为什么map {}.compact比each_with_object([])更快？

Question

我做了一些基准测试:

require 'benchmark'

words = File.open('/usr/share/dict/words', 'r') do |file|
  file.each_line.take(1_000_000).map(&:chomp)
end

Benchmark.bmbm(20) do |x|
  GC.start
  x.report(:map) do
    words.map do |word|
      word.size if word.size > 5
    end.compact
  end

  GC.start
  x.report(:each_with_object) do
    words.each_with_object([]) do |word, long_sizes|
      long_sizes << word.size if word.size > 5
    end
  end
end

输出(ruby 2.3.0):

Rehearsal --------------------------------------------------------
map                    0.020000   0.000000   0.020000 (  0.016906)
each_with_object       0.020000   0.000000   0.020000 (  0.024695)
----------------------------------------------- total: 0.040000sec

                           user     system      total        real
map                    0.010000   0.000000   0.010000 (  0.015004)
each_with_object       0.020000   0.000000   0.020000 (  0.024183)

因为我觉得我无法理解这一点each_with_object应该会更快:它只需要1环和1个新对象的情况下,而不是2环和2个新的对象创建一个新的阵列时,我们结合map和compact.有任何想法吗？

Answer 1

Array#<<如果原始内存空间没有足够的空间来容纳新项目,则需要重新分配内存.请参阅实现,尤其是此行

VALUE target_ary = ary_ensure_room_for_push(ary, 1);

虽然Array#map不必重新分配内存,因为它已经知道结果数组的大小.特别参见实施

collect = rb_ary_new2(RARRAY_LEN(ary));

它分配与原始数组相同大小的内存.