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From "Stephan Ewen (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (FLINK-3291) Object reuse bug in MergeIterator.HeadStream.nextHead
Date Fri, 12 Feb 2016 14:52:18 GMT

    [ https://issues.apache.org/jira/browse/FLINK-3291?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15144654#comment-15144654
] 

Stephan Ewen commented on FLINK-3291:
-------------------------------------

Thanks, @Greg and [~ggevay] for looking into this. It's good some people start going in-depth
there.

Here are a few comments from what I thought initially when we implemented the first object
reuse versions:

  - The initial idea of a contract for {{MutableObjectiterator.next(reuse)}} was the following:
      1. The caller may not hold onto {{reuse}} any more
      2. The iterator implementor may not hold onto the returned object any more.
     Given that this was long ago (5 years probably, since I created that interface), I am
pretty sure that contract is not obeyed everywhere.

  - In all cases, the non-reuse implementations should work without {{serializer.createInstance()}},
because in some corner cases, neither Flink nor Kryo/Objenesis manages to instantiate the
object.

  - Copying objects is sometimes necessary, but should be avoided where possible. In microbenchmarks,
the copies are often cheap (tuples of Strings, Integers), in practice many objects are JSON,
Avro, Thrift, and are hellishly expensive to copy.
    Especially in the non-reusing mode, I should be completely avoidable. The object reuse-mode
is mainly for programs with types that are mutable and efficient to copy. Reuse can become
more expensive the non-reuse if copies are expensive.

> Object reuse bug in MergeIterator.HeadStream.nextHead
> -----------------------------------------------------
>
>                 Key: FLINK-3291
>                 URL: https://issues.apache.org/jira/browse/FLINK-3291
>             Project: Flink
>          Issue Type: Bug
>          Components: Distributed Runtime
>    Affects Versions: 1.0.0
>            Reporter: Gabor Gevay
>            Assignee: Gabor Gevay
>            Priority: Critical
>
> MergeIterator.HeadStream.nextHead saves a reference into `this.head` of the `reuse` object
that it got as an argument. This object might be modified later by the caller.
> This actually happens when ReduceDriver.run calls input.next (which will actually be
MergeIterator.next(E reuse)) in the inner while loop of the objectReuseEnabled branch, and
that calls top.nextHead with the reference that it got from ReduceDriver, which erroneously
saves the reference, and then ReduceDriver later uses that same object for doing the reduce.
> Another way in which this fails is when MergeIterator.next(E reuse) gives `reuse` to
different `top`s in different calls, and then the heads end up being the same object.
> You can observe the latter situation in action by running ReducePerformance here:
> https://github.com/ggevay/flink/tree/merge-iterator-object-reuse-bug
> Set memory to -Xmx200m (so that the MergeIterator actually has merging to do), put a
breakpoint at the beginning of MergeIterator.next(reuse), and then watch `reuse`, and the
heads of the first two elements of `this.heap` in the debugger. They will get to be the same
object after hitting continue about 6 times.
> You can also look at the count that is printed at the end, which shouldn't be larger
than the key range. Also, if you look into the output file /tmp/xxxobjectreusebug, for example
the key 999977 appears twice.
> The good news is that I think I can see an easy fix that doesn't affect performance:
MergeIterator.HeadStream could have a reuse object of its own as a member, and give that to
iterator.next in nextHead(E reuse). And then we wouldn't need the overload of nextHead that
has the reuse parameter, and MergeIterator.next(E reuse) could just call its other overload.



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