[GitHub] [incubator-druid] leventov opened a new issue #8335: Skip `positions` indirection in `PooledTopNAlgorithm` when the aggregation size is small

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leventov opened a new issue #8335: Skip `positions` indirection in `PooledTopNAlgorithm` when the aggregation size is small
URL: https://github.com/apache/incubator-druid/issues/8335
 
 
   `int[] positions` indirection in [`PooledTopNAlgorithm`](https://github.com/apache/incubator-druid/blob/566dc8c719489283f9190cefd6346bbb3f12955f/processing/src/main/java/org/apache/druid/query/topn/PooledTopNAlgorithm.java) seems wasteful, especially when the aggregation size itself is just 4 or 8 bytes, as is the case of float/double/long Min/Max/Sum aggregations, leading to 33%/50% higher memory usage than needed for processing. It's role to initialize the aggregation at the right moment 1) can be replaced with `BitSet dimIndexInitialized`; 2) may be unnecessary/wasteful itself for aggregators which zero the memory as their initialization step: it may be faster to just stream set the whole buffer's memory to zero at the beginning of processing.
   
   There is a locality concern for larger aggregations: `positions` facilitate putting the hottest aggregations together at the beginning of the buffer, thus improving the cache and the TLB utilization. This positive effect is completely canceled by the `positions` itself (access to which is still random) for aggregations of 4 bytes and almost for sure for aggregations of 8 bytes. After that, there should be experiments showing at which aggregation size the positive effect of `positions` outweigh its negative effect (which also diminishes with the growth of the aggregation size): its likely to be somewhere between 12 and 32 bytes, but benchmarking is required to determine the threshold more precisely.

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