0.040000 (  0.044585)
 0.136591)

The metaprogramming version of the same implementation is almost 3 times slower!

Moral of the story: be careful when using metaprogramming, you might end up slowing down your code considerably.

By Matt Aimonetti