Benchmarking GHC compilation times

Question

My Haskell program takes too long to compile.

By that, I mean: It's been over a year since I've been able to compile with -O1. Last time I tried, I set up a 64gb server just to compile my program, and let it run overnight. It ran out of memory.

I'm about to embark on some big re-engineering changes which I hope will speed up compilation times, but first I need to know what's taking so long, and get some good measurements so I can track progress.

I currently have two ideas for how to do this:

1. Cover my program in flags so I can try compiling different pieces and timing them.
2. Writing a wrapper program for stack build that looks for lines like "[75 of 83] Compiling ," and times how long between them.

(1) sounds really ugly, and I don't expect (2) to give good numbers because of imprecision and because of cross-module compilation. Anyone have better ideas?

Answer 1

Better and easier than watching text output, I realized I could simply look at the timestamps on files in the build folder.

This command gives me a nice list of build files sorted by timestamp, so I can roughly tell how long it spent on each:

ls -l -c -T -R .stack-work/dist/x86_64-osx/Cabal-1.22.5.0/build/<source folder> | grep "rw" | grep "\." | awk '{print $8,$10}' | sort