APCng storage - avoid worst-case memory usage in buildPermutationTree #123
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| if (isset($metaData['buckets'])) { | ||
| $metaData['buckets'][] = 'sum'; | ||
| $labels[] = $metaData['buckets']; | ||
| $metaData['labelNames'][] = '__histogram_buckets'; |
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Since I don't use the number of label names anymore, I removed that parameter from buildPermutationTree and removed this line as it seemed unused. I don't see any change in the output for histograms but if this actually used then I can re-add it.
Signed-off-by: Josh Hoffer <jhoffer@datava.com>
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This would be great to land, we are hitting the PHP limit as well. |
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I've tested a copy of this PR and it works well to reduce memory usage with histograms |
Signed-off-by: Lukas Kämmerling <lukas.kaemmerling@hetzner-cloud.de>
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Thank you a lot! Great change! Sorry for the really long review time. I just fixed a few phpstan issues and will merge it when the tests are green :)!
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The peak memory usage of the APCng storage adapter while collecting the metrics for display is currently always the worst-case for the cardinality of the metric label values.
This change replaces the buildPermutationTree implementation with a (cartesian product) generator which avoids keeping every metric label value permutation in memory at once. This can reduce peak memory usage by orders of magnitude for metrics with high cardinality but sparse actual usage. This helps avoid exceeding the PHP memory_limit.
Besides the reduced memory usage, I haven't seen much of a difference in how long
buildPermutationTreetakes to run. (To improve that, only the actually used metric label pairs could be to tracked separately to avoid building the permutations completely.)