Github user WenboZhao commented on a diff in the pull request: https://github.com/apache/spark/pull/21593#discussion_r196939992 --- Diff: common/network-common/src/main/java/org/apache/spark/network/protocol/MessageWithHeader.java --- @@ -137,30 +137,15 @@ protected void deallocate() { } private int copyByteBuf(ByteBuf buf, WritableByteChannel target) throws IOException { - ByteBuffer buffer = buf.nioBuffer(); - int written = (buffer.remaining() <= NIO_BUFFER_LIMIT) ? - target.write(buffer) : writeNioBuffer(target, buffer); + // SPARK-24578: cap the sub-region's size of returned nio buffer to improve the performance + // for the case that the passed-in buffer has too many components. + int length = Math.min(buf.readableBytes(), NIO_BUFFER_LIMIT); + ByteBuffer buffer = buf.nioBuffer(buf.readerIndex(), length); --- End diff -- Thanks @squito and @zsxwing. I would prefer to do it in a different PR with more careful benchmark and testing. As @squito, that change is more prone to bugs.
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