Incendium

Easy profiling for your Phoenix controller actions (and other functions) using flamegraphs.

Example flamegraph

Installation

The package can be installed by adding incendium to your list of dependencies in mix.exs:

def deps do
  [
    {:incendium, "~> x.y.z"}
  ]
end

Documentation can be found at https://hexdocs.pm/incendium.

Rationale

Profiling Elixir code is easy using the default Erlang tools, such as fprof. These tools produce a lot of potentially useful data, but visualizing and interpreting all that data is not easy. The erlang tool eflame contains some utilities to generate flamegraphs from the results of profiling your code

But... eflame expects you to manually run a bash script, which according to my reading seems to call a perl (!) script that generates an interactive SVG. And although the generated SVGs support some minimal interaction, it's possible to do better.

When developping a web application, one can take advantage of the web browser as a highly dynamic tool for visualization using SVG, HTML and Javascript. Fortunately there is a very good javascrtip library to generate flamegraphs: d3-flame-graph.

By reading :eflame stacktrace samples and converting them into a format that d3-flame-graph can understand, we can render the flamegraph in a webpage. That way, instead of the manual steps above you can just visit an URL in your web application.

Batch usage (usage with benchmarks)

Incendium can be used to run benchmarks with integrated profiling data (in the form of flamegraphs). It uses Benchee under the hood, and the API is actually quite similar to Benchee's.

It provides a single function, namely Incendium.run/2, which takes the same arguments as Benchee.run/2, plus some incendium-specific ones. The main difference is that the suite title is a required keyword argument instead of an optional one.

An example:

defmodule Incendium.Benchmarks.Example do
  defp map_fun(i) do
    [i, i * i]
  end

  def run() do
    list = Enum.to_list(1..10_000)

    Incendium.run(%{
      "flat_map" => fn -> Enum.flat_map(list, &map_fun/1) end,
      "map.flatten" => fn -> list |> Enum.map(&map_fun/1) |> List.flatten() end
      },
      title: "Example",
      incendium_flamegraph_widths_to_scale: true
    )
  end
end

Incendium.Benchmarks.Example.run()

The output of the script above can be found here.

Interactive Usage (intgrated with a Phoenix web application)

To use incendium in your web application, you need to follow these steps:

1. Add Incendium as a dependency

def deps do
  [
    {:incendium, "~> 0.2.0"}
  ]
end

You can make it a :dev only dependency if you wish, but Incendium will only decorate your functions if you're in :dev mode. Incendium decorators won't decorate your functions in :prod (profilers such as eflame should never be used in :prod because they add a very significant overhead; your code will be ~10-12 times slower)

2. Create an Incendium Controller for your application

# lib/my_app_web/controllers/incencdium_controller.ex

defmodule MyApp.IncendiumController do
  use Incendium.Controller,
    routes_module: MyAppWeb.Router.Helpers,
    otp_app: :my_app
end

There is no need to define an accompanying view. Currently the controller is not extensible (and there aren't many natural extension points anyway).

Upon compilation, the controller will automcatically add the files incendium.js and incendium.css to your priv/static directory so that those static files will be served using the normal Phoenix mechanisms. On unusual Phoenix apps which have static files in other places, this might not work as expected. Currently there isn't a way to override the place where the static files should be added.

3. Add the controller to your Router

# lib/my_app_web/controllers/router.ex

  require Incendium

  scope "/incendium", MyAppWeb do
    Incendium.routes(IncendiumController)
  end

4. Decorate the functions you want to profile

Incendium decorators depend on the decorator package.

defmodule MyAppWeb.MyContext.ExampleController do
  use MyAppWeb.Mandarin, :controller

  # Activate the incendium decorators
  use Incendium.Decorator

  # Each invocation of the `index/2` function will be traced and profiled.
  @decorate incendium_profile_with_tracing()
  def index(conn, params) do
    resources = MyContext.list_resources(params)
    render(conn, "index.html", resources: resources)
  end
end

Currently incendium only supports tracing profilers (which are very slow and not practical in production). In the future we may support better options such as sampling profilers.

5. Visit the `/incendium` route to see the generated flamegraph

Each time you run a profiled function, a new stacktrace will be generated. Stacktraces are not currently saves, you can only access the latest one. In the future we might add a persistence layer that stores a number of stacktraces instead of keeping just the last one.

Here you can find an example flamegraph with explanations about how to interact with it .

tmbb/incendium