Github user viirya commented on a diff in the pull request:

    https://github.com/apache/spark/pull/18159#discussion_r125803128
  
    --- Diff: 
sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/FileFormatWriter.scala
 ---
    @@ -314,21 +339,40 @@ object FileFormatWriter extends Logging {
     
               recordsInFile = 0
               releaseResources()
    +          numOutputRows += recordsInFile
               newOutputWriter(fileCounter)
             }
     
             val internalRow = iter.next()
    +        val startTime = System.nanoTime()
             currentWriter.write(internalRow)
    +        timeOnCurrentFile += (System.nanoTime() - startTime)
    --- End diff --
    
    I'm neutral about the per-row time tracking. Comparing with actual data 
writing, it should be faster. IMHO, so the performance penalty may be ignored?
    
    In both `SingleDirectoryWriteTask` and `DynamicPartitionWriteTask`, in each 
iteration, we first pull the row from the iterator and then write the row out 
(and measure the timing). So the time spent on pulling rows from data pipeline 
should be already excluded. That's why I'm not worried it may be inaccurate due 
to data pipelining.
    



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