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https://issues.apache.org/jira/browse/DRILL-5636?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Paul Rogers updated DRILL-5636:
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Summary: Reduce external sort memory use when spilling (was: External sort
should not copy data prior to spilling)
> Reduce external sort memory use when spilling
> ---------------------------------------------
>
> Key: DRILL-5636
> URL: https://issues.apache.org/jira/browse/DRILL-5636
> Project: Apache Drill
> Issue Type: Improvement
> Affects Versions: 1.8.0
> Reporter: Paul Rogers
> Assignee: Paul Rogers
> Priority: Minor
>
> The external sort spills data to disk under memory pressure. The sort code
> uses a generic mechanism to do the spilling:
> * Use a "priority queue copier" to copy sorted records into a new batch
> * Spill the new batch by writing the vectors for the newly-created batch
> The above works fine when memory is plentiful. But, under low-memory
> conditions, the intermediate copy can cause OOM errors.
> An improved algorithm is:
> * Priority queue copier works vector-by-vector
> * Serialize each vector to disk
> * Release its memory
> * Repeat for the next vector
> The advantages of the above:
> * Less intermediate memory use
> * Perhaps better CPU cache performance through greater locality (all writes
> happen to a single vector at a time, rather than row by row)
> * No change in disk format or disk write performance (because data is
> buffered prior to write anyway.)
> *
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