[RFC] Compiler pipeline timings visualization

[Ben Gamari]

Sergej Jaskiewicz via ghc-devs <ghc-devs at haskell.org> writes:

> tl;dr: I propose adding a new GHC flag for generating a log that allows
> visualizing how much time each stage in the compiler pipleline took, similar
> to Clang's -ftime-trace.
>
> Hello, everyone.

Hi Sergej,


[snip]

> The latter option is much closer to what we need. If we link the GHC executable
> with the -eventlog flag, then various significant events will be written to
> a special log file. For example, "Parser began parsing the Main.hs file after
> 5 ms since GHC has started, and ended parsing it 3 ms after that".
> The resulting log file can be parsed with the ghc-events library [4], and also
> visualized using the ThreadScope app [5].
>
I'm a bit short on time but here are a few points in no particular order:

Out of curiosity, have you seen Alp's work [1] in this area? This work
allows use to the ghc-events-analyze package [2] to visualize (e.g. [3])
the existing withTiming eventlog output in a reasonably easy-to-consume
format.

That being said, I can certainly see the benefit of using the Chrome
tracing infrastructure for this; it would make for a nicer user
experience than the static image that ghc-events-analyze spits out.

I also know that Matthew Pickering has been working in this area and I'm
sure will have something interesting to add.

I will admit that I am a bit reluctant to add support for a *particular*
tracing format to GHC itself. I think it would make the most sense for
GHC to produce a consumer-agnostic trace representation (via our
existing eventlog mechanism) and for users to transform this into the
format their tools require.

Our current withTimings infrastructure is quite ad-hoc and could perhaps
expose more detail. It would be great to know what sorts of detail
people would find useful.

Cheers,

- Ben


[1] https://www.haskell.org/ghc/blog/20190924-eventful-ghc.html
[2] https://hackage.haskell.org/package/ghc-events-analyze
[3] https://www.haskell.org/ghc/blog/images/eventful-ghc/ghc-events-viz.svg
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