Benchmarking for Hash#compact and Hash#compact!

hash_ext.rb

require 'benchmark'

class Hash
  def compact_via_each
    result = {}
    each { |k, v| result[k] = v unless v.nil? }
    result
  end
  
  def compact_via_reject
    reject { |_, v| v.nil? }
  end
  
  def compact_via_select
    select { |_, v| !v.nil? }
  end

  def compact_via_reject!
    reject! { |_, v| v.nil? }
  end
  
  def compact_via_select!
    select! { |_, v| !v.nil? }
  end
end

CYCLES = 1_000_000
HASH = { a: 1, b: 2, c: nil }

Benchmark.bmbm do |x|
  x.report('via each')   { CYCLES.times{ HASH.compact_via_each } }
  x.report('via reject') { CYCLES.times{ HASH.compact_via_reject } }
  x.report('via select') { CYCLES.times{ HASH.compact_via_select } }
  x.report('via reject!') { CYCLES.times{ HASH.compact_via_reject! } }
  x.report('via select!') { CYCLES.times{ HASH.compact_via_select! } }
end

result.txt

Rehearsal -----------------------------------------------
via each      2.290000   0.040000   2.330000 (  2.343092)
via reject    2.940000   0.080000   3.020000 (  3.040754)
via select    1.790000   0.090000   1.880000 (  1.889668) *
via reject!   0.590000   0.010000   0.600000 (  0.590372) **
via select!   0.630000   0.000000   0.630000 (  0.636196)
-------------------------------------- total: 8.460000sec

                  user     system      total        real
via each      1.900000   0.020000   1.920000 (  1.923040)
via reject    2.570000   0.060000   2.630000 (  2.638467)
via select    1.490000   0.090000   1.580000 (  1.586627) *
via reject!   0.580000   0.000000   0.580000 (  0.592631) **
via select!   0.630000   0.000000   0.630000 (  0.630784)

* better for list
** better for replace

Why is there so big difference?
I've made come changes and here is my results.

...
Benchmark.bmbm do |x|
  x.report('via each') { CYCLES.times { HASH.dup.compact_via_each } }
  x.report('via reject') { CYCLES.times { HASH.dup.compact_via_reject } }
  x.report('via select') { CYCLES.times { HASH.dup.compact_via_select } }
  x.report('via reject!') { CYCLES.times { HASH.dup.compact_via_reject! } }
  x.report('via select!') { CYCLES.times { HASH.dup.compact_via_select! } }
end

MRI 2.1.4

Rehearsal -----------------------------------------------
via each      1.290000   0.010000   1.300000 (  1.296412)
via reject    1.150000   0.000000   1.150000 (  1.158505)
via select    1.160000   0.000000   1.160000 (  1.158948)
via reject!   0.960000   0.000000   0.960000 (  0.965918)
via select!   0.980000   0.000000   0.980000 (  0.983304)
-------------------------------------- total: 5.550000sec

                  user     system      total        real
via each      1.200000   0.000000   1.200000 (  1.206583)
via reject    1.160000   0.000000   1.160000 (  1.156540)
via select    1.240000   0.010000   1.250000 (  1.247227)
via reject!   0.900000   0.000000   0.900000 (  0.904802)
via select!   0.940000   0.000000   0.940000 (  0.941423)

jRuby-head

Rehearsal -----------------------------------------------
via each      6.500000   0.200000   6.700000 (  2.500000)
via reject    2.150000   0.030000   2.180000 (  1.262000)
via select    2.140000   0.020000   2.160000 (  1.265000)
via reject!   1.860000   0.040000   1.900000 (  1.115000)
via select!   1.890000   0.010000   1.900000 (  1.082000)
------------------------------------- total: 14.840000sec

                  user     system      total        real
via each      2.020000   0.010000   2.030000 (  1.171000)
via reject    2.060000   0.020000   2.080000 (  1.288000)
via select    2.090000   0.020000   2.110000 (  1.269000)
via reject!   1.820000   0.010000   1.830000 (  1.090000)
via select!   1.830000   0.010000   1.840000 (  1.070000)