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From "Jason Dai (JIRA)" <>
Subject [jira] [Commented] (SPARK-4672) Cut off the super long serialization chain in GraphX to avoid the StackOverflow error
Date Wed, 03 Dec 2014 04:09:13 GMT


Jason Dai commented on SPARK-4672:

I can see two possible ways to fix this:
# Define customized closure serialization mechanisms in task serializations, which can use
reflections to carefully choose which to serialize (i.e., only those referenced by RDD.iterator);
this potentially needs to deal with many details and can be error prone.
# In task serialization, each "base" RDD can generate a dual, "shippable" RDD, which has all
transient member variables, and only implements the compute() function (which in turn calls
the compute() function of the "base" RDD through ClosureCleaner.clean()); we can then probably
rely on the Java serializer to handle this correctly.

> Cut off the super long serialization chain in GraphX to avoid the StackOverflow error
> -------------------------------------------------------------------------------------
>                 Key: SPARK-4672
>                 URL:
>             Project: Spark
>          Issue Type: Bug
>          Components: GraphX, Spark Core
>    Affects Versions: 1.1.0
>            Reporter: Lijie Xu
>            Priority: Critical
>             Fix For: 1.2.0
> While running iterative algorithms in GraphX, a StackOverflow error will stably occur
in the serialization phase at about 300th iteration. In general, these kinds of algorithms
have two things in common:
> # They have a long computing chain.
> {code:borderStyle=solid}
> (e.g., “degreeGraph=>subGraph=>degreeGraph=>subGraph=>…=>”)
> {code}
> # They will iterate many times to converge. An example:
> {code:borderStyle=solid}
> //K-Core Algorithm
> val kNum = 5
> var degreeGraph = graph.outerJoinVertices(graph.degrees) {
> 		(vid, vd, degree) => degree.getOrElse(0)
> }.cache()
> do {
> 	val subGraph = degreeGraph.subgraph(
> 		vpred = (vid, degree) => degree >= KNum
> 	).cache()
> 	val newDegreeGraph = subGraph.degrees
> 	degreeGraph = subGraph.outerJoinVertices(newDegreeGraph) {
> 		(vid, vd, degree) => degree.getOrElse(0)
> 	}.cache()
> 	isConverged = check(degreeGraph)
> } while(isConverged == false)
> {code}
> After about 300 iterations, StackOverflow will definitely occur with the following stack
> {code:borderStyle=solid}
> Exception in thread "main" org.apache.spark.SparkException: Job aborted due to stage
failure: Task serialization failed: java.lang.StackOverflowError
> {code}
> It is a very tricky bug, which only occurs with enough iterations. Since it took us a
long time to find out its causes, we will detail the causes in the following 3 paragraphs.

> h3. Phase 1: Try using checkpoint() to shorten the lineage
> It's easy to come to the thought that the long lineage may be the cause. For some RDDs,
their lineages may grow with the iterations. Also, for some magical references,  their lineage
lengths never decrease and finally become very long. As a result, the call stack of task's
serialization()/deserialization() method will be very long too, which finally exhausts the
whole JVM stack.
> In deed, the lineage of some RDDs (e.g., EdgeRDD.partitionsRDD) increases 3 OneToOne
dependencies in each iteration in the above example. Lineage length refers to the  maximum
length of OneToOne dependencies (e.g., from the finalRDD to the ShuffledRDD) in each stage.
> To shorten the lineage, a checkpoint() is performed every N (e.g., 10) iterations. Then,
the lineage will drop down when it reaches a certain length (e.g., 33). 
> However, StackOverflow error still occurs after 300+ iterations!
> h3. Phase 2:  Abnormal f closure function leads to a unbreakable serialization chain
> After a long-time debug, we found that an abnormal _*f*_ function closure and a potential
bug in GraphX (will be detailed in Phase 3) are the "Suspect Zero". They together build another
serialization chain that can bypass the broken lineage cut by checkpoint() (as shown in Figure
1). In other words, the serialization chain can be as long as the original lineage before
> Figure 1 shows how the unbreakable serialization chain is generated. Yes, the OneToOneDep
can be cut off by checkpoint(). However, the serialization chain can still access the previous
RDDs through the (1)->(2) reference chain. As a result, the checkpoint() action is meaningless
and the lineage is as long as that before. 
> !|width=100%!
> The (1)->(2) chain can be observed in the debug view (in Figure 2).
> {code:borderStyle=solid}
> _rdd (i.e., A in Figure 1, checkpointed) -> f -> $outer (VertexRDD) -> partitionsRDD:MapPartitionsRDD
-> RDDs in  the previous iterations
> {code}
> !|width=100%!
> More description: While a RDD is being serialized, its f function 
> {code:borderStyle=solid}
> e.g., f: (Iterator[A], Iterator[B]) => Iterator[V]) in ZippedPartitionsRDD2
> {code}
> will be serialized too. This action will be very dangerous if the f closure has a member
“$outer” that references its outer class (as shown in Figure 1). This reference will be
another way (except the OneToOneDependency) that a RDD (e.g., PartitionsRDD) can reference
the other RDDs (e.g., VertexRDD). Note that checkpoint() only cuts off the direct lineage,
while the function reference is still kept. So, serialization() can still access the other
RDDs along the f references. 
> h3. Phase 3: Non-transient member variable of VertexRDD makes things worse
> "Reference (1)" in Figure 1 is caused by the abnormal f clousre, while "Reference (2)"
is caused by the potential bug in GraphX: *PartitionsRDD is a non-transient member variable
of VertexRDD*. 
> With this _small_ bug, the f closure itself (without OneToOne dependency) can cause StackOverflow
error, as shown in the red box in Figure 3:
> # While _vertices:VertexRDD_ is being serialized, its member _PartitionsRDD_ will be
serialized too.
> # Next, while serializing this _partitionsRDD_, serialization() will simultaneously serialize
its f’s referenced $outer. Here, it is another _partitionsRDD_.
> # Finally, the chain 
> {code:borderStyle=solid}
> "f => f$3 => f$3 => $outer => vertices: VertexRDD => partitionsRDD =>
… => ShuffledRDD"
> {code}
> comes into shape. As a result, the serialization chain can be as long as the original
lineage and finally triggers StackOverflow error.
> !|width=100%!
> h2. Conclusions
> In conclusion, the root cause of StackOverflow error is the long serialization chain,
which cannot be cut off by _checkpoint()_. This long chain is caused by the multiple factors,
> # long lineage
> # $outer reference in the f closure
> # non-transient member variable
> h2. How to fix this error
> We propose three pull requests as follows to solve this problem thoroughly.
> # PR-3544
> In this pr, we change the "val PartitionsRDD" to be transient in EdgeRDDImpl and VertexRDDImpl.
As a result, while _vertices:VertexRDD_ is being serialized, its member _PartitionsRDD_ will
not be serialized. In other words, the "Reference (2)" in Figure 1 will be cut off.
> # PR-3545
> In this pr, we set "f = null" if ZippedPartitionsRDD is checkpointed. As a result, when
PartitionsRDD is checkpointed, its f closure will be cleared and the "Reference (1)" (i.e.,
f => $outer) in Figure 1 will no exist.
> # PR-3549
> To cut off the long lineage, we need to perform checkpoint()  on PartitionsRDD. However,
current checkpoint() is performed on VertexRDD and EdgeRDD themselves. As a result, we need
to override the checkpoint() methods in VertexRDDImpl and EdgeRDDImpl to perform checkpoint()
on PartitionsRDD.

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