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  1. Spark
  2. SPARK-24356

Duplicate strings in File.path managed by FileSegmentManagedBuffer

    Details

    • Type: Improvement
    • Status: Resolved
    • Priority: Major
    • Resolution: Fixed
    • Affects Version/s: 2.3.0
    • Fix Version/s: 2.4.0
    • Component/s: Shuffle
    • Labels:
      None

      Description

      I recently analyzed a heap dump of Yarn Node Manager that was suffering from high GC pressure due to high object churn. Analysis was done with the jxray tool (www.jxray.com) that checks a heap dump for a number of well-known memory issues. One problem that it found in this dump is 19.5% of memory wasted due to duplicate strings. Of these duplicates, more than a half come from FileInputStream.path and File.path. All the FileInputStream objects that JXRay shows are garbage - looks like they are used for a very short period and then discarded (I guess there is a separate question of whether that's a good pattern). But File instances are traceable to org.apache.spark.network.buffer.FileSegmentManagedBuffer.file field. Here is the full reference chain:
       

      ↖java.io.File.path
      ↖org.apache.spark.network.buffer.FileSegmentManagedBuffer.file
      ↖{j.u.ArrayList}
      ↖j.u.ArrayList$Itr.this$0
      ↖org.apache.spark.network.server.OneForOneStreamManager$StreamState.buffers
      ↖{java.util.concurrent.ConcurrentHashMap}.values
      ↖org.apache.spark.network.server.OneForOneStreamManager.streams
      ↖org.apache.spark.network.shuffle.ExternalShuffleBlockHandler.streamManager
      ↖org.apache.spark.network.yarn.YarnShuffleService.blockHandler
      ↖Java Static org.apache.spark.network.yarn.YarnShuffleService.instance
      

       
      Values of these File.path's and FileInputStream.path's look very similar, so I think FileInputStream}}s are generated by the {{FileSegmentManagedBuffer code. Instances of File, in turn, likely come from
      https://github.com/apache/spark/blob/master/common/network-shuffle/src/main/java/org/apache/spark/network/shuffle/ExternalShuffleBlockResolver.java#L258-L263
       
      To avoid duplicate strings in File.path's in this case, it is suggested that in the above code we create a File with a complete, normalized pathname, that has been already interned. This will prevent the code inside java.io.File from modifying this string, and thus it will use the interned copy, and will pass it to FileInputStream. Essentially the current line

      return new File(new File(localDir, String.format("%02x", subDirId)), filename);

      should be replaced with something like

      String pathname = localDir + File.separator + String.format(...) + File.separator + filename;
      pathname = fileSystem.normalize(pathname).intern();
      return new File(pathname);

       

        Attachments

        1. SPARK-24356.01.patch
          6 kB
          Misha Dmitriev
        2. dup-file-strings-details.png
          204 kB
          Misha Dmitriev

          Activity

            People

            • Assignee:
              misha@cloudera.com Misha Dmitriev
              Reporter:
              misha@cloudera.com Misha Dmitriev
            • Votes:
              2 Vote for this issue
              Watchers:
              8 Start watching this issue

              Dates

              • Created:
                Updated:
                Resolved: