com.netflix.hystrix.exception.HystrixRuntimeException: xxxx fallback execution rejected.

[clongulin]

When the interface request qps is 3k and I configure the parameter[FallbackIsolationSemaphoreMaxConcurrentRequests] to 30k, the fallback execution rejected error still exists.
How to estimate the FallbackIsolationSemaphoreMaxConcurrentRequests?

com.netflix.hystrix.exception.HystrixRuntimeException: xxxx fallback execution rejected.
at com.netflix.hystrix.AbstractCommand.handleFallbackRejectionByEmittingError(AbstractCommand.java:1043)
at com.netflix.hystrix.AbstractCommand.getFallbackOrThrowException(AbstractCommand.java:875)
at com.netflix.hystrix.AbstractCommand.handleShortCircuitViaFallback(AbstractCommand.java:982)
at com.netflix.hystrix.AbstractCommand.applyHystrixSemantics(AbstractCommand.java:557)
at com.netflix.hystrix.AbstractCommand.access$200(AbstractCommand.java:60)
at com.netflix.hystrix.AbstractCommand$4.call(AbstractCommand.java:419)
at com.netflix.hystrix.AbstractCommand$4.call(AbstractCommand.java:413)