Github user Ben-Zvi commented on a diff in the pull request:
https://github.com/apache/drill/pull/761#discussion_r103067805
--- Diff:
exec/java-exec/src/main/java/org/apache/drill/exec/physical/impl/xsort/managed/ExternalSortBatch.java
---
@@ -392,22 +448,31 @@ private void configure(DrillConfig config) {
// Set too large and the ratio between memory and input data sizes
becomes
// small. Set too small and disk seek times dominate performance.
- spillBatchSize =
config.getBytes(ExecConstants.EXTERNAL_SORT_SPILL_BATCH_SIZE);
- spillBatchSize = Math.max(spillBatchSize, MIN_SPILL_BATCH_SIZE);
+ preferredSpillBatchSize =
config.getBytes(ExecConstants.EXTERNAL_SORT_SPILL_BATCH_SIZE);
+
+ // In low memory, use no more than 1/4 of memory for each spill batch.
Ensures we
+ // can merge.
+
+ preferredSpillBatchSize = Math.min(preferredSpillBatchSize,
memoryLimit / 4);
--- End diff --
Why restrict the spill batch size so low ? This would create more runs and
increase the risk of needing those intermediate merges. Otherwise during a
merge, only a single batch at a time is read from each run, not the whole run
(I believe -- if we spill all the remaining batches at the end ...)
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