punAhuja opened a new pull request, #4724: URL: https://github.com/apache/solr/pull/4724
https://issues.apache.org/jira/browse/SOLR-17841 <!-- _(If you are a project committer then you may remove some/all of the following template.)_ Before creating a pull request, please file an issue in the ASF Jira system for Solr: * https://issues.apache.org/jira/projects/SOLR For something minor (i.e. that wouldn't be worth putting in release notes), you can skip JIRA. To create a Jira issue, you will need to create an account there first. The title of the PR should reference the Jira issue number in the form: * SOLR-17841: <short description of problem or changes> SOLR must be fully capitalized. A short description helps people scanning pull requests for items they can work on. Properly referencing the issue in the title ensures that Jira is correctly updated with code review comments and commits. --> # Description `multiThreaded=true` can search Lucene segments in parallel, but the DocSet path (what faceting uses) went through a custom `FixedBitSetCollector`. That collector was much slower than the `DocSetCollector` the single-threaded path already uses, so enabling MT made faceted search slower. On a 1M-doc index I saw facet MT at ~15 ms mean / ~66 ms p95 vs ~7 ms / ~12 ms for ST. Plain (non-facet) queries were already about the same either way. This showed up during SOLR-13350 review as well (search faster, faceting slower). The custom collector was there for thread safety — one bitset per slice, merge at the end — not because `DocSetCollector` couldn't be used that way. # Solution Two changes in `MultiThreadedSearcher` / `SolrIndexSearcher`: 1. Each MT slice now uses `DocSetCollector` (same as ST). `reduce()` ORs the per-slice DocSets into one `FixedBitSet`. `FixedBitSetCollector` was only used here; it's removed. 2. `allowMT` also requires a searcher executor. `multiThreaded=true` with `indexSearcherExecutorThreads=0` used to enter the CollectorManager path anyway and could get very slow. It now falls back to ST. I used an AI coding assistant (Cursor) while investigating and drafting the change. I reviewed the diff, ran the tests, and did the benchmarks myself. # Tests Unit test: `TestMultiThreadedSearcher.testMultiThreadedDocSetMatchesSingleThreaded` — same query, `needDocSet=true`, ST vs MT DocSets must match on a multi-segment index. Latency was measured with solr-bench (one client query at a time; searcher pool `-Dsolr.searchThreads=-1`). Suites, generator, NoMerge configset, and how to run it: https://github.com/SearchScale/solr-bench/tree/puneet/SOLR-17841-mt-search I compared the same 1M-doc index merged vs forced to ~500 segments (`NoMergePolicy`, flush every 2000 docs). Queries are `event_time` range + `event_type` filter, with and without facets. | | mean | p50 | p95 | |---|---|---|---| | facet, few segments, MT before | ~15.1 | ~6.0 | ~66 | | facet, few segments, after (ST / MT) | 7.14 / 7.65 | 6.38 / 6.86 | 12.43 / 13.80 | | plain, many segments (ST / MT) | 4.55 / 3.36 | 2.75 / 2.78 | 21.56 / 7.75 | | facet, many segments (ST / MT) | 15.79 / 14.78 | 12.22 / 12.44 | 38.28 / 30.59 | After the fix, facet MT is no longer worse on a normal index. With many segments MT can win, especially on plain queries and p95. # Checklist Please review the following and check all that apply: - [ ] I have reviewed the guidelines for [How to Contribute](https://github.com/apache/solr/blob/main/CONTRIBUTING.md) and my code conforms to the standards described there to the best of my ability. - [ ] I have created a Jira issue and added the issue ID to my pull request title. - [ ] I have given Solr maintainers [access](https://help.github.com/en/articles/allowing-changes-to-a-pull-request-branch-created-from-a-fork) to contribute to my PR branch. (optional but recommended, not available for branches on forks living under an organisation) - [ ] I have developed this patch against the `main` branch. - [ ] I have run `./gradlew check`. - [ ] I have added tests for my changes. - [ ] I have added documentation for the [Reference Guide](https://github.com/apache/solr/tree/main/solr/solr-ref-guide) - [ ] I have added a [changelog entry](https://github.com/apache/solr/blob/main/dev-docs/changelog.adoc) for my change -- This is an automated message from the Apache Git Service. To respond to the message, please log on to GitHub and use the URL above to go to the specific comment. To unsubscribe, e-mail: [email protected] For queries about this service, please contact Infrastructure at: [email protected] --------------------------------------------------------------------- To unsubscribe, e-mail: [email protected] For additional commands, e-mail: [email protected]
