Perf analysis of `Performance/HashEachMethods` cop

benchmark.rb

require 'securerandom'
require 'benchmark/ips'

hash = {}

1000.times do
  hash[SecureRandom.uuid] = SecureRandom.uuid
end

array = hash.to_a

Benchmark.ips do |x|
  x.report("hash.each do |k, _|") do
    hash.each do |k, _|
      k
    end
  end

  x.report("hash.each do |_, v|") do
    hash.each do |_, v|
      v
    end
  end

  x.report("hash.each_key do |k|") do
    hash.each_key do |k|
      k
    end
  end

  x.report("hash.each_value do |v|") do
    hash.each_value do |v|
      v
    end
  end

  x.compare!
end

Benchmark.ips do |x|
  x.report("array.each do |k, _|") do
    array.each do |k, _|
      k
    end
  end

  x.report("array.each do |_, v|") do
    array.each do |_, v|
      v
    end
  end

  x.report("array.each do |(k, _)|") do
    array.each do |(k, _)|
      k
    end
  end

  x.report("array.each do |(_, v)|") do
    array.each do |(_, v)|
      v
    end
  end

  x.compare!
end

ruby_2.2.txt

Warming up --------------------------------------
 hash.each do |k, _|     1.427k i/100ms
 hash.each do |_, v|     1.395k i/100ms
hash.each_key do |k|     1.441k i/100ms
hash.each_value do |v|
                         1.473k i/100ms
Calculating -------------------------------------
 hash.each do |k, _|     13.932k (± 6.4%) i/s -     69.923k in   5.038565s
 hash.each do |_, v|     14.688k (± 4.5%) i/s -     73.935k in   5.044267s
hash.each_key do |k|     16.219k (± 4.2%) i/s -     82.137k in   5.073343s
hash.each_value do |v|
                         16.176k (± 6.4%) i/s -     81.015k in   5.034800s

Comparison:
hash.each_key do |k|:    16219.5 i/s
hash.each_value do |v|:    16175.6 i/s - same-ish: difference falls within error
 hash.each do |_, v|:    14687.5 i/s - 1.10x  slower
 hash.each do |k, _|:    13932.4 i/s - 1.16x  slower

Warming up --------------------------------------
array.each do |k, _|     1.996k i/100ms
array.each do |_, v|     1.983k i/100ms
array.each do |(k, _)|
                         1.867k i/100ms
array.each do |(_, v)|
                         1.903k i/100ms
Calculating -------------------------------------
array.each do |k, _|     19.611k (± 4.7%) i/s -     99.800k in   5.100368s
array.each do |_, v|     19.239k (± 5.6%) i/s -     97.167k in   5.067313s
array.each do |(k, _)|
                         18.622k (± 5.6%) i/s -     93.350k in   5.029384s
array.each do |(_, v)|
                         18.081k (± 5.9%) i/s -     91.344k in   5.070445s

Comparison:
array.each do |k, _|:    19611.5 i/s
array.each do |_, v|:    19238.6 i/s - same-ish: difference falls within error
array.each do |(k, _)|:    18621.9 i/s - same-ish: difference falls within error
array.each do |(_, v)|:    18080.8 i/s - same-ish: difference falls within error

ruby_2.3.txt

Warming up --------------------------------------
 hash.each do |k, _|     1.504k i/100ms
 hash.each do |_, v|     1.516k i/100ms
hash.each_key do |k|     1.732k i/100ms
hash.each_value do |v|
                         1.845k i/100ms
Calculating -------------------------------------
 hash.each do |k, _|     16.026k (± 2.8%) i/s -     81.216k in   5.071822s
 hash.each do |_, v|     15.805k (± 6.6%) i/s -     78.832k in   5.017760s
hash.each_key do |k|     18.245k (± 4.9%) i/s -     91.796k in   5.044159s
hash.each_value do |v|
                         17.658k (± 5.9%) i/s -     88.560k in   5.033126s

Comparison:
hash.each_key do |k|:    18244.7 i/s
hash.each_value do |v|:    17657.8 i/s - same-ish: difference falls within error
 hash.each do |k, _|:    16025.8 i/s - 1.14x  slower
 hash.each do |_, v|:    15804.8 i/s - 1.15x  slower

Warming up --------------------------------------
array.each do |k, _|     2.497k i/100ms
array.each do |_, v|     2.455k i/100ms
array.each do |(k, _)|
                         2.173k i/100ms
array.each do |(_, v)|
                         2.146k i/100ms
Calculating -------------------------------------
array.each do |k, _|     25.050k (± 2.9%) i/s -    127.347k in   5.088374s
array.each do |_, v|     24.868k (± 2.5%) i/s -    125.205k in   5.038083s
array.each do |(k, _)|
                         21.744k (± 3.4%) i/s -    108.650k in   5.003050s
array.each do |(_, v)|
                         21.706k (± 3.4%) i/s -    109.446k in   5.048154s

Comparison:
array.each do |k, _|:    25049.8 i/s
array.each do |_, v|:    24867.7 i/s - same-ish: difference falls within error
array.each do |(k, _)|:    21744.2 i/s - 1.15x  slower
array.each do |(_, v)|:    21706.4 i/s - 1.15x  slower

ruby_2.4.txt

Warming up --------------------------------------
 hash.each do |k, _|     2.330k i/100ms
 hash.each do |_, v|     2.182k i/100ms
hash.each_key do |k|     2.240k i/100ms
hash.each_value do |v|
                         2.345k i/100ms
Calculating -------------------------------------
 hash.each do |k, _|     22.848k (± 4.9%) i/s -    114.170k in   5.009208s
 hash.each do |_, v|     22.620k (± 5.1%) i/s -    113.464k in   5.029007s
hash.each_key do |k|     23.297k (± 5.2%) i/s -    116.480k in   5.013482s
hash.each_value do |v|
                         22.473k (± 4.9%) i/s -    112.560k in   5.020628s

Comparison:
hash.each_key do |k|:    23297.0 i/s
 hash.each do |k, _|:    22847.6 i/s - same-ish: difference falls within error
 hash.each do |_, v|:    22620.4 i/s - same-ish: difference falls within error
hash.each_value do |v|:    22473.5 i/s - same-ish: difference falls within error

Warming up --------------------------------------
array.each do |k, _|     2.197k i/100ms
array.each do |_, v|     2.178k i/100ms
array.each do |(k, _)|
                         1.863k i/100ms
array.each do |(_, v)|
                         1.946k i/100ms
Calculating -------------------------------------
array.each do |k, _|     22.343k (± 4.8%) i/s -    112.047k in   5.026678s
array.each do |_, v|     23.139k (± 4.8%) i/s -    115.434k in   5.000323s
array.each do |(k, _)|
                         19.211k (± 4.4%) i/s -     96.876k in   5.052660s
array.each do |(_, v)|
                         20.029k (± 5.6%) i/s -    101.192k in   5.068734s

Comparison:
array.each do |_, v|:    23139.3 i/s
array.each do |k, _|:    22342.9 i/s - same-ish: difference falls within error
array.each do |(_, v)|:    20028.9 i/s - 1.16x  slower
array.each do |(k, _)|:    19210.9 i/s - 1.20x  slower

ruby_2.5.txt

Warming up --------------------------------------
 hash.each do |k, _|     2.133k i/100ms
 hash.each do |_, v|     1.981k i/100ms
hash.each_key do |k|     2.173k i/100ms
hash.each_value do |v|
                         2.119k i/100ms
Calculating -------------------------------------
 hash.each do |k, _|     21.165k (± 5.5%) i/s -    106.650k in   5.054973s
 hash.each do |_, v|     20.046k (± 4.9%) i/s -    101.031k in   5.052132s
hash.each_key do |k|     20.912k (± 6.3%) i/s -    104.304k in   5.007624s
hash.each_value do |v|
                         20.806k (± 6.0%) i/s -    103.831k in   5.008755s

Comparison:
 hash.each do |k, _|:    21164.5 i/s
hash.each_key do |k|:    20912.4 i/s - same-ish: difference falls within error
hash.each_value do |v|:    20805.6 i/s - same-ish: difference falls within error
 hash.each do |_, v|:    20046.3 i/s - same-ish: difference falls within error

Warming up --------------------------------------
array.each do |k, _|     1.996k i/100ms
array.each do |_, v|     1.941k i/100ms
array.each do |(k, _)|
                         1.682k i/100ms
array.each do |(_, v)|
                         1.713k i/100ms
Calculating -------------------------------------
array.each do |k, _|     19.417k (± 5.0%) i/s -     97.804k in   5.049637s
array.each do |_, v|     19.945k (± 5.6%) i/s -    100.932k in   5.076490s
array.each do |(k, _)|
                         17.511k (± 4.3%) i/s -     87.464k in   5.004078s
array.each do |(_, v)|
                         18.023k (± 4.2%) i/s -     90.789k in   5.046341s

Comparison:
array.each do |_, v|:    19944.6 i/s
array.each do |k, _|:    19416.7 i/s - same-ish: difference falls within error
array.each do |(_, v)|:    18023.1 i/s - 1.11x  slower
array.each do |(k, _)|:    17511.2 i/s - 1.14x  slower