Commits on Oct 5, 2017

1. fix quadratic performance in FileTask#out_of_date? …

   FileTask#out_of_date? has been changed in 462e403 to call #needed?
   which calls #out_of_date? recursively. In some cases, where the
   graph of dependencies is fairly dense, this leads to quadratic
   performance when it was linear before.
   
   Use #all_prerequisite_tasks to avoid the problem. This also saves
   a File.exist? test as #timestamp already takes it into account.fix quadratic performance in FileTask#out_of_date?
   
   FileTask#out_of_date? has been changed in 462e403 to call #needed?
   which calls #out_of_date? recursively. In some cases, where the
   graph of dependencies is fairly dense, this leads to quadratic
   performance when it was linear before.
   
   Use #all_prerequisite_tasks to avoid the problem. This also saves
   a File.exist? test as #timestamp already takes it into account.
   
   I made a benchmark that measures the difference in duration for
   out_of_date? on 12.0, 12.1 and 12.1 with this commit applied.
   
   The benchmark used to validate the performance creates 5 layers
   of FileTask, where all tasks of the parent layer are connected
   to all tasks of the child. A root task is added at the top
   and #out_of_date? is called on it.
   
   The benchmark varies the number of tasks per layer.
   
   **12.0**
   
   | Tasks per layers | Duration (s) | Standard deviation |
   |------------------|--------------|--------------------|
   | 1                | 1.32e-05     | 0.96e-05           |
   | 2                | 8.69e-05     | 1.11e-06           |
   | 5                | 1.84e-05     | 2.43e-06           |
   | 8                | 2.89e-05     | 1.05e-05           |
   | 10               | 3.35e-05     | 4.12e-06           |
   | 15               | 4.97e-05     | 6.74e-06           |
   | 20               | 6.19e-05     | 6.23e-06           |
   
   **12.1**
   
   | Tasks per layers | Duration (s) | Standard deviation |
   |------------------|--------------|--------------------|
   | 1                | 7.00e-05     | 5.62e-05           |
   | 2                | 3.98e-04     | 7.38e-05           |
   | 5                | 2.32e-02     | 1.02e-03           |
   | 8                | 0.22         | 0.006              |
   | 10               | 0.65         | 0.006              |
   | 15               | 4.78         | 0.048              |
   | 20               | 20           | 0.49               |
   
   **PR 224**
   
   | Tasks per layers | Duration (s) | Standard deviation |
   |------------------|--------------|--------------------|
   | 1                | 4.47e-05     | 2.68e-05           |
   | 2                | 7.56e-05     | 2.92e-05           |
   | 5                | 2.42e-03     | 4.16e-05           |
   | 8                | 0.51e-03     | 7.21e-05           |
   | 10               | 0.77e-03     | 0.13e-03           |
   | 15               | 14.2e-03     | 0.11e-03           |
   | 20               | 24.2e-03     | 0.16e-03           |
   
   Benchmarking code:
   
   ~~~ ruby
   require 'rake'
   
   LAYER_MAX_SIZE = 20
   LAYER_COUNT = 5
   
   def measure(size)
       app = Rake::Application.new
       layers = (0...LAYER_COUNT).map do |layer_i|
           (0...size).map do |i|
               app.define_task(Rake::FileTask, "#{layer_i}_#{i}")
           end
       end
       layers.each_cons(2) do |parent, child|
           child_names = child.map(&:name)
           parent.each { |t| t.enhance(child_names) }
       end
       root = app.define_task(Rake::FileTask, "root")
       root.enhance(layers[0].map(&:name))
   
       tic = Time.now
       root.send(:out_of_date?, tic)
       Time.now - tic
   end
   
   FileUtils.touch "root"
   sleep 0.1
   LAYER_COUNT.times do |layer_i|
       LAYER_MAX_SIZE.times do |i|
           FileUtils.touch "#{LAYER_COUNT - layer_i - 1}_#{i}"
       end
       sleep 0.1
   end
   
   COUNT = 100
   [1, 2, 5, 8, 10, 15, 20].each do |size|
       mean = 0
       sum_squared_deviations = 0
   
       COUNT.times do |count|
           duration = measure(size)
           old_mean = mean
           mean     = old_mean + (duration - old_mean) / (count + 1)
           sum_squared_deviations = sum_squared_deviations + (duration - old_mean) * (duration - mean)
       end
   
       puts "#{size} #{mean} #{Math.sqrt(sum_squared_deviations / (COUNT - 1))}"
   end
   ~~~

   

   doudou committed Oct 5, 2017
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