Github user liyezhang556520 commented on the pull request:
https://github.com/apache/spark/pull/12083#issuecomment-204207886
Hi @vanzin , the memory copy place is given out by @zsxwing , the call
stack is as follows:
```
at java.nio.Bits.copyFromArray(Bits.java:754)
at java.nio.DirectByteBuffer.put(DirectByteBuffer.java:371)
at java.nio.DirectByteBuffer.put(DirectByteBuffer.java:342)
at sun.nio.ch.IOUtil.write(IOUtil.java:60)
at sun.nio.ch.SocketChannelImpl.write(SocketChannelImpl.java:466)
- locked <0x00007f8a8a28d400> (a java.lang.Object)
at
org.apache.spark.network.protocol.MessageWithHeader.copyByteBuf(MessageWithHeader.java:131)
at
org.apache.spark.network.protocol.MessageWithHeader.transferTo(MessageWithHeader.java:114)
```
The whole buffer copy is in line
http://www.grepcode.com/file/repository.grepcode.com/java/root/jdk/openjdk/7u40-b43/sun/nio/ch/IOUtil.java#60,
but the buffer cannot be totally written if its side more than the available
underlying buffer. Which is in line
http://www.grepcode.com/file/repository.grepcode.com/java/root/jdk/openjdk/7u40-b43/sun/nio/ch/IOUtil.java#65.
So each time we will make a copy of the input `ByteBuf`, and write only a part
of it if the input size is big relatively. This results in multiply copies of
the input `ByteBuf` that is not necessary.
The method of handling the issue in this PR is the same as that in Hadoop,
please refer to
https://github.com/apache/hadoop/blob/trunk/hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/ipc/Server.java#L2957
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