[GitHub] [flink] AHeise commented on pull request #13027: [FLINK-16245] Decoupling user classloader from context classloader.

[GitBox <gi...@apache.org>]

AHeise commented on pull request #13027:
URL: https://github.com/apache/flink/pull/13027#issuecomment-668501769


   > Could you summarize what the problem was that caused the performance regression?
   
   For some reason, not setting a parent classloader in the wrapper is much slower. I cannot see an obvious reason and added a couple of measurements.
   
   First, I wanted to know if the performance difference was caused by slower class loading. So I measured the total time spent in `ClassLoader#loadClass` for the previous version and this version. I found no difference. There is also no difference by the amount of classes loaded.
   
   Second, a profiler revealed that much time in the slow version was spent in `Class#newInstance`. I added the time measurements to `org.apache.flink.table.runtime.generated.GeneratedClass`. Interestingly, there is again no difference in compiling the class. There is a huge difference in calling `newInstance` on the generated class though that can easily account for the time difference. 
   
   Without the parent, creating an instance of the 4 specific blink operators `BatchNestedLoopJoin`, `LongHashJoinOperator`, `LocalHashAggregateWithKeys`, and `HashAggregateWithKeys` slows down over time taking up over 10s to finish at the end of the. 
   
   Interestingly, they use only the instantiation method with explicit arguments `GeneratedClass#newInstance(ClassLoader classLoader, Object... args)`. However, there are also operators that use that method that are "well-behaved".
   
   I have also forced class resolution through reflection on the generated classes, but resolution is fast in all cases. It's really just about creating the instances.


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