Roger Pack wrote in post #1114319:
> What doesn't work about it?

So I've made the run again with a solution finally below:

See info here:
https://github.com/ruby-prof/ruby-prof/issues/86
https://github.com/skaes/rvm-patchsets

Solution: patch the MRI using RVM:

rvm reinstall 1.9.3 --patch railsexpress
gem-ruby-1.9.3-p448 install ruby-prof

Now the output of my test code looks like:

$ ruby-1.9.3-p448 test.rb
Thread ID: 14925700
Fiber ID: 17378400
Total: 3842.242188
Sort by: self_time

 %self      total      self      wait     child     calls  name
 56.94   2187.766  2187.766     0.000     0.000    20000   String#chars
 32.79   1259.953  1259.953     0.000     0.000    10000   Array#join
 10.17    390.812   390.812     0.000     0.000    10000   Array#shuffle
  0.07   3842.242     2.852     0.000  3839.391        1   Global#[No 
method]
  0.01   1563.102     0.336     0.000  1562.766    10000 
Enumerable#to_a
  0.01   3839.391     0.336     0.000  3839.055        1   Integer#times
  0.00   1562.766     0.188     0.000  1562.578    10000 
Enumerator#each

* indicates recursively called methods

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