time-ghc-modules/README.md

time-ghc-modules

Figure out why your builds are slow. This tool analyzes how long it takes GHC to compile your Haskell modules, broken down by phase.

Quick start

cd <my-project>

stack clean
stack build --ghc-options "-ddump-to-file -ddump-timings"
# ----- OR -----
cabal clean
cabal build all --ghc-options "-ddump-to-file -ddump-timings"

If you have Nix, you can simply run time-ghc-modules from Nixpkgs!

nix run nixpkgs#time-ghc-modules

Or, clone the repo first:

git clone git@github.com:codedownio/time-ghc-modules.git /path/to/time-ghc-modules

# If you have Nix, you can use the fully reproducible version
/path/to/time-ghc-modules/time-ghc-modules-nix

# Otherwise, your system needs to have SQLite >= 3.33.0, Python 3, and sed
/path/to/time-ghc-modules/time-ghc-modules

The script will search for all your *.dump-timings files and analyze them. It will finish by printing out the path to an HTML file:

...
--> Wrote report at file:///tmp/tmp.pvnp4FYmLa/report.html

Example: hledger

You can generate the time report below for hledger by running the following commands (assuming you have Nix).

set -e
cd $(mktemp -d)
git clone git@github.com:simonmichael/hledger.git
git clone git@github.com:codedownio/time-ghc-modules.git
cd hledger
stack build --ghc-options "-ddump-to-file -ddump-timings"
../time-ghc-modules/time-ghc-modules-nix

Tips

• The script will output its log messages to stderr and print the final report path to stdout (assuming it didn't exit with a failure). This makes it easy to use the output in scripts. For example:

# Build the report and open it in your browser
> firefox $(/path/to/time-ghc-modules/time-ghc-modules)

# Build the report in CI and stash it somewhere
> cp $(/path/to/time-ghc-modules/time-ghc-modules) $MY_CI_ARTIFACTS_DIR/

• You can also look at the timing of individual components, but doing e.g. stack build some-component:lib. But, make sure to clean up any old .dump-timings files from previous runs:

find . -name "*.dump-timings" | xargs rm

• GHC's -dumpdir option can be used to consolidate the .dump-timings files, so they aren't left all over your source tree. For example:

stack build --ghc-options "-ddump-to-file -ddump-timings -dumpdir .ghcdump"

Compatibility

The flag -ddump-timings is available for GHC >= 8.4.1.