• davide's avatar
    testsuite: Save performance metrics in git notes. · 932cd41d
    davide authored
    This patch makes the following improvement:
      - Automatically records test metrics (per test environment) so that
        the programmer need not supply nor update expected values in *.T
        files.
        - On expected metric changes, the programmer need only indicate the
          direction of change in the git commit message.
      - Provides a simple python tool "perf_notes.py" to compare metrics
        over time.
    
    Issues:
      - Using just the previous commit allows performance to drift with each
        commit.
        - Currently we allow drift as we have a preference for minimizing
          false positives.
        - Some possible alternatives include:
          - Use metrics from a fixed commit per test: the last commit that
            allowed a change in performance (else the oldest metric)
          - Or use some sort of aggregate since the last commit that allowed
            a change in performance (else all available metrics)
          - These alternatives may result in a performance issue (with the
            test driver) having to heavily search git commits/notes.
      - Run locally, performance tests will trivially pass unless the tests
        were run locally on the previous commit. This is often not the case
        e.g.  after pulling recent changes.
    
    Previously, *.T files contain statements such as:
    ```
    stats_num_field('peak_megabytes_allocated', (2, 1))
    compiler_stats_num_field('bytes allocated',
                             [(wordsize(64), 165890392, 10)])
    ```
    This required the programmer to give the expected values and a tolerance
    deviation (percentage). With this patch, the above statements are
    replaced with:
    ```
    collect_stats('peak_megabytes_allocated', 5)
    collect_compiler_stats('bytes allocated', 10)
    ```
    So that programmer must only enter which metrics to test and a tolerance
    deviation. No expected value is required. CircleCI will then run the
    tests per test environment and record the metrics to a git note for that
    commit and push them to the git.haskell.org ghc repo. Metrics will be
    compared to the previous commit. If they are different by the tolerance
    deviation from the *.T file, then the corresponding test will fail. By
    adding to the git commit message e.g.
    ```
     # Metric (In|De)crease <metric(s)> <options>: <tests>
    Metric Increase ['bytes allocated', 'peak_megabytes_allocated'] \
             (test_env='linux_x86', way='default'):
        Test012, Test345
    Metric Decrease 'bytes allocated':
        Test678
    Metric Increase:
        Test711
    ```
    This will allow the noted changes (letting the test pass). Note that by
    omitting metrics or options, the change will apply to all possible
    metrics/options (i.e. in the above, an increase for all metrics in all
    test environments is allowed for Test711)
    
    phabricator will use the message in the description
    
    Reviewers: bgamari, hvr
    
    Reviewed By: bgamari
    
    Subscribers: rwbarton, carter
    
    GHC Trac Issues: #12758
    
    Differential Revision: https://phabricator.haskell.org/D5059
    932cd41d
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