Nix: optimizing Haskell build size

Nearly three months ago I wrote a log about reducing Docker size, mainly by relying on static builds.

While perfectly relevant in this context, when I introduced Continuous Integration (CI) at my work (which was building with a vanilla callCabal2nix), the CI server disk filled-up in few hours.

For each build:

• Duration: 5 minutes
• Output size:
  • Total: 313 MiB
  • bin (9 executables): 101 MiB
  • lib: 212 MiB

After a quick look at the size per files extension, we got:

• hi (interface file): 15 MiB
• dyn_hi (interface file for dynamic linking): 15 MiB
• so (dynamic library): 17 MiB (1 file)
• p_hi (interface file for profiling): 18 MiB
• a (static library): 149 MiB (2 files)

Let's go for the most obvious:

• We only run dynamically linked executables
• We do not run profiled executables

We can wrap our final executable with:

exeOnly = b:
  with pkgs.haskell.lib;
  enableSharedExecutables (enableSharedLibraries
    (disableStaticLibraries
      (disableExecutableProfiling (disableLibraryProfiling b))));

Let's see the results:

• Duration: 2 minutes
• Output size:
  • Total: 91 MiB
  • bin (9 executables): 1.5 MiB
  • lib: 89 MiB

Note: the build time drop comes mainly from the profiling support drop

Let's have look at the size per files extension, we got:

• hi (interface file): 15 MiB
• dyn_hi (interface file for dynamic linking): 15 MiB
• so (dynamic library): 17 MiB (1 file)
• a (static library): 44 MiB (1 file)

Note: the remaining a file seems to come from a previous derivation.

That's a good first step, but actually, I only intend to run it, not to use it as the input of another build, let's remove files manually:

exeOnly = b:
  with pkgs.haskell.lib;
  overrideCabal (enableSharedExecutables (enableSharedLibraries
    (disableStaticLibraries
      (disableExecutableProfiling (disableLibraryProfiling b)))))
  (drv: {
    postFixup = drv.postFixup or "" + ''
      ${pkgs.findutils}/bin/find $out -name '*.a' -exec rm \{\} \+
      ${pkgs.findutils}/bin/find $out -name '*.hi' -exec rm \{\} \+
    '';
  });

And the final results are:

• Duration: 2 minutes 10 seconds
• Output size:
  • Total: 34 MiB
  • bin (9 executables): 1.5 MiB
  • lib: 32 MiB

Note: I guess the 10 seconds increase is due to find

Finally we only have the necessary files:

• dyn_hi (interface file for dynamic linking): 15 MiB
• so (dynamic library): 17 MiB (1 file)

To sum-up:

• Time: 5 minutes -> 2 minutes 10 seconds (57% drop)
• Size: 313 MiB -> 34 MiB (89% drop)