[jira] [Created] (SPARK-19222) Limit Query Performance issue

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

Sujith created SPARK-19222:
------------------------------

             Summary: Limit Query Performance issue
                 Key: SPARK-19222
                 URL: https://issues.apache.org/jira/browse/SPARK-19222
             Project: Spark
          Issue Type: Bug
          Components: SQL
    Affects Versions: 2.1.0
         Environment: Linux/Windows
            Reporter: Sujith
            Priority: Minor


Performance/memory bottle neck occurs in the below mentioned query
case 1:
create table t1 as select * from dest1 limit 10000000;
case 2:
create table t1 as select * from dest1 limit 1000;
pre-condition : partition count >=10000

In above cases limit is being added in the terminal of the physical plan 

== Physical Plan  ==
ExecutedCommand
   +- CreateHiveTableAsSelectCommand [Database:spark}, TableName: t2, InsertIntoHiveTable]
         +- GlobalLimit 10000000
            +- LocalLimit 10000000
               +- Project [imei#101, age#102, task#103L, num#104, level#105, productdate#106, name#107, point#108]
                  +- SubqueryAlias hive
                     +- Relation[imei#101,age#102,task#103L,num#104,level#105,productdate#106,name#107,point#108] csv  |
Issue Hints: 

Possible Bottleneck snippet in limit.scala file under spark-sql package.
  protected override def doExecute(): RDD[InternalRow] = {
    val locallyLimited = child.execute().mapPartitionsInternal(_.take(limit))
    val shuffled = new ShuffledRowRDD(
      ShuffleExchange.prepareShuffleDependency(
        locallyLimited, child.output, SinglePartition, serializer))
    shuffled.mapPartitionsInternal(_.take(limit))
  }

As mentioned in above case 1  (where limit value is 10000000 or partition count is > 10000) and case 2(limit value is small(around 1000)), As per the above snippet when the ShuffledRowRDD
is created by grouping all the limit data from different partitions to a single partition in executer,  memory issue occurs since all the partition limit data will be collected and 
grouped  in a single partition for processing, in both former/later case the data count  can go very high which can create the memory bottleneck.

Proposed solution for case 2:
An accumulator value can be to send to all partitions, all executor will be updating the accumulator value based on the  data fetched , 
eg: Number of partition = 100, number of cores =10
Ideally tasks will be launched in a group of 10 task/core, once the first group finishes the tasks driver will check whether the accumulator value is been reached the limit value if its reached then no further tasks will be launched to executors and the result after applying limit will be returned.

Please let me now for any suggestions or solutions for the below problems



--
This message was sent by Atlassian JIRA
(v6.3.4#6332)

---------------------------------------------------------------------
To unsubscribe, e-mail: issues-unsubscribe@spark.apache.org
For additional commands, e-mail: issues-help@spark.apache.org