ElasticSearch 6.7: Big difference between parent circuit breaker and jvm used heap

We are using ES 6.7. We are looking at 2 metrics:

  • Parent Circuit Breaker Used memory
  • JVM used heap

We see that ES circuit breaker are way off. We see that for the same timestamp:
Used heap returns 17.85 GB whereas parent circuit breaker for the same node shows only 178 MB. Is not the parent circuit breaker breaker be roughly equal to JVM used heap? Why are they way off? In that case, isn't the circuit breaker will always fail to trigger?

Elasticsearch 6.x only tracks a limited subset of the heap usage for circuit breaking purposes; although it was good at preventing some overloads, it was not perfect. Elasticsearch 7.x tracks the actual heap usage too.

In 7.x, will they mean the same value?

  • Parent Circuit Breaker Used memory
  • JVM used heap

Technically no, since I think the other circuit breakers might track memory that's potentially needed in future but not currently allocated. In practice they're normally quite close.

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