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  1. Core Server
  2. SERVER-27406

Reduce DocumentSourceCursor batch size from 16MB to 4MB

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    Details

    • Type: Improvement
    • Status: Closed
    • Priority: Major - P3
    • Resolution: Fixed
    • Affects Version/s: 3.4.0
    • Fix Version/s: 3.4.2, 3.5.2
    • Component/s: Aggregation Framework
    • Labels:
      None
    • Backwards Compatibility:
      Fully Compatible
    • Sprint:
      Query 2017-01-23
    • Linked BF Score:
      0

      Description

      DocumentSourceCursor batch size was changed from 4MB to 16MB for MongoDB 3.4 as part of a broader effort to unify user request batch size at 16MB under SERVER-21797. This change led to a mongo-perf performance regression for the "Aggregation.skip" which appears as slower memcpy operations under VTune under DocumentSourceCursor::loadBatch and is likely due to the larger batches exceeding L2/L3 caches.

      Testing across our aggregation micro-benchmarks shows a 4MB DocumentSourceCursor batch size restores previous performance across affected tests and does not penalize tests that may benefit from the batching behavior.

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                  Updated:
                  Resolved: