HDFS-11345. Document the configuration key for FSNamesystem lock fairness. Contributed by Erik Krogen.

(cherry picked from commit 2c769167db)
(cherry picked from commit 6b3c13d543)

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

@@ -383,6 +383,10 @@ public class DFSConfigKeys extends CommonConfigurationKeys {
   public static final long
       DFS_NAMENODE_MAX_LOCK_HOLD_TO_RELEASE_LEASE_MS_DEFAULT = 25;
 
+  public static final String DFS_NAMENODE_FSLOCK_FAIR_KEY =
+      "dfs.namenode.fslock.fair";
+  public static final boolean DFS_NAMENODE_FSLOCK_FAIR_DEFAULT = true;
+
   public static final String  DFS_NAMENODE_LOCK_DETAILED_METRICS_KEY =
       "dfs.namenode.lock.detailed-metrics.enabled";
   public static final boolean DFS_NAMENODE_LOCK_DETAILED_METRICS_DEFAULT =

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

@@ -32,6 +32,8 @@ import org.apache.hadoop.util.Timer;
 
 import static org.apache.hadoop.hdfs.DFSConfigKeys.DFS_LOCK_SUPPRESS_WARNING_INTERVAL_DEFAULT;
 import static org.apache.hadoop.hdfs.DFSConfigKeys.DFS_LOCK_SUPPRESS_WARNING_INTERVAL_KEY;
+import static org.apache.hadoop.hdfs.DFSConfigKeys.DFS_NAMENODE_FSLOCK_FAIR_DEFAULT;
+import static org.apache.hadoop.hdfs.DFSConfigKeys.DFS_NAMENODE_FSLOCK_FAIR_KEY;
 import static org.apache.hadoop.hdfs.DFSConfigKeys.DFS_NAMENODE_LOCK_DETAILED_METRICS_DEFAULT;
 import static org.apache.hadoop.hdfs.DFSConfigKeys.DFS_NAMENODE_LOCK_DETAILED_METRICS_KEY;
 import static org.apache.hadoop.hdfs.DFSConfigKeys.DFS_NAMENODE_READ_LOCK_REPORTING_THRESHOLD_MS_DEFAULT;
@@ -113,7 +115,8 @@ class FSNamesystemLock {
   @VisibleForTesting
   FSNamesystemLock(Configuration conf,
       MutableRatesWithAggregation detailedHoldTimeMetrics, Timer timer) {
-    boolean fair = conf.getBoolean("dfs.namenode.fslock.fair", true);
+    boolean fair = conf.getBoolean(DFS_NAMENODE_FSLOCK_FAIR_KEY,
+        DFS_NAMENODE_FSLOCK_FAIR_DEFAULT);
     FSNamesystem.LOG.info("fsLock is fair: " + fair);
     this.coarseLock = new ReentrantReadWriteLock(fair);
     this.timer = timer;

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

@@ -2652,6 +2652,16 @@
   </description>
 </property>
 
+<property>
+  <name>dfs.namenode.fslock.fair</name>
+  <value>true</value>
+  <description>If this is true, the FS Namesystem lock will be used in Fair mode,
+    which will help to prevent writer threads from being starved, but can provide
+    lower lock throughput. See java.util.concurrent.locks.ReentrantReadWriteLock
+    for more information on fair/non-fair locks.
+  </description>
+</property>
+
 <property>
   <name>dfs.namenode.startup.delay.block.deletion.sec</name>
   <value>0</value>

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

@@ -40,6 +40,7 @@ import java.util.concurrent.TimeoutException;
 import java.util.regex.Pattern;
 
 import static org.junit.Assert.*;
+import static org.apache.hadoop.hdfs.DFSConfigKeys.DFS_NAMENODE_FSLOCK_FAIR_KEY;
 import static org.apache.hadoop.test.MetricsAsserts.assertCounter;
 import static org.apache.hadoop.test.MetricsAsserts.assertGauge;
 
@@ -53,11 +54,11 @@ public class TestFSNamesystemLock {
   public void testFsLockFairness() throws IOException, InterruptedException{
     Configuration conf = new Configuration();
 
-    conf.setBoolean("dfs.namenode.fslock.fair", true);
+    conf.setBoolean(DFS_NAMENODE_FSLOCK_FAIR_KEY, true);
     FSNamesystemLock fsnLock = new FSNamesystemLock(conf, null);
     assertTrue(fsnLock.coarseLock.isFair());
 
-    conf.setBoolean("dfs.namenode.fslock.fair", false);
+    conf.setBoolean(DFS_NAMENODE_FSLOCK_FAIR_KEY, false);
     fsnLock = new FSNamesystemLock(conf, null);
     assertFalse(fsnLock.coarseLock.isFair());
   }
@@ -103,7 +104,7 @@ public class TestFSNamesystemLock {
     final int threadCount = 3;
     final CountDownLatch latch = new CountDownLatch(threadCount);
     final Configuration conf = new Configuration();
-    conf.setBoolean("dfs.namenode.fslock.fair", true);
+    conf.setBoolean(DFS_NAMENODE_FSLOCK_FAIR_KEY, true);
     final FSNamesystemLock rwLock = new FSNamesystemLock(conf, null);
     rwLock.writeLock();
     ExecutorService helper = Executors.newFixedThreadPool(threadCount);