Github user liyezhang556520 commented on a diff in the pull request:
https://github.com/apache/spark/pull/12083#discussion_r58287444
--- Diff:
common/network-common/src/main/java/org/apache/spark/network/protocol/MessageWithHeader.java
---
@@ -44,6 +45,14 @@
private long totalBytesTransferred;
/**
+ * When the write buffer size is larger than this limit, I/O will be
done in chunks of this size.
+ * The size should not be too large as it will waste underlying memory
copy. e.g. If network
+ * avaliable buffer is smaller than this limit, the data cannot be sent
within one single write
+ * operation while it still will make memory copy with this size.
+ */
+ private static final int NIO_BUFFER_LIMIT = 512 * 1024;
--- End diff --
>On my machine, /proc/sys/net/core/wmem_default is around 200k, which (I
assume) means you'd be copying about half of the buffer with no need here.
@vanzin , on my machine, both `wmem_default` and `wmem_max` are also around
200K, but in my test, I can successfully write more than 512K for each
`WritableByteChannel.write()`, this size should be the same with return size of
`writeFromNativeBuffer` as in line
http://www.grepcode.com/file/repository.grepcode.com/java/root/jdk/openjdk/7u40-b43/sun/nio/ch/IOUtil.java#65.
I don't know why. Can you also make a test?
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