HDFS-15892. Add metric for editPendingQ in FSEditLogAsync (#2770)

Signed-off-by: Takanobu Asanuma <tasanuma@apache.org>

hadoop-common-project/hadoop-common/src/site/markdown/Metrics.md

@@ -228,6 +228,7 @@ Each metrics record contains tags such as ProcessName, SessionId, and Hostname a
 | `EditLogTailIntervalNumOps` | Total number of intervals between edit log tailings by standby NameNode |
 | `EditLogTailIntervalAvgTime` | Average time of intervals between edit log tailings by standby NameNode in milliseconds |
 | `EditLogTailInterval`*num*`s(50/75/90/95/99)thPercentileLatency` | The 50/75/90/95/99th percentile of time between edit log tailings by standby NameNode in milliseconds (*num* seconds granularity). Percentile measurement is off by default, by watching no intervals. The intervals are specified by `dfs.metrics.percentiles.intervals`. |
+| `PendingEditsCount` | Current number of pending edits |
 
 FSNamesystem
 ------------

hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/FSEditLogAsync.java

@@ -28,6 +28,8 @@
 import java.util.concurrent.atomic.AtomicBoolean;
 import java.util.concurrent.atomic.AtomicInteger;
 
+import org.apache.hadoop.hdfs.server.namenode.metrics.NameNodeMetrics;
+import org.apache.hadoop.util.Time;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 import org.apache.hadoop.conf.Configuration;
@@ -53,6 +55,8 @@ class FSEditLogAsync extends FSEditLog implements Runnable {
   // of the edit log buffer - ie. a sync will eventually be forced.
   private final Deque<Edit> syncWaitQ = new ArrayDeque<Edit>();
 
+  private long lastFull = 0;
+
   FSEditLogAsync(Configuration conf, NNStorage storage, List<URI> editsDirs) {
     super(conf, storage, editsDirs);
     // op instances cannot be shared due to queuing for background thread.
@@ -194,6 +198,11 @@ private void enqueueEdit(Edit edit) {
       if (!editPendingQ.offer(edit)) {
         Preconditions.checkState(
             isSyncThreadAlive(), "sync thread is not alive");
+        long now = Time.monotonicNow();
+        if (now - lastFull > 4000) {
+          lastFull = now;
+          LOG.info("Edit pending queue is full");
+        }
         if (Thread.holdsLock(this)) {
           // if queue is full, synchronized caller must immediately relinquish
           // the monitor before re-offering to avoid deadlock with sync thread
@@ -231,15 +240,18 @@ private Edit dequeueEdit() throws InterruptedException {
   public void run() {
     try {
       while (true) {
+        NameNodeMetrics metrics = NameNode.getNameNodeMetrics();
         boolean doSync;
         Edit edit = dequeueEdit();
         if (edit != null) {
           // sync if requested by edit log.
           doSync = edit.logEdit();
           syncWaitQ.add(edit);
+          metrics.setPendingEditsCount(editPendingQ.size() + 1);
         } else {
           // sync when editq runs dry, but have edits pending a sync.
           doSync = !syncWaitQ.isEmpty();
+          metrics.setPendingEditsCount(0);
         }
         if (doSync) {
           // normally edit log exceptions cause the NN to terminate, but tests

hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/metrics/NameNodeMetrics.java

@@ -87,6 +87,8 @@ public class NameNodeMetrics {
   MutableGaugeInt blockOpsQueued;
   @Metric("Number of blockReports and blockReceivedAndDeleted batch processed")
   MutableCounterLong blockOpsBatched;
+  @Metric("Number of pending edits")
+  MutableGaugeInt pendingEditsCount;
 
   @Metric("Number of file system operations")
   public long totalFileOps(){
@@ -343,6 +345,10 @@ public void addBlockOpsBatched(int count) {
     blockOpsBatched.incr(count);
   }
 
+  public void setPendingEditsCount(int size) {
+    pendingEditsCount.set(size);
+  }
+
   public void addTransaction(long latency) {
     transactions.add(latency);
   }