Also, you’re the freedom_formatter guy! Thanks for the trailing commas !

Thanks @josevalim!

Findings:

• 1.19 slowdown confirmed to be in “after compile callback”.
• 1.20 introduces additional slowdown in “group pass check” (1.4 → 6.4 seconds)
• OTP version not relevant.

Details:
I re-ran it in 1.18 just to confirm that the protocol consolidation / file writing was faster in 1.18:

[profile] Finished compilation cycle of 1648 modules in 20759ms
# About 3 seconds in 1.18, which matches the overall time
[profile] Finished group pass check of 1648 modules in 1069ms
<- Ran mix compile.elixir in 23795ms

I can also confirm that OTP has no impact (i.e. 1.19 with older OTP version is also slow)

Compiling with the latest changes gives the following output:

[profile] Finished cycle resolution in 0ms
[profile] Finished compilation cycle of 1648 modules in 21185ms
[profile] Finished writing modules to disk in 284ms
[profile] Finished after compile callback in 8216ms
[profile] Finished group pass check of 1648 modules in 6402ms
<- Ran mix compile.elixir in 36360ms

So “after compile callback” is indeed slow, but strangely “group pass check” is also slower in 1.20.

So I started from 1.19.1’s source and just added the “after compile callback” output and got back the quicker “group pass check”:

[profile] Finished cycle resolution in 0ms
[profile] Finished compilation cycle of 1648 modules in 21974ms
[profile] Finished after compile callback in 9124ms
[profile] Finished group pass check of 1648 modules in 1398ms
<- Ran mix compile.elixir in 34360ms

The compiler is definitely doing more work overall in 1.19.X, so make sure you’re leveraging the MIX_OS_DEPS_COMPILE_PARTITION_COUNT env var, described here: elixir/CHANGELOG.md at v1.19 · elixir-lang/elixir · GitHub

If the compilation is still longer overall after setting that value to around half your cores, I’d open a bug, as I don’t think that’s expected.

This is very weird indeed, because extract_protocols did not change at all! I can only think that somehow :beam_lib.chunks is way more expensive now? Do you see a large difference in .beam file sizes?

PS: I pushed a new commit to the v1.19 branch if you want to validate this theory.

No. I have a hunch this is protocol consolidation. If you open up mix/compilers/elixir.ex, you will see each_cycle and after_compile callbacks, measure those!