GHC can output profiling information as JSON and you should use it.

Posted on March 2, 2018 by Fūzetsu

This is just a small PSA. As of GHC 8.2, when GHC is able to accept -pj RTS flag which will output things in a nice machine readable format. So, use it. I’d even be inclined to say that you should always use it and if you want the old format, we can have a tool that can produce that.

Yesterday I released two packages with relation to this. First is just a very simple and naive package that reads the output: ghc-prof-aeson. It’s just about what you’d expect, type definition then aeson deriving. I have thrown in Linux, OSX and Windows CI as a bonus.

The second package is ghc-prof-aeson-flamegraph. This one is similar to ghc-prof-flamegraph except that it doesn’t bundle FlameGraph tool and that I didn’t have to try to impose additional dependencies on the existing package. It was also not completely straight-forward to integrate with it. Primarily this package is motivated by this long-standing issue where parsing the profile would just fail.

The -p and -P &c. profiling output format is not reliable. If you’re doing retainer profiling, you now have all this extra stuff in your profile. Just use -pj and save everyone’s sanity.


Running on a random profile I had lying around. Generated with +RTS -pj.

[shana@lenalee:~/programming/ghc-prof-aeson-flamegraph]$ nix run nixpkgs.stack nixpkgs.perl.FlameGraph
[shana@lenalee:~/programming/ghc-prof-aeson-flamegraph]$ stack exec --no-nix-pure -- bash -c 'cat /tmp/ | ghc-prof-aeson-flamegraph | > /tmp/secret.svg'

Censored to protect the innocent.



You should start using -pj if you’re using GHC profiling. Better, you should (re)write tools to work with this format instead of the awful ad-hoc parsing that existing tools do.