Currently, FST loads all the terms into heap memory during index open. This causes frequent JVM OOM issues if the term size gets big. A better way of doing this will be to lazily load FST using mmap. That ensures only the required terms get loaded into memory.
Lucene can expose API for providing list of fields to load terms offheap. I'm planning to take following approach for this:
- Add a boolean property fstOffHeap in FieldInfo
- Pass list of offheap fields to lucene during index open (ALL can be special keyword for loading ALL fields offheap)
- Initialize the fstOffHeap property during lucene index open
- FieldReader invokes default FST constructor or OffHeap constructor based on fstOffHeap field
I created a patch (that loads all fields offheap), did some benchmarks using es_rally and results look good.