MarkLogic forest memory and cache memory, resulting in low memory warnings

we are seeing lot of low memory warnings in MarkLogic logs.

forest + cache is exceeding 100% of memory.

what exactly is forest memory why it is spiking up drastically during loading content. we are seeing spike in meters db from 4GB to 21-25GB during migration which is resulting in low memory warnings.

can any one shed some light on what exactly is forest data?

some stats from our DB, we have 32GB RAM per host and we have 10 such nodes.
each host is having ondisk size of 19GB and in-memory-size of 3GB.