[HDFS-11047] Remove deep copies of FinalizedReplica to alleviate heap consumption on DataNode

• Type: Improvement
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Fix Version/s: 2.8.0, 3.0.0-alpha2
• Component/s: datanode
• Hadoop Flags: Reviewed
• Assignee: Xiaobing Zhou
• Reporter: Xiaobing Zhou
• Created: 24/Oct/16 17:03
• Updated: 22/Jun/17 11:34
• Resolved: 28/Oct/16 18:18

Description

DirectoryScanner does scan by deep copying FinalizedReplica. In a deployment with 500,000+ blocks, we've seen the DN heap usage being accumulated to high peaks very quickly. Deep copies of FinalizedReplica will make DN heap usage even worse if directory scans are scheduled more frequently. This proposes removing unnecessary deep copies since DirectoryScanner#scan already holds lock of dataset.

DirectoryScanner#scan

    try(AutoCloseableLock lock = dataset.acquireDatasetLock()) {
      for (Entry<String, ScanInfo[]> entry : diskReport.entrySet()) {
        String bpid = entry.getKey();
        ScanInfo[] blockpoolReport = entry.getValue();
        
        Stats statsRecord = new Stats(bpid);
        stats.put(bpid, statsRecord);
        LinkedList<ScanInfo> diffRecord = new LinkedList<ScanInfo>();
        diffs.put(bpid, diffRecord);
        
        statsRecord.totalBlocks = blockpoolReport.length;
        List<ReplicaInfo> bl = dataset.getFinalizedBlocks(bpid); /* deep copies here*/

FsDatasetImpl#getFinalizedBlocks

  public List<ReplicaInfo> getFinalizedBlocks(String bpid) {
    try (AutoCloseableLock lock = datasetLock.acquire()) {
      ArrayList<ReplicaInfo> finalized =
          new ArrayList<ReplicaInfo>(volumeMap.size(bpid));
      for (ReplicaInfo b : volumeMap.replicas(bpid)) {
        if (b.getState() == ReplicaState.FINALIZED) {
          finalized.add(new ReplicaBuilder(ReplicaState.FINALIZED)
              .from(b).build()); /* deep copies here*/
        }
      }
      return finalized;
    }
  }