Dandandan commented on PR #21138:
URL: https://github.com/apache/datafusion/pull/21138#issuecomment-4187523225

   > no separate IO threads
   
   My current view: I think we currently are using `spawn_blocking` fine to 
keep it of the main / query threads, which is not optimal (because we create 
too much/unbounded threads), but OK from separating IO calls / CPU.
   
   I think though that we: 
   
   * Don't perfectly balance work - that's where morselization will help to 
balance work evenly across threads (i.e. all threads are 90% busy most of the 
time
   * Don't interleave IO and CPU - that's where prefetching might help to get 
the last ~10-15% or so extra utilization
   * There is also some IO-level efficiency improvements, like file open / 
close on each read, which might help a bit (although it will matter even less 
when it is prefetched). 


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