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https://issues.apache.org/jira/browse/DRILL-5014?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15843523#comment-15843523
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Rahul Challapalli commented on DRILL-5014:
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The fact that the existing sort forces a spill when the memory usage reaches
95% of the allocation covers up this issue. Otherwise we would have seen an OOM
error on large data sets where a lot of spilling and merging happens.
Maybe a performance test would expose this?
> ExternalSortBatch cache size, spill count differs from config setting
> ---------------------------------------------------------------------
>
> Key: DRILL-5014
> URL: https://issues.apache.org/jira/browse/DRILL-5014
> Project: Apache Drill
> Issue Type: Sub-task
> Affects Versions: 1.8.0
> Reporter: Paul Rogers
> Assignee: Paul Rogers
> Priority: Minor
>
> 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:
> {code}
> 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
> {code}
> 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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