[jira] [Created] (SPARK-21401) add poll function for BoundedPriorityQueue

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

Peng Meng created SPARK-21401:
---------------------------------

             Summary: add poll function for BoundedPriorityQueue
                 Key: SPARK-21401
                 URL: https://issues.apache.org/jira/browse/SPARK-21401
             Project: Spark
          Issue Type: Improvement
          Components: ML, MLlib
    Affects Versions: 2.3.0
            Reporter: Peng Meng
            Priority: Minor


The most of BoundedPriorityQueue usages in ML/MLLIB are:
Get the value of BoundedPriorityQueue, then sort it.
For example, in Word2Vec: pq.toSeq.sortBy(-_._2)
in ALS, pq.toArray.sorted()

The test results show using pq.poll() is much faster than sort the value.
For example, in PR https://github.com/apache/spark/pull/18624
We get the sorted value of pq by the following code:
{quote}
var size = pq.size
while(size > 0) {
     size -= 1
     val factor = pq.poll
}{quote} 

If using the generally used methods: pq.toArray.sorted() to get the sorted value of pq. There is about 10% performance reduction. 

It is good to add the poll function for BoundedPriorityQueue, since many usages of PQ need  the sorted value.




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