[jira] [Created] (PIG-3212) Race Conditions in POSort and (Internal)SortedBag during Proactive Spill.

["Kai Londenberg (JIRA)" <ji...@apache.org>]

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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