Hi zhenhua,
Thanks for the idea.
Actually, I think we can completely avoid shuffling the data in a limit
operation, no matter LocalLimit or GlobalLimit.
wangzhenhua (G) wrote
> How about this:
> 1. we can make LocalLimit shuffle to mutiple partitions, i.e. create a new
> partitioner to uniformly dispatch the data
>
> class LimitUniformPartitioner(partitions: Int) extends Partitioner {
>
> def numPartitions: Int = partitions
>
> var num = 0
>
> def getPartition(key: Any): Int = {
> num = num + 1
> num % partitions
> }
>
> override def equals(other: Any): Boolean = other match {
> case h: HashPartitioner =>
> h.numPartitions == numPartitions
> case _ =>
> false
> }
>
> override def hashCode: Int = numPartitions
> }
>
> 2. then in GlobalLimit, we only take the first
> limit_number/num_of_shufflepartitions elements in each partition.
>
> One issue left is how to decide shuffle partition number.
> We can have a config of the maximum number of elements for each
> GlobalLimit task to process,
> then do a factorization to get a number most close to that config.
> E.g. the config is 2000:
> if limit=10000, 10000 = 2000 * 5, we shuffle to 5 partitions
> if limit=9999, 9999 = 1111 * 9, we shuffle to 9 partitions
> if limit is a prime number, we just fall back to single partition
>
> best regards,
> -zhenhua
>
>
> -----邮件原件-----
> 发件人: Liang-Chi Hsieh [mailto:
> viirya@
> ]
> 发送时间: 2017年1月18日 15:48
> 收件人:
> dev@.apache
> 主题: Re: Limit Query Performance Suggestion
>
>
> Hi Sujith,
>
> I saw your updated post. Seems it makes sense to me now.
>
> If you use a very big limit number, the shuffling before `GlobalLimit`
> would be a bottleneck for performance, of course, even it can eventually
> shuffle enough data to the single partition.
>
> Unlike `CollectLimit`, actually I think there is no reason `GlobalLimit`
> must shuffle all limited data from all partitions to one single machine
> with respect to query execution. In other words, I think we can avoid
> shuffling data in `GlobalLimit`.
>
> I have an idea to improve this and may update here later if I can make it
> work.
>
>
> sujith71955 wrote
>> Dear Liang,
>>
>> Thanks for your valuable feedback.
>>
>> There was a mistake in the previous post i corrected it, as you
>> mentioned the `GlobalLimit` we will only take the required number of
>> rows from the input iterator which really pulls data from local blocks
>> and remote blocks.
>> but if the limit value is very high >= 10000000, and when there will
>> be a shuffle exchange happens between `GlobalLimit` and `LocalLimit`
>> to retrieve data from all partitions to one partition, since the limit
>> value is very large the performance bottleneck still exists.
>>
>> soon in next post i will publish a test report with sample data and
>> also figuring out a solution for this problem.
>>
>> Please let me know for any clarifications or suggestions regarding
>> this issue.
>>
>> Regards,
>> Sujith
>
>
>
>
>
> -----
> Liang-Chi Hsieh | @viirya
> Spark Technology Center
> http://www.spark.tc/
> --
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> http://apache-spark-developers-list.1001551.n3.nabble.com/Limit-Query-Performance-Suggestion-tp20570p20652.html
> Sent from the Apache Spark Developers List mailing list archive at
> Nabble.com.
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Liang-Chi Hsieh | @viirya
Spark Technology Center
http://www.spark.tc/
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