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. It can draw both bar charts and treemaps, of time or space usage.

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.