test.rb

#!/usr/bin/env ruby

require 'benchmark'

# global executorの並列度を高めにしてから...
def concurrent_download(files, future_provider: Concurrent::Future)
  require 'concurrent/future'

  # 最大20ファイルずつ並列ダウンロード
  n = 20

  download_sets = files.each_slice(n).map { |fs|
    fs.map { |f| future_provider.execute { sleep(rand); puts "Downloading #{f}"; f } }
  }

  downloaded_files = future_provider.execute {
    download_sets.inject([]) { |completed, ds|
      ds.each { |d|
        completed << d.value
      }
      completed
    }
  }.value
end

class Task
  attr_accessor :thread, :result

  def self.execute(&block)
    task = Task.new
    t = Thread.start(task, block) do |t, b|
      t.result = b.call
    end
    task.thread = t
  end

  def value
    thread.join
    result
  end
end

Benchmark.bm do |x|
  x.report do
    files = 1.upto(30).map { |num| "file#{num}" }
    r = concurrent_download(files, future_provider: Task)
    puts "Completed downloading #{files.count} files: #{r}"
  end
end