RivenSun created KAFKA-13799:
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Summary: 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
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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