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From "Josh Elser (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (ACCUMULO-3745) deadlock in SourceSwitchingIterator
Date Thu, 23 Apr 2015 18:03:40 GMT

    [ https://issues.apache.org/jira/browse/ACCUMULO-3745?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14509471#comment-14509471
] 

Josh Elser commented on ACCUMULO-3745:
--------------------------------------

Keith helped me out on IRC. It took a third time to realize that his comment answered my question
:)

At his request, I tried to expand it to cover some more of what we covered (and what made
it click for me)
{code}
  // Synchronization on copies synchronizes operations across all deep copies of this instance.
  //
  // This implementation assumes that there is one thread reading data (a scan) from all deep
copies
  // and that another thread may call switch at any point. A single scan may have multiple
deep
  // copies of this iterator if other iterators above this one duplicate their source. For
example,
  // if an IntersectingIterator over two columns was configured, `copies` would contain two
SSIs
  // instead of just one SSI. The two instances in `copies` would both be at the same "position"
  // in the tree of iterators for the scan. If multiple instances of SSI are configure in
the iterator
  // tree (e.g. priority 8 and priority 12), each instance would share their own `copies`
e.g.
  // SSI@priority8:copies1[...], SSI@priority12:copies2[...]
{code}

> deadlock in SourceSwitchingIterator
> -----------------------------------
>
>                 Key: ACCUMULO-3745
>                 URL: https://issues.apache.org/jira/browse/ACCUMULO-3745
>             Project: Accumulo
>          Issue Type: Bug
>          Components: tserver
>    Affects Versions: 1.5.0, 1.5.1, 1.5.2, 1.6.0, 1.6.1, 1.6.2
>         Environment: Large production cluster, with complex iterator trees.
>            Reporter: Eric Newton
>            Priority: Blocker
>             Fix For: 1.5.3, 1.7.0, 1.6.3
>
>         Attachments: ACCUMULO-3745-1.patch, ACCUMULO-3745-2.patch
>
>          Time Spent: 40m
>  Remaining Estimate: 0h
>
> Details come from an offline cluster, so it's difficult to reproduce the exact details.
 A very complex iterator was running over tablet. "deepCopy" may have been called a couple
dozen times, which may have contributed to the problem.
> Relevant facts:
> A scan and a minor compaction created a deadlock which was detected by the java runtime.
> {noformat}
> "Query... ":
>   waiting to lock monitor 0x1234 (object 0x1234, a java.util.Collections$SynchronizedRandomAccessList),

>   which is held by "minor compactor 1"
> "minor compactor 1":
>  waiting to lock monitor 0x9876 (object 0x9876, a org.apache.accumulo.core.iterators.system.SourceSwitchingIterator),

>  which is held by "Query..."
> {noformat}
> Java stacks:
> {noformat}
> "Query..."
>   at java.util.Collections@SynchronizedCollection.add(Collections.java:1636)
>   - waiting to lock <0x1234> (a java.util.Collections$SynchronizedRandomAccessList)
>   at org.apache.accumulo.core.iterators.system.SourceSwitchingIterator.<init>(SourceSwitchingIterator.java:72)
>  at org.apache.accumulo.core.iterators.system.SourceSwitchingIterator.deepCopy(SourceSwitchingIterator:85)
>  - locked <0x9876> (a org.apache.accumulo.core.iterators.system.SourceSwitchingIterator)
>   ... PartialMutationSkippingIterator.deepCopy(InMememoryMap.java:113)
>  ... InMemoryMap#MemoryIterator.deepCopy(InnMemoryMap.java:623)
>  ...
> {noformat}
> and:
> {noformat}
> "minor compactor 1":
>  at org.apache.accumulo.core.iterators.system.SourceSwitchingIterarot._switchNow(SourceSwitchingIterator:171)
>  - waiting to lock <0x9876> (a org.apache.accumulo.core.iterators.system.SourceSwitchingIterator)
>  at org.apache.accumulo.iterators.system.SourceSwitchingIterator.switchNow(SourceSwitchingIterator.java:184)
>  locked <0x1234> (a java.util.Collections#SynhronizedRandomAccessList)
>  at org.apache.accumulo.tserver.InMemoryMap$MemoryIterator.switchNow(InMemoryMap.java:647)
>  ...
> {noformat}



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