HDFS-14370. Add exponential backoff to the edit log tailer to avoid spinning on empty edit tail requests. Contributed by Erik Krogen.

hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/DFSConfigKeys.java

@@ -1004,6 +1004,8 @@ public class DFSConfigKeys extends CommonConfigurationKeys {
   public static final int DFS_HA_LOGROLL_PERIOD_DEFAULT = 2 * 60; // 2m
   public static final String DFS_HA_TAILEDITS_PERIOD_KEY = "dfs.ha.tail-edits.period";
   public static final int DFS_HA_TAILEDITS_PERIOD_DEFAULT = 60; // 1m
+  public static final String DFS_HA_TAILEDITS_PERIOD_BACKOFF_MAX_KEY = "dfs.ha.tail-edits.period.backoff-max";
+  public static final int DFS_HA_TAILEDITS_PERIOD_BACKOFF_MAX_DEFAULT = 0; // disabled
   public static final String DFS_HA_TAILEDITS_ALL_NAMESNODES_RETRY_KEY = "dfs.ha.tail-edits.namenode-retries";
   public static final int DFS_HA_TAILEDITS_ALL_NAMESNODES_RETRY_DEFAULT = 3;
   public static final String  DFS_HA_TAILEDITS_INPROGRESS_KEY =

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

@@ -134,10 +134,17 @@ public class EditLogTailer {
   private final ExecutorService rollEditsRpcExecutor;
 
   /**
-   * How often the Standby should check if there are new finalized segment(s)
+   * How often the tailer should check if there are new edit log entries
-   * available to be read from.
+   * ready to be consumed. This is the initial delay before any backoff.
    */
   private final long sleepTimeMs;
+  /**
+   * The maximum time the tailer should wait between checking for new edit log
+   * entries. Exponential backoff will be applied when an edit log tail is
+   * performed but no edits are available to be read. If this is less than or
+   * equal to 0, backoff is disabled.
+   */
+  private final long maxSleepTimeMs;
 
   private final int nnCount;
   private NamenodeProtocol cachedActiveProxy = null;
@@ -206,6 +213,19 @@ public EditLogTailer(FSNamesystem namesystem, Configuration conf) {
         DFSConfigKeys.DFS_HA_TAILEDITS_PERIOD_KEY,
         DFSConfigKeys.DFS_HA_TAILEDITS_PERIOD_DEFAULT,
         TimeUnit.SECONDS, TimeUnit.MILLISECONDS);
+    long maxSleepTimeMsTemp = conf.getTimeDuration(
+        DFSConfigKeys.DFS_HA_TAILEDITS_PERIOD_BACKOFF_MAX_KEY,
+        DFSConfigKeys.DFS_HA_TAILEDITS_PERIOD_BACKOFF_MAX_DEFAULT,
+        TimeUnit.SECONDS, TimeUnit.MILLISECONDS);
+    if (maxSleepTimeMsTemp > 0 && maxSleepTimeMsTemp < sleepTimeMs) {
+      LOG.warn("{} was configured to be {} ms, but this is less than {}."
+              + "Disabling backoff when tailing edit logs.",
+          DFSConfigKeys.DFS_HA_TAILEDITS_PERIOD_BACKOFF_MAX_KEY,
+          maxSleepTimeMsTemp, DFSConfigKeys.DFS_HA_TAILEDITS_PERIOD_KEY);
+      maxSleepTimeMs = 0;
+    } else {
+      maxSleepTimeMs = maxSleepTimeMsTemp;
+    }
 
     rollEditsTimeoutMs = conf.getTimeDuration(
         DFSConfigKeys.DFS_HA_TAILEDITS_ROLLEDITS_TIMEOUT_KEY,
@@ -291,7 +311,7 @@ public Void run() throws Exception {
   }
   
   @VisibleForTesting
-  public void doTailEdits() throws IOException, InterruptedException {
+  public long doTailEdits() throws IOException, InterruptedException {
     // Write lock needs to be interruptible here because the 
     // transitionToActive RPC takes the write lock before calling
     // tailer.stop() -- so if we're not interruptible, it will
@@ -316,7 +336,7 @@ public void doTailEdits() throws IOException, InterruptedException {
         // edits file hasn't been started yet.
         LOG.warn("Edits tailer failed to find any streams. Will try again " +
             "later.", ioe);
-        return;
+        return 0;
       } finally {
         NameNode.getNameNodeMetrics().addEditLogFetchTime(
             Time.monotonicNow() - startTime);
@@ -347,6 +367,7 @@ public void doTailEdits() throws IOException, InterruptedException {
         lastLoadTimeMs = monotonicNow();
       }
       lastLoadedTxnId = image.getLastAppliedTxId();
+      return editsLoaded;
     } finally {
       namesystem.writeUnlock();
     }
@@ -407,6 +428,11 @@ void triggerActiveLogRoll() {
     }
   }
 
+  @VisibleForTesting
+  void sleep(long sleepTimeMillis) throws InterruptedException {
+    Thread.sleep(sleepTimeMillis);
+  }
+
   /**
    * The thread which does the actual work of tailing edits journals and
    * applying the transactions to the FSNS.
@@ -435,7 +461,9 @@ public Object run() {
     }
     
     private void doWork() {
+      long currentSleepTimeMs = sleepTimeMs;
       while (shouldRun) {
+        long editsTailed  = 0;
         try {
           // There's no point in triggering a log roll if the Standby hasn't
           // read any more transactions since the last time a roll was
@@ -461,7 +489,7 @@ private void doWork() {
           try {
             NameNode.getNameNodeMetrics().addEditLogTailInterval(
                 startTime - lastLoadTimeMs);
-            doTailEdits();
+            editsTailed = doTailEdits();
           } finally {
             namesystem.cpUnlock();
             NameNode.getNameNodeMetrics().addEditLogTailTime(
@@ -481,7 +509,17 @@ private void doWork() {
         }
 
         try {
-          Thread.sleep(sleepTimeMs);
+          if (editsTailed == 0 && maxSleepTimeMs > 0) {
+            // If no edits were tailed, apply exponential backoff
+            // before tailing again. Double the current sleep time on each
+            // empty response, but don't exceed the max. If the sleep time
+            // was configured as 0, start the backoff at 1 ms.
+            currentSleepTimeMs = Math.min(maxSleepTimeMs,
+                (currentSleepTimeMs == 0 ? 1 : currentSleepTimeMs) * 2);
+          } else {
+            currentSleepTimeMs = sleepTimeMs; // reset to initial sleep time
+          }
+          EditLogTailer.this.sleep(currentSleepTimeMs);
         } catch (InterruptedException e) {
           LOG.warn("Edit log tailer interrupted: {}", e.getMessage());
         }

hadoop-hdfs-project/hadoop-hdfs/src/main/resources/hdfs-default.xml

@@ -1662,9 +1662,25 @@
   <name>dfs.ha.tail-edits.period</name>
   <value>60s</value>
   <description>
-    How often, in seconds, the StandbyNode should check for new
+    How often, the StandbyNode and ObserverNode should check if there are new
-    finalized log segments in the shared edits log.
+    edit log entries ready to be consumed. This is the minimum period between
-    Support multiple time unit suffix(case insensitive), as described
+    checking; exponential backoff will be applied if no edits are found and
+    dfs.ha.tail-edits.period.backoff-max is configured. By default, no
+    backoff is applied.
+    Supports multiple time unit suffix (case insensitive), as described
+    in dfs.heartbeat.interval.
+  </description>
+</property>
+
+<property>
+  <name>dfs.ha.tail-edits.period.backoff-max</name>
+  <value>0</value>
+  <description>
+    The maximum time the tailer should wait between checking for new edit log
+    entries. Exponential backoff will be applied when an edit log tail is
+    performed but no edits are available to be read. Values less than or
+    equal to zero disable backoff entirely; this is the default behavior.
+    Supports multiple time unit suffix (case insensitive), as described
     in dfs.heartbeat.interval.
   </description>
 </property>

hadoop-hdfs-project/hadoop-hdfs/src/site/markdown/ObserverNameNode.md

@@ -140,13 +140,33 @@ few configurations to your **hdfs-site.xml**:
    If too large, RPC time will increase as client requests will wait
    longer in the RPC queue before Observer tails edit logs and catches
    up the latest state of Active. The default value is 1min. It is
-   **highly recommend** to configure this to a much lower value.
+   **highly recommend** to configure this to a much lower value. It is also
+   recommended to configure backoff to be enabled when using low values; please
+   see below.
 
         <property>
           <name>dfs.ha.tail-edits.period</name>
           <value>0ms</value>
         </property>
 
+*  **dfs.ha.tail-edits.period.backoff-max** - whether the Standby/Observer
+   NameNodes should perform backoff when tailing edits.
+
+   This determines the behavior of a Standby/Observer when it attempts to
+   tail edits from the JournalNodes and finds no edits available. This is a
+   common situation when the edit tailing period is very low, but the cluster
+   is not heavily loaded. Without this configuration, such a situation will
+   cause high utilization on the Standby/Observer as it constantly attempts to
+   read edits even though there are none available. With this configuration
+   enabled, exponential backoff will be performed when an edit tail attempt
+   returns 0 edits. This configuration specifies the maximum time to wait
+   between edit tailing attempts.
+
+        <property>
+          <name>dfs.ha.tail-edits.period</name>
+          <value>10s</value>
+        </property>
+
 *  **dfs.journalnode.edit-cache-size.bytes** - the in-memory cache size,
    in bytes, on the JournalNodes.
 

hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/server/namenode/ha/TestEditLogTailer.java

@@ -20,6 +20,7 @@
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertTrue;
 import static org.junit.Assert.fail;
+import static org.mockito.Mockito.mock;
 import static org.mockito.Mockito.when;
 
 import java.io.File;
@@ -27,9 +28,14 @@
 import java.net.BindException;
 import java.net.URI;
 import java.util.ArrayList;
+import java.util.Arrays;
 import java.util.Collection;
+import java.util.List;
+import java.util.Queue;
 import java.util.Random;
 import java.util.concurrent.Callable;
+import java.util.concurrent.ConcurrentLinkedQueue;
+import java.util.concurrent.TimeUnit;
 import java.util.concurrent.TimeoutException;
 import java.util.concurrent.atomic.AtomicInteger;
 
@@ -42,8 +48,10 @@
 import org.apache.hadoop.hdfs.HdfsConfiguration;
 import org.apache.hadoop.hdfs.MiniDFSCluster;
 import org.apache.hadoop.hdfs.MiniDFSNNTopology;
+import org.apache.hadoop.hdfs.server.common.HdfsServerConstants;
 import org.apache.hadoop.hdfs.server.namenode.FSEditLog;
 import org.apache.hadoop.hdfs.server.namenode.FSImage;
+import org.apache.hadoop.hdfs.server.namenode.FSNamesystem;
 import org.apache.hadoop.hdfs.server.namenode.NNStorage;
 import org.apache.hadoop.hdfs.server.namenode.NameNode;
 import org.apache.hadoop.hdfs.server.namenode.NameNodeAdapter;
@@ -151,6 +159,46 @@ public void testTailer() throws IOException, InterruptedException,
     }
   }
 
+  @Test
+  public void testTailerBackoff() throws Exception {
+    Configuration conf = new Configuration();
+    NameNode.initMetrics(conf, HdfsServerConstants.NamenodeRole.NAMENODE);
+    conf.setTimeDuration(DFSConfigKeys.DFS_HA_TAILEDITS_PERIOD_KEY,
+        1, TimeUnit.MILLISECONDS);
+    conf.setTimeDuration(DFSConfigKeys.DFS_HA_TAILEDITS_PERIOD_BACKOFF_MAX_KEY,
+        10, TimeUnit.MILLISECONDS);
+    FSNamesystem mockNamesystem = mock(FSNamesystem.class);
+    FSImage mockImage = mock(FSImage.class);
+    NNStorage mockStorage = mock(NNStorage.class);
+    when(mockNamesystem.getFSImage()).thenReturn(mockImage);
+    when(mockImage.getStorage()).thenReturn(mockStorage);
+    final Queue<Long> sleepDurations = new ConcurrentLinkedQueue<>();
+    final int zeroEditCount = 5;
+    final AtomicInteger tailEditsCallCount = new AtomicInteger(0);
+    EditLogTailer tailer = new EditLogTailer(mockNamesystem, conf) {
+      @Override
+      void sleep(long sleepTimeMs) {
+        if (sleepDurations.size() <= zeroEditCount) {
+          sleepDurations.add(sleepTimeMs);
+        }
+      }
+
+      @Override
+      public long doTailEdits() {
+        return tailEditsCallCount.getAndIncrement() < zeroEditCount ? 0 : 1;
+      }
+    };
+    tailer.start();
+    try {
+      GenericTestUtils.waitFor(
+          () -> sleepDurations.size() > zeroEditCount, 50, 10000);
+    } finally {
+      tailer.stop();
+    }
+    List<Long> expectedDurations = Arrays.asList(2L, 4L, 8L, 10L, 10L, 1L);
+    assertEquals(expectedDurations, new ArrayList<>(sleepDurations));
+  }
+
   @Test
   public void testNN0TriggersLogRolls() throws Exception {
     testStandbyTriggersLogRolls(0);