[jira] [Created] (DRILL-5014) ExternalSortBatch spills only half the batches requested in config

["Paul Rogers (JIRA)" <ji...@apache.org>]

Paul Rogers created DRILL-5014:
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             Summary: ExternalSortBatch spills only half the batches requested in config
                 Key: DRILL-5014
                 URL: https://issues.apache.org/jira/browse/DRILL-5014
             Project: Apache Drill
          Issue Type: Bug
    Affects Versions: 1.8.0
            Reporter: 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:

* When the accumulated number of batches exceeds the threshold, spill the requested number of batches.

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.

So, 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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