Kai Londenberg created PIG-3212:
-----------------------------------
Summary: Race Conditions in POSort and (Internal)SortedBag during
Proactive Spill.
Key: PIG-3212
URL: https://issues.apache.org/jira/browse/PIG-3212
Project: Pig
Issue Type: Bug
Reporter: Kai Londenberg
Priority: Critical
The following bug exists in the latest release of Pig 0.11.0
While running some large jobs involving groups and sorts like these:
{code}
events_by_user = GROUP events BY user_id;
sorted_events_by_user = FOREACH events_by_user {
A = ORDER events BY ts, split_idx, line_num;
GENERATE group, A;
}
{code}
I got a pretty strange behaviour: While this worked on small datasets, if I ran
it on large datasets, the results were sometimes not sorted perfectly.
So after a long debugging session, I tracked it down to at least one race
condition:
The following partial stack trace shows how a proactive spill gets triggered on
an InternalSortedBag. A spill in turn triggers a sort of that InternalSortedBag.
{code}
at
org.apache.pig.data.SortedSpillBag.proactive_spill(SortedSpillBag.java:83)
at
org.apache.pig.data.InternalSortedBag.spill(InternalSortedBag.java:455)
at
org.apache.pig.impl.util.SpillableMemoryManager.handleNotification(SpillableMemoryManager.java:243)
at
sun.management.NotificationEmitterSupport.sendNotification(NotificationEmitterSupport.java:138)
at sun.management.MemoryImpl.createNotification(MemoryImpl.java:171)
at
sun.management.MemoryPoolImpl$PoolSensor.triggerAction(MemoryPoolImpl.java:272)
at sun.management.Sensor.trigger(Sensor.java:120)
{code}
At the same time, the same InternalSortedBag might be sorted or accessed within
a POSort Operation. For example using the following Code path (line numbers
might be off, I had to add debug statements to diagnose this)
{code}
at
org.apache.pig.backend.hadoop.executionengine.physicalLayer.relationalOperators.POSort.getNext(POSort.java:346)
at
org.apache.pig.backend.hadoop.executionengine.physicalLayer.PhysicalOperator.getNext(PhysicalOperator.java:492)
at
org.apache.pig.backend.hadoop.executionengine.physicalLayer.expressionOperators.POProject.processInputBag(POProject.java:582)
at
org.apache.pig.backend.hadoop.executionengine.physicalLayer.expressionOperators.PORelationToExprProject.getNext(PORelationToExprProject.java:107)
at
org.apache.pig.backend.hadoop.executionengine.physicalLayer.PhysicalOperator.getNext(PhysicalOperator.java:394)
at
org.apache.pig.backend.hadoop.executionengine.physicalLayer.relationalOperators.POForEach.processPlan(POForEach.java:372)
at
org.apache.pig.backend.hadoop.executionengine.physicalLayer.relationalOperators.POForEach.getNext(POForEach.java:297)
at
org.apache.pig.backend.hadoop.executionengine.physicalLayer.PhysicalOperator.processInput(PhysicalOperator.java:368)
at
org.apache.pig.backend.hadoop.executionengine.physicalLayer.relationalOperators.POSplit.getNext(POSplit.java:214)
at
org.apache.pig.backend.hadoop.executionengine.mapReduceLayer.PigGenericMapReduce$Reduce.runPipeline(PigGenericMapReduce.java:465)
at
org.apache.pig.backend.hadoop.executionengine.mapReduceLayer.PigGenericMapReduce$Reduce.processOnePackageOutput(PigGenericMapReduce.java:433)
at
org.apache.pig.backend.hadoop.executionengine.mapReduceLayer.PigGenericMapReduce$Reduce.reduce(PigGenericMapReduce.java:413)
at
org.apache.pig.backend.hadoop.executionengine.mapReduceLayer.PigGenericMapReduce$Reduce.reduce(PigGenericMapReduce.java:257)
at org.apache.hadoop.mapreduce.Reducer.run(Reducer.java:176)
at
org.apache.hadoop.mapred.ReduceTask.runNewReducer(ReduceTask.java:566)
at org.apache.hadoop.mapred.ReduceTask.run(ReduceTask.java:408)
at org.apache.hadoop.mapred.Child.main(Child.java:170)
{code}
The key here is: Both operations try to compare and modify elements of the
SortedBag simultaneously. This leads to all kinds of problems, most notably
incorrectly sorted data.
POSort.SortComparator that's passed as a Comparison function to
(Internal)SortedBag is not thread safe, since it works by attaching single
input tuples to PhysicalOperator's - these Operators in turn are part of the
POSort.sortPlans and are re-used among each thread accessing the
(Internal)SortedBag.
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