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https://issues.apache.org/jira/browse/KAFKA-13799?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17517382#comment-17517382
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RivenSun commented on KAFKA-13799:
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Hi [~showuon] [~dengziming] and [~guozhang]
I think we can state in Kafka's documentation that when Kafka reads disk file
messages, Kafka combines pagecache and zero-copy to greatly improve message
consumption efficiency. But zero-copy only works in PlaintextTransportLayer.
Maybe add this declaration in the sendfile module section of the documentation
below.
[https://kafka.apache.org/documentation/#maximizingefficiency]
WDYT？
Thanks.
> Improve documentation for Kafka zero-copy
> -----------------------------------------
>
> Key: KAFKA-13799
> URL: https://issues.apache.org/jira/browse/KAFKA-13799
> Project: Kafka
> Issue Type: Improvement
> Components: documentation
> Reporter: RivenSun
> Priority: Major
>
> Via documentation https://kafka.apache.org/documentation/#maximizingefficiency
> and [https://kafka.apache.org/documentation/#networklayer] ,
> We can know that Kafka combines pagecache and zero-copy when reading messages
> in files on disk, which greatly improves the consumption rate of messages.
> But after browsing the source code:
> Look directly at the *FileRecords.writeTo(...)* method,
> 1. Only PlaintextTransportLayer.transferFrom() uses fileChannel.transferTo(),
> and the bottom layer calls the sendfile method to implement zero-copy data
> transfer.
> 2. The logic of the SslTransportLayer.transferFrom() method:
> {code:java}
> fileChannel.read(fileChannelBuffer, pos)
> ->
> sslEngine.wrap(src, netWriteBuffer)
> ->
> flush(ByteBuffer buf) && socketChannel.write(buf){code}
> That is, first read the data on the disk or directly from the page cache,
> then encrypt the data, and finally send the encrypted data to the network.
> {*}FileChannel.transferTo() is not used in the whole process{*}.
>
> Conclusion:
> PlaintextTransportLayer and SslTransportLayer both use pagecache, but
> SslTransportLayer does not implement zero-copy.
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