ES periodically crashed under high load.
I wonder how to diagnose ES's problems? Is there any ways or some tips for using ES?
Check your logs for OOM or long GC.
What version are you on? What is "high load"? What is causing the high load? Are you monitoring Elasticsearch?
You can install Marvel
bin/plugin -i elasticsearch/marvel/latest
and keep an eye on memory consumption, fielddata and filter cache size and evictions and other metrics
Only in Development environment
You can also use http://sematext.com/spm/integrations/elasticsearch-monitoring.html - there is a free-forever plan without node limits, even for production.