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From Ben-Zvi <...@git.apache.org>
Subject [GitHub] drill pull request #761: DRILL-5284: Roll-up of final fixes for managed sort
Date Sat, 25 Feb 2017 03:36:25 GMT
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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