DiskBBQ tail centroids should always be block encoded too

[tteofili]

DiskBBQ should always block encode (and bulk score) centroids, even for tails smaller than BULK_SIZE (16).

see #138296

[tteofili]

this provides some speedups at lower visit percentage, while retaining the same recall on hotpotQA E5 small.

baseline

index_name                       index_type  num_docs  doc_add_time(ms)  total_index_time(ms)  force_merge_time(ms)  num_segments
-------------------------------  ----------  --------  ----------------  --------------------  --------------------  ------------  
corpus-hotpotqa-E5-small-0.fvec         ivf   5000000             24590                108283                     0             5

index_name                       index_type  visit_percentage(%)  latency(ms)  net_cpu_time(ms)  avg_cpu_count     QPS  recall     visited  filter_selectivity  filter_cached  oversampling_factor
-------------------------------  ----------  -------------------  -----------  ----------------  -------------  ------  ------  ----------  ------------------  -------------  -------------------  
corpus-hotpotqa-E5-small-0.fvec         ivf                1.000         1.58              0.00           0.00  632.91    0.70   101969.61                1.00           true                 3.00
corpus-hotpotqa-E5-small-0.fvec         ivf                5.000         6.40              0.00           0.00  156.31    0.80   501993.56                1.00           true                 3.00
corpus-hotpotqa-E5-small-0.fvec         ivf               10.000        12.51              0.00           0.00   79.92    0.82  1001983.24                1.00           true                 3.00
corpus-hotpotqa-E5-small-0.fvec         ivf               30.000        35.32              0.00           0.00   28.31    0.84  3001917.90                1.00           true                 3.00
corpus-hotpotqa-E5-small-0.fvec         ivf               50.000        59.67              0.00           0.00   16.76    0.85  5001867.57                1.00           true                 3.00
corpus-hotpotqa-E5-small-0.fvec         ivf               70.000        81.80              0.00           0.00   12.22    0.85  7001670.77                1.00           true                 3.00
corpus-hotpotqa-E5-small-0.fvec         ivf              100.000       117.33              0.00           0.00    8.52    0.85  9999825.00                1.00           true                 3.00

candidate

index_name                       index_type  num_docs  doc_add_time(ms)  total_index_time(ms)  force_merge_time(ms)  num_segments
-------------------------------  ----------  --------  ----------------  --------------------  --------------------  ------------  
corpus-hotpotqa-E5-small-0.fvec         ivf   5000000             23018                110338                     0             5

index_name                       index_type  visit_percentage(%)  latency(ms)  net_cpu_time(ms)  avg_cpu_count     QPS  recall     visited  filter_selectivity  filter_cached  oversampling_factor
-------------------------------  ----------  -------------------  -----------  ----------------  -------------  ------  ------  ----------  ------------------  -------------  -------------------  
corpus-hotpotqa-E5-small-0.fvec         ivf                1.000         1.39              0.00           0.00  720.72    0.70   101969.61                1.00           true                 3.00
corpus-hotpotqa-E5-small-0.fvec         ivf                5.000         6.24              0.00           0.00  160.26    0.80   501993.56                1.00           true                 3.00
corpus-hotpotqa-E5-small-0.fvec         ivf               10.000        12.00              0.00           0.00   83.32    0.82  1001983.24                1.00           true                 3.00
corpus-hotpotqa-E5-small-0.fvec         ivf               30.000        35.17              0.00           0.00   28.43    0.84  3001917.90                1.00           true                 3.00
corpus-hotpotqa-E5-small-0.fvec         ivf               50.000        57.58              0.00           0.00   17.37    0.85  5001867.57                1.00           true                 3.00
corpus-hotpotqa-E5-small-0.fvec         ivf               70.000        80.82              0.00           0.00   12.37    0.85  7001670.77                1.00           true                 3.00
corpus-hotpotqa-E5-small-0.fvec         ivf              100.000       118.54              0.00           0.00    8.44    0.85  9999825.00                1.00           true                 3.00

[elasticsearchmachine]

Pinging @elastic/es-search-relevance (Team:Search Relevance)

[elasticsearchmachine]

Hi @tteofili, I've created a changelog YAML for you.

[john-wagster]

been tracking; this lgtm