GHC perf

[Simon Peyton Jones]

Ben, David
I'm still baffled by how to reliably get GHC perf metrics on my local machine.
The wiki page https://gitlab.haskell.org/ghc/ghc/wikis/building/running-tests/performance-tests helps, but not enough!

  *   There are two things going on:

     *   CI perf measurements
     *   Local machine perf measurements

I think that they are somehow handled differently (why?) but they are all muddled up on the wiki page.

  *   My goal is this:

     *   Start with a master commit, say from Dec 2019.
     *   Implement some change, on a branch.
     *   sh validate -legacy (or something else if you like)
     *   Look at perf regressions.
  *   I believe I have first to utter the incantation
$ git fetch https://gitlab.haskell.org/ghc/ghc-performance-notes.git refs/notes/perf:refs/notes/ci/perf

  *   But then:
     *   How do I ensure that the baseline perf numbers I get relate to the master commit I started from, back in Dec 2019?  I don't want numbers from Jan 2020.
     *   If I rebase my branch on top of HEAD, say, how do I update the perf baseline numbers to be for HEAD?
     *   Generally, how can I tell the commit to which the baseline numbers relate?
  *   Also, in my tree I have a series of incremental changes; I want to see if any of them have perf regressions.    How do I do that?
Thanks
Simon
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