clintropolis opened a new pull request #12293:
URL: https://github.com/apache/druid/pull/12293
### Description
On very large historical servers with a lot of processors, queries using
lookups can find a decent amount of time being wasted on lock contention, when
the majority of these lookups are populated offline and then swapped to the new
version, making this synchronization unnecessary, at least when using 'on heap'
lookups.
I've re-arranged some code in `lookups-cached-global` to allow the
"versioned" family of lookups to now "allocate" a cache and then later "attach"
it after it is fully populated, which for on-heap lookups means that a
non-concurrent map will be used, and wrapped in a `Collections.unmodifiableMap`
when "attaching" it to the lookups cache. Since all consumers after initial
population are read only, and version updates populate a completely new cache
prior to attaching, this should be safe as far as I can tell. Off heap lookups
have not been changed and will continue to use the concurrent map
implementation at this time.
Kafka extraction lookups still use the old "createCache" method, since they
do in fact incrementally update lookups as the consumer finds new values, so
they should be unchanged by this patch.
<hr>
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- [ ] added integration tests.
- [ ] been tested in a test Druid cluster.
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