bm.rb

require 'benchmark'

FooBar = Struct.new(:foo, :bar)
data   = (0..9999).map{ |i| FooBar.new(i, i+1) }

Benchmark.bmbm do |b|
  b.report('inject(fold)'){
    1000.times{ data.inject({}){|h, d| h[d.foo] = d.bar; h }}}

  b.report('Hash+map'){
    1000.times{ Hash[data.map {|d| [d.foo, d.bar]} ]}}
end

JRuby

00:58 /p/tmp> jruby bm.rb
Rehearsal ------------------------------------------------
inject(fold)   2.898000   0.000000   2.898000 (  2.821000)
Hash+map       2.412000   0.000000   2.412000 (  2.412000)
--------------------------------------- total: 5.310000sec

                   user     system      total        real
inject(fold)   2.477000   0.000000   2.477000 (  2.477000)
Hash+map       2.292000   0.000000   2.292000 (  2.292000)

JRuby (with 5 times more data)

01:16 /p/tmp> jruby bm.rb
Rehearsal ------------------------------------------------
inject(fold)  16.940000   0.000000  16.940000 ( 16.864000)
Hash+map      16.694000   0.000000  16.694000 ( 16.694000)
-------------------------------------- total: 33.634000sec

                   user     system      total        real
inject(fold)  16.385000   0.000000  16.385000 ( 16.385000)
Hash+map      16.606000   0.000000  16.606000 ( 16.606000)


01:19 /p/tmp> jruby --server bm.rb
Rehearsal ------------------------------------------------
inject(fold)  15.480000   0.000000  15.480000 ( 15.405000)
Hash+map      13.348000   0.000000  13.348000 ( 13.348000)
-------------------------------------- total: 28.828000sec

                   user     system      total        real
inject(fold)  15.089000   0.000000  15.089000 ( 15.089000)
Hash+map      12.999000   0.000000  12.999000 ( 12.999000)

MRI 1.9.2

01:07 /p/tmp> ruby bm.rb
Rehearsal ------------------------------------------------
inject(fold)   6.120000   0.150000   6.270000 (  6.256518)
Hash+map       6.650000   0.020000   6.670000 (  6.672158)
-------------------------------------- total: 12.940000sec

                   user     system      total        real
inject(fold)   6.100000   0.140000   6.240000 (  6.231792)
Hash+map       6.500000   0.020000   6.520000 (  6.499045)

Rubinius

01:08 /p/tmp> rbx bm.rb
Rehearsal ------------------------------------------------
inject(fold)  14.370269   0.136047  14.506316 ( 14.640777)
Hash+map      26.914388   0.131548  27.045936 ( 27.099733)
-------------------------------------- total: 41.552252sec

                   user     system      total        real
inject(fold)  14.399943   0.115520  14.515463 ( 14.516802)
Hash+map      27.300467   0.124162  27.424629 ( 27.511583)