[jira] [Created] (TEZ-3967) DAGImpl: dag lock is unfair and can starve the writers

[Gopal V (JIRA)]

Gopal V created TEZ-3967:
----------------------------

             Summary: DAGImpl: dag lock is unfair and can starve the writers
                 Key: TEZ-3967
                 URL: https://issues.apache.org/jira/browse/TEZ-3967
             Project: Apache Tez
          Issue Type: Bug
            Reporter: Gopal V


Found when debugging HIVE-20103, that a reader arriving when another reader is 
active can postpone a writer from obtaining a write-lock.

This is fundamentally bad for the DAGImpl as useful progress can only happen 
when the writeLock is held.

{code}
  public void handle(DAGEvent event) {
...
    try {
      writeLock.lock();
{code}

{code}
   java.lang.Thread.State: WAITING (parking)
        at sun.misc.Unsafe.park(Native Method)
        - parking to wait for  <0x00007efb02246f40> (a 
java.util.concurrent.locks.ReentrantReadWriteLock$NonfairSync)
        at java.util.concurrent.locks.LockSupport.park(LockSupport.java:175)
        at 
java.util.concurrent.locks.AbstractQueuedSynchronizer.parkAndCheckInterrupt(AbstractQueuedSynchronizer.java:836)
        at 
java.util.concurrent.locks.AbstractQueuedSynchronizer.acquireQueued(AbstractQueuedSynchronizer.java:870)
        at 
java.util.concurrent.locks.AbstractQueuedSynchronizer.acquire(AbstractQueuedSynchronizer.java:1199)
        at 
java.util.concurrent.locks.ReentrantReadWriteLock$WriteLock.lock(ReentrantReadWriteLock.java:943)
        at org.apache.tez.dag.app.dag.impl.DAGImpl.handle(DAGImpl.java:1162)
        at org.apache.tez.dag.app.dag.impl.DAGImpl.handle(DAGImpl.java:149)
        at 
org.apache.tez.dag.app.DAGAppMaster$DagEventDispatcher.handle(DAGAppMaster.java:2251)
        at 
org.apache.tez.dag.app.DAGAppMaster$DagEventDispatcher.handle(DAGAppMaster.java:2242)
        at 
org.apache.tez.common.AsyncDispatcher.dispatch(AsyncDispatcher.java:180)
        at org.apache.tez.common.AsyncDispatcher$1.run(AsyncDispatcher.java:115)
        at java.lang.Thread.run(Thread.java:745)
{code}

while read-lock is passed around between 

{code}
       at org.apache.tez.dag.app.dag.impl.DAGImpl.getDAGStatus(DAGImpl.java:901)
        at 
org.apache.tez.dag.app.dag.impl.DAGImpl.getDAGStatus(DAGImpl.java:940)
        at 
org.apache.tez.dag.api.client.DAGClientHandler.getDAGStatus(DAGClientHandler.java:73)
{code}

calls.



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