Investigating high heap usage

What is the best approach or way to investigate why heap usage is so high? I'm watching the node stats and I can see the fielddata and filter cache which I understand are a part of the heap, but if those two are low, no indexing is currently being done and heap is still high, where should I be looking?

Another thing I tried is getting the hot_threads, but I'm not sure what to make of it:

Is there a way to get a snapshot of whats currently in the heap in a human readable way?

Are you using Marvel to monitor things?

Nope :frowning:

Unfortunately I'm not using Marvel, is there sort of a cheap sheet of things to check in order to diagnose heap issuse in Elasticsearch? Is there a page that explains how to read the hot_threads output?

No and no :frowning:
But a quick check doesn't show anything bad in that hot threads.