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https://issues.apache.org/jira/browse/KAFKA-20915?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Lianet Magrans resolved KAFKA-20915.
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Resolution: Duplicate
> Unneeded app thread wakeup when a fetch already in flight and nothing has
> changed in AsyncConsumer
> --------------------------------------------------------------------------------------------------
>
> Key: KAFKA-20915
> URL: https://issues.apache.org/jira/browse/KAFKA-20915
> Project: Kafka
> Issue Type: Bug
> Components: clients, consumer
> Reporter: Lianet Magrans
> Assignee: Lianet Magrans
> Priority: Blocker
> Fix For: 4.4.0
>
>
> On the async consumer, the fetch req manager has logic to wake up the fetch
> buffer if it doesn't generate any fetch request to send.
> [https://github.com/apache/kafka/blob/dacff9c85a29806fcafe617feb2db0c9677e6de7/clients/src/main/java/org/apache/kafka/clients/consumer/internals/FetchRequestManager.java#L149-L151]
>
> That wake up is needed in some cases (to unblock app thread if the case was
> no fetch req generated because partitions were paused, not subscribed etc).,
> but the wakeup is useless and harmful if the case was that no requests was
> generated because they were already in-flight.
> * Useless because nothing has changed yet regarding data (it is the fetch
> response for the inflights that should wake up the app thread)
> * harmful because it leads to a spin (high CPU) between the app thread and
> the background (background wakes ups app thread when nothing really changed
> regarding data, we're just waiting for in-flight requests that will
> themselves wake up the buffer when complete).
> This wakeup behaviour was introduced in 4.2, but it's impact on CPU was
> revealed with a recent correctness fix
> https://issues.apache.org/jira/browse/KAFKA-20780
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