mccullocht commented on PR #15169:
URL: https://github.com/apache/lucene/pull/15169#issuecomment-3294250569
Average visited count in the query path actually is exposed in luceneutil
today, it just appears in the iteration summary and not the overall summary.
TIL. I've extracted it for my most recent run here:
```
-4 3794
-8 3857
4 4294
8 3849
```
4 bit is doing ~10% more comparisons than 8 bit for the same fanout. More
work in 4 bit but doesn't explain the size of the win in OSQ4. Workload CPU
usage and latency is mostly driven scoring costs so let's start by looking at
microbenchmark results for dot product on the same hardware:
```
VectorUtilBenchmark.binaryDotProductVector 128 thrpt 15 47.741
± 0.502 ops/us
VectorUtilBenchmark.binaryDotProductVector 256 thrpt 15 26.198
± 0.608 ops/us
VectorUtilBenchmark.binaryDotProductVector 300 thrpt 15 22.857
± 0.130 ops/us
VectorUtilBenchmark.binaryDotProductVector 512 thrpt 15 13.864
± 0.443 ops/us
VectorUtilBenchmark.binaryDotProductVector 702 thrpt 15 10.003
± 0.205 ops/us
VectorUtilBenchmark.binaryDotProductVector 1024 thrpt 15 6.795
± 0.098 ops/us
VectorUtilBenchmark.binaryHalfByteVectorPacked 128 thrpt 15 72.590
± 1.089 ops/us
VectorUtilBenchmark.binaryHalfByteVectorPacked 256 thrpt 15 50.962
± 0.197 ops/us
VectorUtilBenchmark.binaryHalfByteVectorPacked 300 thrpt 15 39.587
± 0.159 ops/us
VectorUtilBenchmark.binaryHalfByteVectorPacked 512 thrpt 15 31.660
± 0.187 ops/us
VectorUtilBenchmark.binaryHalfByteVectorPacked 702 thrpt 15 22.251
± 0.135 ops/us
VectorUtilBenchmark.binaryHalfByteVectorPacked 1024 thrpt 15 18.117
± 0.129 ops/us
```
At 702 dimensions (next closest to our test data set) half byte is about
twice as fast. The performance different between -4 and -8 makes sense with
this context. I don't know why 4 is so slow, these numbers suggest it shouldn't
be worse than 8 yet somehow it is 🤷. Not sure this is worth figuring out.
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