[GitHub] [arrow] jorgecarleitao edited a comment on pull request #9271: ARROW-11300: [Rust][DataFusion] Further performance improvements on hash aggregation with small groups

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jorgecarleitao edited a comment on pull request #9271:
URL: https://github.com/apache/arrow/pull/9271#issuecomment-765865323


   Thanks a lot for your points. I am learning a lot! :)
   
   Note that for small arrays, we are basically in the metadata problem on which the "payload size" of transmitting 1 element is driven by its metadata, not the data itself. This will always be a problem, as the arrow format was designed to be performant for large arrays.
   
   For example, all our buffers are shared via an `Arc`. There is a tradeoff between this indirection and mem-copying the memory region. The tradeoff works in `Arc`'s favor for large memory regions and vice-versa.
   
   With that said, we could consider replacing `Arc<ArrayData>` by `ArrayData` on all our arrays, to avoid the extra `Arc`: cloning an `ArrayData` is actually cheap. I am not sure if that would work for FFI, but we could certainly try.
   
   Another idea is to use `buffer1: Buffer`, `buffer2: Buffer` instead of `buffers: Vec<Buffer>` to avoid the `Vec`. This is possible because arrow arrays support at most 2 buffers (3 with the null). For types of a single buffer, we are already incurring the cost of the `Vec` and thus adding a `Buffer` instead should not be a big issue (memory-wise). The advantage of this is that we avoid cloning the `Vec` on every operation as well as the extra bound check. The disadvantage is that we have to be more verbose when we want to apply an operation to every buffer.
   
   


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