GroupBy Optimization: Trim on the Fly

[wuwenw]

Description

Group-by issue: If there are too many groups, then it takes up too much memory and the garbage collection process becomes a major performance bottleneck.

Current Approach: When the number of groups reaches a limit (by default 100k), the key generator stops to add new groups. Produces inaccurate results since we only calculate the first 100k groups

trim on the fly approach: Whenever the number of groups reaches the limit (by default 100k now), trim the results to 5k using OrderBy information. - Pays the overhead of orderBy and trimming, so it’s expensive if the cardinality is relatively low (< 5m), but boost the performance when it’s large - Threshold and trim size are configurable

Detailed benchmark number will be added later

Upgrade Notes

Does this PR prevent a zero down-time upgrade? (Assume upgrade order: Controller, Broker, Server, Minion)

• [ ] Yes (Please label as backward-incompat, and complete the section below on Release Notes)

Does this PR fix a zero-downtime upgrade introduced earlier?

• [ ] Yes (Please label this as backward-incompat, and complete the section below on Release Notes)

Does this PR otherwise need attention when creating release notes? Things to consider:

• New configuration options
• Deprecation of configurations
• Signature changes to public methods/interfaces
• New plugins added or old plugins removed

• [ ] Yes (Please label this PR as release-notes and complete the section on Release Notes)

Release Notes

Documentation

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Impacted Files	Coverage Δ
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...e/pinot/common/minion/MinionTaskMetadataUtils.java	55.00% <ø> (ø)
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...inot/common/tier/TimeBasedTierSegmentSelector.java	0.00% <0.00%> (ø)
...n/utils/LikeToRegexpLikePatternConverterUtils.java	0.00% <0.00%> (ø)
...e/pinot/controller/helix/SegmentStatusChecker.java	57.25% <0.00%> (ø)
...e/assignment/segment/OfflineSegmentAssignment.java	17.32% <0.00%> (ø)
...realtime/segment/DefaultFlushThresholdUpdater.java	83.33% <ø> (ø)
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...core/retention/strategy/TimeRetentionStrategy.java	10.00% <0.00%> (ø)
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[gortiz]

Interesting contribution. Do you think this mode could be enabled using a hint so it won't break backward compatibility?

[Jackie-Jiang]

Right now we have different behavior within segment vs cross segments, which is a kind of confusing:

• Within segment, there is no on-the-fly trim, and we simply ignore groups when it is over the threshold
• Cross segment, we perform on-the-fly trim, and keep trimming the groups when it is over the threshold
• The threshold is different within/cross segment, and is configured with different keys

IMO it is easier to use if we make the behavior consistent: Always trim on-the-fly when the threshold is reached, and trim the inner segment result before doing the cross-segment merge. Essentially make the server side segment merge behavior the same as broker side server result merge.