We should be able to scale-up better with Solr/Lucene by utilizing multiple CPU cores, and not have to resort to scaling-out by sharding (with all the associated distributed system pitfalls) when the index size does not warrant it.
Presently, IndexSearcher has an optional constructor arg for an ExecutorService, which gets used for searching in parallel for call paths where one of the TopDocCollector's is created internally. The per-atomic-reader search happens in parallel and then the TopDocs/TopFieldDocs results are merged with locking around the merge bit.
However there are some problems with this approach:
- If arbitary Collector args come into play, we can't parallelize. Note that even if ultimately results are going to a TopDocCollector it may be wrapped inside e.g. a EarlyTerminatingCollector or TimeLimitingCollector or both.
- The special-casing with parallelism baked on top does not scale, there are many Collector's that could potentially lend themselves to parallelism, and special-casing means the parallelization has to be re-implemented if a different permutation of collectors is to be used.
A refactoring of collectors that allows for parallelization at the level of the collection protocol.
Some requirements that should guide the implementation:
- easy migration path for collectors that need to remain serial
- the parallelization should be composable (when collectors wrap other collectors)
- allow collectors to pick the optimal solution (e.g. there might be memory tradeoffs to be made) by advising the collector about whether a search will be parallelized, so that the serial use-case is not penalized.
- encourage use of non-blocking constructs and lock-free parallelism, blocking is not advisable for the hot-spot of a search, besides wasting pooled threads.