OpenSearch/server
Nik Everett 2f38aeb5e2
Save memory when numeric terms agg is not top (#55873) (#56454)
Right now all implementations of the `terms` agg allocate a new
`Aggregator` per bucket. This uses a bunch of memory. Exactly how much
isn't clear but each `Aggregator` ends up making its own objects to read
doc values which have non-trivial buffers. And it forces all of it
sub-aggregations to do the same. We allocate a new `Aggregator` per
bucket for two reasons:

1. We didn't have an appropriate data structure to track the
   sub-ordinals of each parent bucket.
2. You can only make a single call to `runDeferredCollections(long...)`
   per `Aggregator` which was the only way to delay collection of
   sub-aggregations.

This change switches the method that builds aggregation results from
building them one at a time to building all of the results for the
entire aggregator at the same time.

It also adds a fairly simplistic data structure to track the sub-ordinals
for `long`-keyed buckets.

It uses both of those to power numeric `terms` aggregations and removes
the per-bucket allocation of their `Aggregator`. This fairly
substantially reduces memory consumption of numeric `terms` aggregations
that are not the "top level", especially when those aggregations contain
many sub-aggregations. It also is a pretty big speed up, especially when
the aggregation is under a non-selective aggregation like
the `date_histogram`.

I picked numeric `terms` aggregations because those have the simplest
implementation. At least, I could kind of fit it in my head. And I
haven't fully understood the "bytes"-based terms aggregations, but I
imagine I'll be able to make similar optimizations to them in follow up
changes.
2020-05-08 20:38:53 -04:00
..
licenses Upgrade to lucene 8.5.1 release (#55229) (#55235) 2020-04-15 17:35:42 +02:00
src Save memory when numeric terms agg is not top (#55873) (#56454) 2020-05-08 20:38:53 -04:00
build.gradle Create plugin for internalClusterTest task (#56067) 2020-05-06 17:20:52 -07:00