zhijiangW commented on issue #6417: [FLINK-9913][runtime] Improve output 
serialization only once in RecordWriter
URL: https://github.com/apache/flink/pull/6417#issuecomment-413468551
 
 
   @pnowojski , I think I got the reason of regression in some non-broadcast 
cases. 
   
   The key point is when to call `RecordSerializer#prune()` which is used to 
decrease the intermediate serialization buffer. In the past, the prune method 
was only called after the target copying buffer is full and the record is full. 
But now after emitting each record we will call to prune method in order to 
narrow down the intermediate buffer ASAP. So the performance may be regression 
in sensitive job scenarios. I will modify to keep the previous behavior of 
calling prune in serializer.
   
   Thanks for your benchmark to find this potential issue. I only verified the 
broadcast scenarios in benchmark before and the obvious advantage may hide the 
potential regression. :)

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