Slow recompilation in development

[liamwhite]

Importing the UserAuth module into the router in development causes very minor changes to the application to end up recompiling the router. This is very slow.

Performance after touch lib/philomena/users.ex:

root@28c98c26a5aa:/srv/philomena# time mix compile
Compiling 2 files (.ex)
Generated philomena app

real    0m6.173s
user    0m6.524s
sys 0m0.888s
root@28c98c26a5aa:/srv/philomena# time mix compile
Compiling 2 files (.ex)

real    0m6.024s
user    0m6.408s
sys 0m0.706s

A small set of changes causes development recompilation performance to return to normal:

--- a/lib/philomena_web/router.ex
+++ b/lib/philomena_web/router.ex
@@ -1,15 +1,13 @@
 defmodule PhilomenaWeb.Router do
   use PhilomenaWeb, :router

-  import PhilomenaWeb.UserAuth
-
   pipeline :browser do
     plug :accepts, ["html"]
     plug :fetch_session
     plug :fetch_flash
     plug :protect_from_forgery
     plug :put_secure_browser_headers
-    plug :fetch_current_user
+    plug PhilomenaWeb.FetchUserPlug
     plug PhilomenaWeb.ContentSecurityPolicyPlug
     plug PhilomenaWeb.CurrentFilterPlug
     plug PhilomenaWeb.ImageFilterPlug
@@ -53,6 +51,14 @@ defmodule PhilomenaWeb.Router do
     plug PhilomenaWeb.FilterBannedUsersPlug
   end

+  pipeline :require_authenticated_user do
+    plug PhilomenaWeb.RequireUserPlug
+  end
+
+  pipeline :redirect_if_user_is_authenticated do
+    plug PhilomenaWeb.RequireNoUserPlug
+  end
+
   scope "/", PhilomenaWeb do
     pipe_through [:browser, :redirect_if_user_is_authenticated]

The added plugs simply call the relevant function in the UserAuth module.

Performance after touch lib/philomena/users.ex:

root@28c98c26a5aa:/srv/philomena# time mix compile
Compiling 1 file (.ex)
Generated philomena app

real    0m1.584s
user    0m1.687s
sys 0m0.599s
root@28c98c26a5aa:/srv/philomena# time mix compile
Compiling 1 file (.ex)

real    0m1.560s
user    0m1.588s
sys 0m0.595s

I've already confirmed that the problem only occurs because the router is recompiled, and recompiling the router alone is quite slow:

root@28c98c26a5aa:/srv/philomena# time mix compile
Compiling 1 file (.ex)

real    0m5.698s
user    0m5.929s
sys 0m0.802s

[josevalim]

This is much improved in Elixir v1.11, which no longer treats imports as compile-time dependencies. Since v1.11 will be out soon, I propose to keep things as is, thanks! :)

[liamwhite]

Okay, good to know!

[liamwhite]

Confirming that this is fixed in 1.11.