from Elasticsearch the Definivte guide
Please Do Not Tweak JVM Settings
The JVM exposes dozens (hundreds even!) of settings, parameters, and configura‐ tions. They allow you to tweak and tune almost every aspect of the JVM.
When a knob is encountered, it is human nature to want to turn it. We implore you to squash this desire and not use custom JVM settings. Elasticsearch is a complex piece of software, and the current JVM settings have been tuned over years of real-world usage.
It is easy to start turning knobs, producing opaque effects that are hard to measure, and eventually detune your cluster into a slow, unstable mess. When debugging clus‐ ters, the first step is often to remove all custom configurations. About half the time, this alone restores stability and performance.