Module compilation taking more than 10s

Questions / Help
1

When I compile my application, I see a lot of messages like:

Compiling some/module/file.ex (it’s taking more than 10s)

I understand that this message is appearing because the compilation of those modules is slow, but I don’t know why is this and where to start looking for possible causes. Any suggestions?

2

Is your project public?

Modules are known to take long when you have a lot of macrocalls or functions with a lot of heads. Before 1.5.somewhat compile time of a function grew squared to its number of heads. I’m not sure when this was fixed (or if at all, maybe fix is master only)

3

Unfortunately no. It belongs to the company I am working for.

I don’t know if it has something to do with it, but in the 95% of those warnings, the affected module is a controller or a view.

4

Another reason for slow compilation is if given module depends on other slow modules. [1] Is a great resource for understanding compilation. In general, you want to avoid compile-time dependencies between modules as much as possible.

On our Phoenix app, switching from import MyApp.Router.Helpers to alias MyApp.Router.Helpers, as: Routes speed up compilation and limited re-compilations greatly, see: [2]

Elixir 1.6 will ship with improvements to mix xref which should make it easier to investigate this stuff.

[1] http://milhouseonsoftware.com/2016/08/11/understanding-elixir-recompilation/
[2] https://github.com/phoenixframework/phoenix/issues/2530
[3] https://github.com/elixir-lang/elixir/blob/master/CHANGELOG.md#mix-xref

5 Likes
5

Hm, we may have something here :thinking:
Running mix xref graph --source on those slow modules usually shows the router as a compile time dependency, which in turn depends on lots of controllers, which depend on models, etc.

I am going to try aliasing the router module instead of importing it and check the result.
Thank you @wojtekmach

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