promql: Re-introduce direct mean calculation #16773
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krajorama
reviewed
Jun 25, 2025
krajorama
approved these changes
Jun 25, 2025
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Cleaned up the commits. |
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@charleskorn might be a suitable reviewer. (His findings triggered ultimately triggered this.) |
bboreham
reviewed
Jun 25, 2025
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Can you summarise what the net difference of #16569 and this PR is, besides adding tests? |
I did that in the commit description: |
charleskorn
approved these changes
Jun 26, 2025
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LGTM modulo suggestion for another test case.
Thanks for fixing this @beorn7.
bboreham
approved these changes
Jun 27, 2025
These demonstrate that direct mean calculation has some merits after all. Signed-off-by: beorn7 <beorn@grafana.com>
The test in question actually worked fine even before #16569. The finding reported in the comment has turned out to be caused by something else. Signed-off-by: beorn7 <beorn@grafana.com>
This commit brings back direct mean calculation (for `avg` and `avg_over_time`) but isn't an outright revert of #16569. It keeps the improved incremental mean calculation and features generally a bit cleaner code than before. Also, this commit... - ...updates the lengthy comment explaining the whole situation and trade-offs. - ...divides the running sum and the Kahan compensation term separately (in direct mean calculation) to avoid the (unlikely) possibility that sum and Kahan compensation together ovorflow float64. - ...uncomments the tests that should now work again on darwin/arm64. - ...uncomments the test that should now reliably yield the (inaccurate) value 0 on all hardware platforms. Also, the test description has been updated accordingly. - ...adds avg_over_time tests for zero and one sample in the range. Signed-off-by: beorn7 <beorn@grafana.com>
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Thanks. I'll merge on green. |
crush-on-anechka
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Jul 10, 2025
Signed-off-by: Aleksandr Smirnov <5targazer@mail.ru>
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See commit description for details, also the discussion in #16714 .
(The very short version is that I was wrong assuming that the incremental mean calculation combined with Kahan summation in the improved way introduced in #16569 is always at least as good as the previous approach.)
/cc @charleskorn @crush-on-anechka