[
https://issues.apache.org/jira/browse/FLINK-13100?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16880999#comment-16880999
]
zhijiang commented on FLINK-13100:
----------------------------------
In netty internal implementation of writeAndFlush(), the write operation could
always be done but the flink buffer is not released yet. The flush operation
could only be done after write operation when the channel interest ops include
SelectionKey.OP_WRITE, that means the socket cache has enough space to hold the
flushed message, otherwise the flush operation would be done with following
writeAndFlush().
In summary, the channel future of writeAndFlush() done only indicates the write
done, the flush is not always done. But the flink buffer is only recycled after
flush done based on zero-copy improvement in netty stack. So it would cause
this issue.
> Fix the unexpected IOException during FileBufferReader#nextBuffer
> -----------------------------------------------------------------
>
> Key: FLINK-13100
> URL: https://issues.apache.org/jira/browse/FLINK-13100
> Project: Flink
> Issue Type: Sub-task
> Components: Runtime / Network
> Reporter: zhijiang
> Priority: Blocker
>
> In the implementation of FileBufferReader#nextBuffer, we expect the next
> memory segment always available based on the assumption that the nextBuffer
> call could only happen when the previous buffer was recycled before.
> Otherwise it would throw an IOException in current implementation.
> In fact, the above assumption is not making sense based on the credit-based
> and zero-copy features in network. The detail processes are as follows:
> * The netty thread finishes calling the channel.writeAndFlush() in
> PartitionRequestQueue and adds a listener to handle the ChannelFuture later.
> Before future done, the corresponding buffer is not recycled because of
> zero-copy improvement.
> * Before the previous future done, the netty thread could trigger next
> writeAndFlush via processing addCredit message, then
> FileBufferReader#nextBuffer would throw exception because of previous buffer
> not recycled.
> We thought of several ways for solving this potential bug:
> * It does not trigger the next writeAndFlush before the previous future
> done. To do so it has to maintain the future state and check it in relevant
> actions. I wonder it might bring performance regression in network throughput
> and bring extra state management.
> * Adjust the implementation of current FileBufferReader. We ever regarded
> the blocking partition view as always available based on the next buffer read
> ahead, so it would be always added into available queue in
> PartitionRequestQueue. Actually this next buffer ahead only simplifies the
> process of BoundedBlockingSubpartitionReader#notifyDataAvailable. The view
> availability could be judged based on available buffers in FileBufferReader
> instead of next buffer ahead. When the buffer is recycled into
> FileBufferReader after writeAndFlush done, it could call notifyDataAvailable
> to add this view into available queue in PartitionRequestQueue.
> I prefer the second way because it would not bring any bad impacts.
--
This message was sent by Atlassian JIRA
(v7.6.3#76005)
