[GHC] #15936: Rethink Choice of Baseline Commit for Performance Tests

[GHC]

#15936: Rethink Choice of Baseline Commit for Performance Tests
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           Reporter:  davide         |             Owner:  (none)
               Type:  task           |            Status:  new
           Priority:  normal         |         Milestone:  8.6.3
          Component:  Test Suite     |           Version:  8.6.2
           Keywords:  performance    |  Operating System:  Unknown/Multiple
  tests git notes                    |
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 = Intro =

 Currently we always use the previous commit when running performance
 tests. This works well in CI where we fully test each commit in sequence
 (and hence always have test results for the previous commit). Remember,
 test results are stored in git notes and are not by default shared between
 repositories (i.e. your local repo will only have performance results run
 locally on your machine). This is by design: we want to avoid comparing
 results form different machines.

 Unfortunately This is not so effective when testing locally. The
 programmer may have only run a subset of performance tests on the previous
 commit, and often have not run the tests at all (this is notably true
 after performing a rebase: the previous commit has changed). We need to
 rethink how we pick a baseline commit.

 = Goals =

 * In all cases, do something sensible.
 * Giving a warning if conditions are not idea. Provide clear and simple
 instructions on how to get to the ideal case.
 * Give control over the baseline commit to the programmer via command line
 options.
 * In general, performance tests should just work! No extra knowledge
 needed by the programmer.
 * If tests pass without warning now, then they should pass without warning
 later.

 = Proposed Solution =

 * Choose baseline commit
   * Provide command line arguments to set the baseline commit
   * If not baseline commit is provided use the "Ideal baseline commit".
 * If local test results for the baseline are available use those results.
 * Else if results from CI are available (without fetching), then use those
 results.
 * Else Warn user that there are no results available (tests trivially
 pass), then suggest fetching CI results (quick and easy, give full
 command, mention you may have to wait for CI to finish if the commit is
 recent) or running locally (more accurate, mention exact commit to
 checkout), or manually select a baseline commit.

 == Ideal baseline commit ==

 * If there are no new changes: 0 ahead and 0 or more behind master.
   * Ideal baseline commit is the previous commit.
 * Else 1 or more ahead and 0 or more behind master.
   * Ideal baseline commit is `merge-base master HEAD`
   * Assume that the intention is to create a patch where all new commits
 will ultimately be squashed and placed on top of master. We only want to
 consider performance changes caused by the new commits, so we use the
 merge base instead of master HEAD (though these may be the same commit).

 = Open issues =

 If commits between HEAD and the baseline commit
 [https://ghc.haskell.org/trac/ghc/wiki/Performance/Tests#ExpectedPerformanceChanges
 allow changes] in performance numbers, what should the behaviour be? What
 happens if this is merged? The commits will be squashed: this could affect
 the commit message and hence the allowed changes which is parsed from the
 commit messages.

 = Use cases =

 TODO This section is a WIP

 We generally will assume that the master branch corresponds to ghc's
 master branch (though may be slightly out of date).

 ||= Case =||= Details =||= Baseline =||
 || 0 commits ahead of master || May be on master or branched off master
 with no new commits || previous commit ||
 || CI pre-merge ||
 || CI post-merge

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15936>
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