ghc-build-analzyer

ghc-build-analzyer is a tool for extracting build timing data from GHC. Add -ddump-json -dshow-passes to your GHC command-line options and then pipe your build tool to ghc-build-analzyer; GHC output will be printed normally and the JSON messages will be decoded and analyzed (and hopefully eventually sent to Honeycomb).

Rationale

The -ddump-timings option shows how long each module takes to build, but doesn't give relative timings, so it's impossible to infer parallelism from the data, which is one of the more important metrics for determining how optimized our build is / how optimized our build can be.

The GHC eventlog looks like it has what we want, but the support for it is poor; in particular, you have to specify an output filename on the command-line, so if your build tool invokes GHC multiple times (common for larger projects) only the last eventlog will be saved. (And, because we specify GHC options in package.yaml or similar files, we can't do anything clever like -ol$(mktemp) to generate per-invocation eventlog files.) -ddump-timings -ddump-to-file is designed as compiler diagnostics, so it saves each timing log to a module-specific file in the build directory, but the eventlog is designed as a generic interface for any program to log events, so it doesn't have that sort of GHC-specific output tooling.

This leaves us with -ddump-json -dshow-passes, which prints a message when each compiler pass starts and ends, for each module; by intercepting the output in real-time, we can then infer information about our build's performance. Unfortunately, -dshow-passes seems to be incompatible with -ddump-to-file, so we can't run GHC “normally” and then analyze the output after-the-fact. Fortunately, it does appear that -ddump-json -dshow-passes causes all compiler output to be printed as JSON, which is perfect for an intercepting tool like this.