File.read vs File.read with size

gistfile1.rb

require 'benchmark'
bigfiles = %w[ddd1 ddd2 ddd3] # all copies of the same 6MB file

puts RUBY_DESCRIPTION
puts "Bigfile size is ", File.size('ddd1')
Benchmark.bmbm do |bm|
 bm.report("straight read") { bigfiles.each {|bigfile| File.read(bigfile) } }
 bm.report("read w/ size")  { bigfiles.each {|bigfile| File.read(bigfile,File.size(bigfile)) } }
end

#=> jruby 1.7.2 (1.9.3p327) 2013-01-04 302c706 on Java HotSpot(TM) 64-Bit Server VM 1.8.0-ea-b55 +indy [darwin-x86_64]
#=> Bigfile size is 6200808
#=> Rehearsal -------------------------------------------------
#=> straight read   0.320000   0.050000   0.370000 (  0.203000)
#=> read w/ size    0.030000   0.000000   0.030000 (  0.023000)
#=> ---------------------------------------- total: 0.400000sec
#=> 
#=>                     user     system      total        real
#=> straight read   0.190000   0.010000   0.200000 (  0.137000)
#=> read w/ size    0.010000   0.010000   0.020000 (  0.012000)


### Faster is jruby --1.8
#=> jruby 1.7.2 (ruby-1.8.7p370) 2013-01-04 302c706 on Java HotSpot(TM) 64-Bit Server VM 1.8.0-ea-b55 +indy [darwin-x86_64]
#=> Bigfile size is 6200808
#=> Rehearsal -------------------------------------------------
#=> straight read   0.080000   0.010000   0.090000 (  0.037000)
#=> read w/ size    0.040000   0.000000   0.040000 (  0.025000)
#=> ---------------------------------------- total: 0.130000sec
#=> 
#=>                     user     system      total        real
#=> straight read   0.030000   0.010000   0.040000 (  0.024000)
#=> read w/ size    0.010000   0.000000   0.010000 (  0.012000)

# Much, much faster in MRI

#=> ruby 1.9.3p362 (2012-12-25 revision 38607) [x86_64-darwin11.4.2]
#=> Bigfile size is 6200808
#=> Rehearsal -------------------------------------------------
#=> straight read   0.000000   0.000000   0.000000 (  0.009195)
#=> read w/ size    0.010000   0.010000   0.020000 (  0.010848)
#=> ---------------------------------------- total: 0.020000sec
#=> 
#=>                     user     system      total        real
#=> straight read   0.000000   0.010000   0.010000 (  0.009717)
#=> read w/ size    0.000000   0.010000   0.010000 (  0.009302)