Ruby Hash#each vs #keys.each vs #each_key

hash_bench.rb

#!/usr/bin/env ruby
require 'benchmark/ips'

STRING_HASH = Hash['a' => 1, 'b' => 2, 'c' => 3, 'd' => 4, 'e' => 5]
SYMBOL_HASH = Hash[a: 1, b: 2, c: 3, d: 4, e: 5]

Benchmark.ips do |x|
  x.report('string each')     { STRING_HASH.each      {|k,_| } }
  x.report('string keys')     { STRING_HASH.keys.each {|k| } }
  x.report('string each_key') { STRING_HASH.each_key  {|k| } }
  x.report('symbol each')     { SYMBOL_HASH.each      {|k,_| } }
  x.report('symbol keys')     { SYMBOL_HASH.keys.each {|k| } }
  x.report('symbol each_key') { SYMBOL_HASH.each_key  {|k| } }
end

__END__
Output:

Calculating -------------------------------------
         string each    78.009k i/100ms
         string keys    69.856k i/100ms
     string each_key    81.488k i/100ms
         symbol each    77.053k i/100ms
         symbol keys    69.434k i/100ms
     symbol each_key    81.968k i/100ms
-------------------------------------------------
         string each      1.820M (± 5.9%) i/s -      9.127M
         string keys      1.501M (±20.0%) i/s -      7.195M
     string each_key      2.032M (± 5.1%) i/s -     10.186M
         symbol each      1.834M (± 4.8%) i/s -      9.169M
         symbol keys      1.492M (±20.4%) i/s -      7.152M
     symbol each_key      2.021M (± 7.6%) i/s -     10.082M

Ruby:

    ruby 2.2.0p0 (2014-12-25 revision 49005) [x86_64-darwin13]

Hardware:

    Hardware Overview:

      Model Name: MacBook Air
      Model Identifier: MacBookAir5,2
      Processor Name: Intel Core i7
      Processor Speed: 2 GHz
      Number of Processors: 1
      Total Number of Cores: 2
      L2 Cache (per Core): 256 KB
      L3 Cache: 4 MB
      Memory: 8 GB
      Boot ROM Version: MBA51.00EF.B02
      SMC Version (system): 2.5f9

Software:

    System Software Overview:

      System Version: OS X 10.9.5 (13F34)
      Kernel Version: Darwin 13.4.0
      Time since boot: 15 days 1:58