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

    https://github.com/apache/spark/pull/9241#discussion_r42953398
  
    --- Diff: 
core/src/main/java/org/apache/spark/unsafe/map/BytesToBytesMap.java ---
    @@ -227,62 +238,147 @@ public BytesToBytesMap(
        */
       public int numElements() { return numElements; }
     
    -  public static final class BytesToBytesMapIterator implements 
Iterator<Location> {
    +  public final class BytesToBytesMapIterator implements Iterator<Location> 
{
     
    -    private final int numRecords;
    -    private final Iterator<MemoryBlock> dataPagesIterator;
    +    private int numRecords;
         private final Location loc;
     
         private MemoryBlock currentPage = null;
    -    private int currentRecordNumber = 0;
    +    private int recordsInPage = 0;
         private Object pageBaseObject;
         private long offsetInPage;
     
         // If this iterator destructive or not. When it is true, it frees each 
page as it moves onto
         // next one.
         private boolean destructive = false;
    -    private BytesToBytesMap bmap;
     
    -    private BytesToBytesMapIterator(
    -        int numRecords, Iterator<MemoryBlock> dataPagesIterator, Location 
loc,
    -        boolean destructive, BytesToBytesMap bmap) {
    +    private LinkedList<UnsafeSorterSpillWriter> spillWriters =
    +      new LinkedList<UnsafeSorterSpillWriter>();
    +    private UnsafeSorterSpillReader reader = null;
    +
    +    private BytesToBytesMapIterator(int numRecords, Location loc, boolean 
destructive) {
           this.numRecords = numRecords;
    -      this.dataPagesIterator = dataPagesIterator;
           this.loc = loc;
           this.destructive = destructive;
    -      this.bmap = bmap;
    -      if (dataPagesIterator.hasNext()) {
    -        advanceToNextPage();
    -      }
    +      destructiveIterator = this;
         }
     
         private void advanceToNextPage() {
    -      if (destructive && currentPage != null) {
    -        dataPagesIterator.remove();
    -        this.bmap.taskMemoryManager.freePage(currentPage);
    -        this.bmap.shuffleMemoryManager.release(currentPage.size());
    +      synchronized (this) {
    +        int nextIdx = dataPages.indexOf(currentPage) + 1;
    +        if (destructive && currentPage != null) {
    +          dataPages.remove(currentPage);
    +          taskMemoryManager.freePage(currentPage);
    +          shuffleMemoryManager.release(currentPage.size());
    +          nextIdx --;
    +        }
    +        if (dataPages.size() > nextIdx) {
    +          currentPage = dataPages.get(nextIdx);
    +          pageBaseObject = currentPage.getBaseObject();
    +          offsetInPage = currentPage.getBaseOffset();
    +          recordsInPage = Platform.getInt(pageBaseObject, offsetInPage);
    +          offsetInPage += 4;
    +        } else {
    +          currentPage = null;
    +          try {
    +            reader = spillWriters.removeFirst().getReader(blockManager);
    +            recordsInPage = -1;
    +          } catch (IOException e) {
    +            // Scala iterator does not handle exception
    +            Platform.throwException(e);
    +          }
    +        }
           }
    -      currentPage = dataPagesIterator.next();
    -      pageBaseObject = currentPage.getBaseObject();
    -      offsetInPage = currentPage.getBaseOffset();
         }
     
         @Override
         public boolean hasNext() {
    -      return currentRecordNumber != numRecords;
    +      return numRecords > 0;
         }
     
         @Override
         public Location next() {
    -      int totalLength = Platform.getInt(pageBaseObject, offsetInPage);
    -      if (totalLength == END_OF_PAGE_MARKER) {
    +      if (recordsInPage == 0) {
             advanceToNextPage();
    -        totalLength = Platform.getInt(pageBaseObject, offsetInPage);
           }
    -      loc.with(currentPage, offsetInPage);
    -      offsetInPage += 4 + totalLength;
    -      currentRecordNumber++;
    -      return loc;
    +      numRecords --;
    --- End diff --
    
    Mind omitting the space between `numRecords` and `--`?


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