Since version 2 Ruby comes with DTrace probes, which lets you dynamically instrument a running program.

Unfortunately the Ruby shipped with MacOS X doesn’t have DTrace enabled, when you try to list the available probes, you’ll see this:

martin$ sudo dtrace  -c "ruby -v" -l -m ruby
   ID   PROVIDER            MODULE                          FUNCTION NAME
dtrace: failed to match :ruby::: No probe matches description

So you need to install one that has. I perfer rbenv and how to install it is documented here so I am not going to show how do it. After a successful install you should see this:

martin$ sudo dtrace  -c "`rbenv which ruby` -v" -l -m ruby
   ID   PROVIDER            MODULE                          FUNCTION NAME
341053  ruby87551              ruby                       rb_ary_drop array-create
341054  ruby87551              ruby                    rb_ary_product array-create
341055  ruby87551              ruby       rb_ary_repeated_combination array-create
341056  ruby87551              ruby       rb_ary_repeated_permutation array-create
341057  ruby87551              ruby                rb_ary_combination array-create
341058  ruby87551              ruby                rb_ary_permutation array-create
341059  ruby87551              ruby                     rb_ary_sample array-create
341060  ruby87551              ruby                        rb_ary_and array-create
341061  ruby87551              ruby                       rb_ary_diff array-create
341062  ruby87551              ruby                      rb_ary_times array-create
341063  ruby87551              ruby                 rb_ary_slice_bang array-create
341064  ruby87551              ruby                     rb_ary_reject array-create
341065  ruby87551              ruby                   empty_ary_alloc array-create
341066  ruby87551              ruby                     rb_ary_subseq array-create
341067  ruby87551              ruby                        rb_ary_new array-create
341068  ruby87551              ruby                           ary_new array-create
341069  ruby87551              ruby                     vm_call_cfunc cmethod-entry
341070  ruby87551              ruby                     vm_call0_body cmethod-entry
341071  ruby87551              ruby                      vm_exec_core cmethod-entry
341072  ruby87551              ruby                     vm_call_cfunc cmethod-return
341073  ruby87551              ruby                     vm_call0_body cmethod-return
341074  ruby87551              ruby             rb_vm_pop_cfunc_frame cmethod-return
341075  ruby87551              ruby                      vm_exec_core cmethod-return
341076  ruby87551              ruby               rb_require_internal find-require-entry
341077  ruby87551              ruby               rb_require_internal find-require-return
341078  ruby87551              ruby                          gc_start gc-mark-begin
341079  ruby87551              ruby                          gc_start gc-mark-end
341080  ruby87551              ruby                     gc_sweep_step gc-sweep-begin
341081  ruby87551              ruby                     gc_sweep_step gc-sweep-end
341082  ruby87551              ruby              m_core_hash_from_ary hash-create
341083  ruby87551              ruby                      vm_exec_core hash-create
341084  ruby87551              ruby                  empty_hash_alloc hash-create
341085  ruby87551              ruby                         rb_f_load load-entry
341086  ruby87551              ruby                         rb_f_load load-return
341087  ruby87551              ruby    rb_clear_method_cache_by_class method-cache-clear
341088  ruby87551              ruby               invoke_block_from_c method-entry
341089  ruby87551              ruby                      vm_exec_core method-entry
341090  ruby87551              ruby               invoke_block_from_c method-return
341091  ruby87551              ruby                           vm_exec method-return
341092  ruby87551              ruby                      vm_exec_core method-return
341093  ruby87551              ruby                      rb_obj_alloc object-create
341094  ruby87551              ruby                        yycompile0 parse-begin
341095  ruby87551              ruby                        yycompile0 parse-end
341096  ruby87551              ruby                   setup_exception raise
341097  ruby87551              ruby               rb_require_internal require-entry
341098  ruby87551              ruby               rb_require_internal require-return
341099  ruby87551              ruby                   empty_str_alloc string-create
341100  ruby87551              ruby                  rb_str_resurrect string-create
341101  ruby87551              ruby                    str_new_static string-create
341102  ruby87551              ruby                          str_new0 string-create
341103  ruby87551              ruby         register_static_symid_str symbol-create
341104  ruby87551              ruby                     dsymbol_alloc symbol-create

DTrace require elevated privileges and thus needs to be run as root, and to avoid having to type your password over and over again, you can update /etc/sudoers (using visudo off course) and add the below line, which will let you use DTrace without a password:

%admin ALL = (root) NOPASSWD: /usr/sbin/dtrace

Trying it out

Now that we have a Ruby version with DTrace probes, we can use it to e.g. see how many object creations are involved in printing a “hello world” message in.

First create a file called hello.rb

puts "hello world from Ruby #{RUBY_VERSION}!"

Next create count.d which will count the number of times a new object is created, grouped by the class. The copyinstr() is needed to convert arg0, which is the class name, from user space to kernel space where the DTrace probes are run.

    @objects[copyinstr(arg0)] = count();

If you forget to use copyinstr(), you’ll get a number like 140576469837096 instead of String.

When you run this, you should get:

martin$ sudo dtrace  -s count.d -c "`rbenv which ruby` `pwd`/hello.rb"
dtrace: script 'count.d' matched 2 probes
hello world from Ruby 2.2.1!
dtrace: pid 87601 has exited

  ARGF.class                                                        1
  Gem::Platform                                                     1
  IOError                                                           1
  Monitor                                                           1
  NoMemoryError                                                     1
  Range                                                             1
  SystemStackError                                                  1
  ThreadGroup                                                       1
  Time                                                              1
  fatal                                                             1
  LoadError                                                         2
  Mutex                                                             3
  Object                                                            3
  Gem::Specification                                                6
  Hash                                                              8
  Gem::Version                                                      9
  Gem::Requirement                                                 19
  Array                                                            72
  String                                                          254

As you can see, there are a lot of String objects created!