Github user adeneche commented on a diff in the pull request:

    https://github.com/apache/drill/pull/317#discussion_r48901914
  
    --- Diff: exec/java-exec/src/main/java/org/apache/drill/exec/physical/impl/xsort/ExternalSortBatch.java ---
    @@ -588,36 +571,29 @@ public BatchGroup mergeAndSpill(LinkedList<BatchGroup> batchGroups) throws Schem
         } finally {
           hyperBatch.clear();
         }
    -    long bufSize = getBufferSize(c1);
    -    totalSizeInMemory += bufSize;
    -    logger.debug("mergeAndSpill: final total size in memory = {}", totalSizeInMemory);
    +    logger.debug("mergeAndSpill: final total size in memory = {}", oAllocator.getAllocatedMemory());
         logger.info("Completed spilling to {}", outputFile);
         return newGroup;
       }
     
    -  private long getBufferSize(VectorAccessible batch) {
    -    long size = 0;
    -    for (VectorWrapper<?> w : batch) {
    -      DrillBuf[] bufs = w.getValueVector().getBuffers(false);
    -      for (DrillBuf buf : bufs) {
    -        size += buf.getPossibleMemoryConsumed();
    -      }
    -    }
    -    return size;
    -  }
    -
       private SelectionVector2 newSV2() throws OutOfMemoryException, InterruptedException {
    -    SelectionVector2 sv2 = new SelectionVector2(oContext.getAllocator());
    +    SelectionVector2 sv2 = new SelectionVector2(oAllocator);
         if (!sv2.allocateNewSafe(incoming.getRecordCount())) {
           try {
    -        // Not being able to allocate sv2 means this operator's allocator reached it's maximum capacity.
    -        // Spilling this.batchGroups won't help here as they are owned by upstream operator,
    -        // but spilling spilledBatchGroups may free enough memory
    -        final BatchGroup merged = mergeAndSpill(spilledBatchGroups);
    +        final BatchGroup merged = mergeAndSpill(batchGroups);
             if (merged != null) {
    -          spilledBatchGroups.addFirst(merged);
    +          spilledBatchGroups.add(merged);
    --- End diff --
    
    will revert the change.
    
    I'm confused though, why do we sometimes (in 3 different places) add the outcome of `mergeAndSpill` at the end of the queue ? 


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