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https://issues.apache.org/jira/browse/TEZ-3967?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17087761#comment-17087761
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László Bodor commented on TEZ-3967:
-----------------------------------
[~gopalv], [~jeagles]: could you please take a look?
> 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 Vijayaraghavan
> Assignee: László Bodor
> Priority: Major
> Attachments: TEZ-3967.01.patch, TEZ-3967.02.patch
>
>
> 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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