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The rails-dtrace gem hooks into the ActiveSupport::Notifications instruments in Rails, turning them into DTrace probes.


Rails > 3.0

An OS that supports DTrace. For example:

  • MacOS X
  • Illumos
  • SmartOS
  • Solaris
  • BSD
  • Oracle Linux ?


Add this line to your application's Gemfile:

gem 'ruby-usdt', :git => '',
        :submodules => true, :branch => 'disable_provider'
gem 'rails-dtrace'

And then execute:

$ bundle


This gem in an experiment in progress. The code does not have automated tests, and the performance impact of ActiveSupport::Notifications combined with ruby-usdt and libusdt are unknown. DO NOT USE THIS IN PRODUCTION unless you love risk.

Also, this gem requires experimental behavior in libusdt/ruby-usdt. Core behavior may drastically change between releases/commits of this gem. When this is no longer the case (and I figure out how to write tests around this gem) these warnings will disappear.


Once you add the gem to your Rails application, it will automatically subscribe to all notifications, turning them into DTrace probes.

Note that notifications are lazy-loaded, so even though rails-dtrace subscribes to all available instruments, they will only be converted to probes as they fire in Rails code. For this reason, in order to get a full picture of what is happening in a Rails process, enough load should be generated to ensure that all important probes are registered before tracing.

Rails 3

Arguments to probes are:

  • arg0 - Unique identifier of notification - String
  • arg1 - Stringified hash of notification payload - String
  • arg2 - Nanoseconds between start and finish of event - Integer

The notification name is turned into the probe function, and the probe name is defined as 'event'. For instance, sql.active_record becomes ruby*:rails:sql.active_record:event. This is to make events somewhat compatible with the edge Rails method of firing events, described below.

Some operating systems (MacOS X, I'm looking at you) don't give you nanosecond time resolution. In this case you get microseconds multipled by 1000. If the time differential is larger than the maximum value of a C int (2147483647), then that will be output instead.

The following dtrace command can be used, as an example:

sudo dtrace -n 'ruby*:rails:sql.active_record: { 
    "%s \n\t %s \n\t %d", 
    copyinstr(arg0), copyinstr(arg1),

Example output:

  1  15407  sql.active_record:event 4595e4d30b17bdb96fe9 
	 {:sql=>"SELECT \"things\".* FROM \"things\" ", :name=>"Thing Load", :connection_id=>70184895988280, :binds=>[]} 

Rails 4 / Edge Rails

In newer Rails, notifications become more dtrace-like. Rather than a notification sending a message to #call (which gets start time and end time), they can send two messages, #start and #finish. This way, you can do your own math on the notification timing.

In rails-dtrace, the notification name becomes the probe function, and we map start -> entry and finish -> exit.

If a subscriber responds to #start and #finish it will work in the new way. If it does not, Rails will try to fall back to the older way with #call. Because of this, rails-dtrace can be used with either codebase.

Arguments to probes are:

  • arg0 - Unique identifier of notification - String
  • arg1 - Stringified hash of notification payload - String
  • arg2 - 0 - This is for seamless compatibility with older Rails - Integer

The following dtrace command can be used, as an example:

sudo dtrace -n 'ruby*:rails:sql.active_record: { 
    "%s \n\t %s", 

Example output:

3  52609  sql.active_record:entry a392841cfd75a132432e 
	 {:sql=>"SELECT \"things\".* FROM \"things\" ", :name=>"Thing Load", :connection_id=>70362294355260, :binds=>[]}
3  52610  sql.active_record:exit a392841cfd75a132432e 
	 {:sql=>"SELECT \"things\".* FROM \"things\" ", :name=>"Thing Load", :connection_id=>70362294355260, :binds=>[]}


  1. Fork it
  2. Create your feature branch (git checkout -b my-new-feature)
  3. Commit your changes (git commit -am 'Added some feature')
  4. Push to the branch (git push origin my-new-feature)
  5. Create new Pull Request

Suggestions for Contributions

  • How the F do you test this thing?
  • Can Rails instruments be detected at initialization time to register all probes at once?
  • The Subscriber turns the Notification payload (a hash) into a string. This can surely be better.


Copyright (c) 2012 Eric Saxby

Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.


Add dtrace probes to Rails using ruby-usdt/libusdt




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