[polyfractal]

Not the OP, but GC issues in Elasticsearch basically boil down to memory pressure (obviously), which is usually caused by facets. Facets eat a lot of memory, especially if you are faceting high-cardinality fields - think fields like "tags" or any analyzed field. High cardinality, analyzed strings is the easiest way to blow out the heap.

There are other reasons, but that is like 90% of GC issues. To solve it, you need to make sure your faceted fields are configured well (usually not_analyzed) and assess how much memory is available. You may be able to index and even full-text search ten billion docs on a single machine, but faceting it may just be too much to ask for a single node.

Omiting norms, disabling bloom filters on old indices and enabling doc values are other ways to help alleviate field-data pressure.

Other GC culprits can be: too large bulk requests, unbounded threadpool queues, or something like parent/child/scripts/filter cache keys eating all your memory. Also don't go above 30gb heaps, the JVM becomes unhappy :)