Details

    • Type: Sub-task
    • Status: Resolved
    • Priority: Minor
    • Resolution: Fixed
    • Affects Version/s: 1.8.0
    • Fix Version/s: 1.11.0
    • Component/s: None
    • Labels:
      None

      Description

      The ExternalSortBatch (ESB) operator sorts data while spilling to disk to remain within a defined memory footprint. Spilling occurs based on a number of factors. Among those are two config parameters:

      • drill.exec.sort.external.spill.group.size: The number of batches to spill per spill event.
      • drill.exec.sort.external.spill.threshold: The number of batches to accumulate in memory before starting a spill event.

      The expected behavior would be:

      • After the accumulated batches exceeds the threshold, and
      • More than "batch size" new batches have arrived since the last spill,
      • Spill half the accumulated records.

      That is if the threshold is 200, and the size is 150, we should accumulate 200 batches, then spill 150 of them (leaving 50) and repeat.

      The actual behavior is:

      • When the accumulated records exceeds the threshold and,
      • More than "batch size" new batches have arrived since the last spill,
      • Spill half the accumulated records.

      The above can leave more batches in memory than desired, and spill more than desired.

      The actual behavior for the (threshold=200, size=150) case is:

      Before spilling, buffered batch count: 201
      After spilling, buffered batch count: 101
      Before spilling, buffered batch count: 251
      After spilling, buffered batch count: 126
      Before spilling, buffered batch count: 276
      After spilling, buffered batch count: 138
      Before spilling, buffered batch count: 288
      After spilling, buffered batch count: 144
      Before spilling, buffered batch count: 294
      After spilling, buffered batch count: 147
      Before spilling, buffered batch count: 297
      After spilling, buffered batch count: 149
      Before spilling, buffered batch count: 299
      After spilling, buffered batch count: 150
      Before spilling, buffered batch count: 300
      After spilling, buffered batch count: 150
      Before spilling, buffered batch count: 300
      

      In short, the actual number of batches retained in memory is twice the spill size, *NOT* the number set in the threshold. As a result, the ESB operator will use more memory than expected.

      The work-around is to set a batch size that is half the threshold so that the batch size (used in spill decisions) matches the actual spill count (as implemented by the code.)

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            • Assignee:
              paul-rogers Paul Rogers
              Reporter:
              paul-rogers Paul Rogers
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              • Created:
                Updated:
                Resolved: