[GitHub] [arrow] Dandandan commented on pull request #9086: [Rust] [DataFusion] [Experiment] Blocking threads filter

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Dandandan commented on pull request #9086:
URL: https://github.com/apache/arrow/pull/9086#issuecomment-754162220


   @jorgecarleitao 
   
   This is really cool, thanks for creating this experiment!
   I am not very deep yet into the Rust way of doing parallelism, the documentation of tokio makes sense to me.
   
   Some ideas:
   
   * In general, I think it is best if the parallelism is on a high level as possible to reduce the amount of overhead related to scheduling / context switching, etc.
   * But in order to utilize parallelism best it should be fine-grained enough.
   * I think there is some balance between total control of large amount of control control over parallelism. I think Spark concurrency via partitions is an example where you can have a larger amount of control over it. It is not always fine-grained enough, e.g. if you have one 1  / a couple of files as input.
   * I think filtering batches is relatively fine-grained, so I am wondering if this a good level for parallelism.
   
   * Tokios default config `max_blocking_threads` is 512, this is I think very large for CPU intensive work (and would have a negative effect on performance) https://docs.rs/tokio/1.0.1/tokio/runtime/struct.Builder.html#method.max_blocking_threads. Maybe if using different "scopes" it makes sense to use a different runtime for CPU-intensive work where you use a different `max_blocking_threads` config?
   * Tokio's documentation seems to hint that Rayon would be a better choice for CPU intensive work?
   * In the `ParquetExec` `thread::spawn` is being used. `task::spawn_blocking` seems a better choice there as it handles errors in a better way and can limit the nr. of threads compared to  thread::spawn` I guess?
   
   * I think just as the statistics @andygrove started to add for `Exec`s it would be good to have something here as well to debug issues and make sure we are not doing things in an inefficient way


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