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https://issues.apache.org/jira/browse/FLINK-14118?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16933359#comment-16933359
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Yingjie Cao commented on FLINK-14118:
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I couldn't come up with any bad case theoretically. A new condition is
considered when flushing, if there is no data available, the netty thread will
not be notified, this can reduce the flush overhead, and if there is data
available, the behavior remains the same. The only difference is that one more
condition will be calculated when flushing, whose overhead should be minor.
I can test the fix using the micro benchmark or some simple test jobs and post
the test result under this issue, do you think this is necessary? [~StephanEwen]
> Reduce the unnecessary flushing when there is no data available for flush
> -------------------------------------------------------------------------
>
> Key: FLINK-14118
> URL: https://issues.apache.org/jira/browse/FLINK-14118
> Project: Flink
> Issue Type: Improvement
> Components: Runtime / Network
> Reporter: Yingjie Cao
> Priority: Critical
> Labels: pull-request-available
> Fix For: 1.10.0, 1.9.1, 1.8.3
>
> Time Spent: 10m
> Remaining Estimate: 0h
>
> The new flush implementation which works by triggering a netty user event may
> cause performance regression compared to the old synchronization-based one.
> More specifically, when there is exactly one BufferConsumer in the buffer
> queue of subpartition and no new data will be added for a while in the future
> (may because of just no input or the logic of the operator is to collect some
> data for processing and will not emit records immediately), that is, there is
> no data to send, the OutputFlusher will continuously notify data available
> and wake up the netty thread, though no data will be returned by the
> pollBuffer method.
> For some of our production jobs, this will incur 20% to 40% CPU overhead
> compared to the old implementation. We tried to fix the problem by checking
> if there is new data available when flushing, if there is no new data, the
> netty thread will not be notified. It works for our jobs and the cpu usage
> falls to previous level.
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