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https://issues.apache.org/jira/browse/TEZ-3967?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16542205#comment-16542205
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Gopal V commented on TEZ-3967:
------------------------------

{code}
    TEZ_DAG_STATUS_CHECK_INTERVAL("hive.tez.dag.status.check.interval", "500ms",
      new TimeValidator(TimeUnit.MILLISECONDS), "Interval between subsequent 
DAG status invocation."),
{code}

bq. I can see unfair locking being better for through put for the exact exact 
same reason you are suggesting

The trouble is that overlapping read-locks are starving out the write-lock 
(because the read-lock can lock over the other read-lock) right now.

Yes, fairness is not free - if you have a different write-preferred locking for 
this, I think that's what we are really looking for.


> 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
>            Priority: Major
>
> 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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