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https://issues.apache.org/jira/browse/HDFS-15869?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17819939#comment-17819939
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ASF GitHub Bot commented on HDFS-15869:
---------------------------------------
ZanderXu commented on PR #2737:
URL: https://github.com/apache/hadoop/pull/2737#issuecomment-1960828209
Thanks for your works and discussions for this problem. I spent a long time
to catch your ideas and concerns😂, it's so hard.
I have some throughs and questions about this ticket.
Some questions in
[HDFS-15869](https://issues.apache.org/jira/browse/HDFS-15869):
> The `channel.write(buffer)` operation in line 3594 may be slow. Although
for this specific stack trace, the channel is initialized in the non-blocking
mode, there is still a chance of being slow depending on native write
implementation in the OS (e.g., a kernel issue). Furthermore, the channelIO
invocation in line 3594 may also get stuck, since it waits until the buffer is
drained:
`ret = (readCh == null) ? writeCh.write(buf) : readCh.read(buf);` will
return 0 if namenode cannot write more data to this connection, right?
`RpcEdit,logSyncNotify` will add this response into the queue of this
connection and let the Responder to take this job, right? So FSEditLogAsync can
go to process the next jobs, right?
Some throughs in
[HDFS-15869](https://issues.apache.org/jira/browse/HDFS-15869):
Actually, I encountered this problems in our prod environment that the
thread `FSEditLogAsync` spends a little more time to send responses to clients,
which had a big performance impact on writing edits to JNs. So I just use a new
single thread to do these send response jobs. Of course, we can use multiple
threads to send responses to client. New task is very expensive, so we use a
producer-consumer mode to fix this problem.
- FSEditLogAsync just put task into a capacity blocking Queue.
- ResponseSender thread take tasks from the Queue and send them to clients.
About "Bug" or "Improvement", I think it should be a performance
improvement, since all processes are worked as expected, no blocking or
hanging, just slow.
Some throughs in
[HDFS-15957](https://issues.apache.org/jira/browse/HDFS-15957):
- I think namenode should directly close this connection if IOException
happens in `RpcEdit,logSyncNotify`, since we cannot let the client hang
forever. It seems that the namenode drops a request.
@functioner Looking forward your ideas and confirm.
@daryn-sharp @Hexiaoqiao @linyiqun @amahussein Looking forward your ideas. I
hope we can push this ticket forward.
> Network issue while FSEditLogAsync is executing RpcEdit.logSyncNotify can
> cause the namenode to hang
> ----------------------------------------------------------------------------------------------------
>
> Key: HDFS-15869
> URL: https://issues.apache.org/jira/browse/HDFS-15869
> Project: Hadoop HDFS
> Issue Type: Improvement
> Components: fs async, namenode
> Affects Versions: 3.2.2
> Reporter: Haoze Wu
> Assignee: Haoze Wu
> Priority: Major
> Labels: pull-request-available
> Attachments: 1.png, 2.png
>
> Time Spent: 6.5h
> Remaining Estimate: 0h
>
> We were doing some testing of the latest Hadoop stable release 3.2.2 and
> found some network issue can cause the namenode to hang even with the async
> edit logging (FSEditLogAsync).
> The workflow of the FSEditLogAsync thread is basically:
> # get EditLog from a queue (line 229)
> # do the transaction (line 232)
> # sync the log if doSync (line 243)
> # do logSyncNotify (line 248)
> {code:java}
> //hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/FSEditLogAsync.java
> @Override
> public void run() {
> try {
> while (true) {
> boolean doSync;
> Edit edit = dequeueEdit(); //
> line 229
> if (edit != null) {
> // sync if requested by edit log.
> doSync = edit.logEdit(); //
> line 232
> syncWaitQ.add(edit);
> } else {
> // sync when editq runs dry, but have edits pending a sync.
> doSync = !syncWaitQ.isEmpty();
> }
> if (doSync) {
> // normally edit log exceptions cause the NN to terminate, but tests
> // relying on ExitUtil.terminate need to see the exception.
> RuntimeException syncEx = null;
> try {
> logSync(getLastWrittenTxId()); //
> line 243
> } catch (RuntimeException ex) {
> syncEx = ex;
> }
> while ((edit = syncWaitQ.poll()) != null) {
> edit.logSyncNotify(syncEx); //
> line 248
> }
> }
> }
> } catch (InterruptedException ie) {
> LOG.info(Thread.currentThread().getName() + " was interrupted,
> exiting");
> } catch (Throwable t) {
> terminate(t);
> }
> }
> {code}
> In terms of the step 4, FSEditLogAsync$RpcEdit.logSyncNotify is
> essentially doing some network write (line 365).
> {code:java}
> //hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/FSEditLogAsync.java
> private static class RpcEdit extends Edit {
> // ...
> @Override
> public void logSyncNotify(RuntimeException syncEx) {
> try {
> if (syncEx == null) {
> call.sendResponse(); // line
> 365
> } else {
> call.abortResponse(syncEx);
> }
> } catch (Exception e) {} // don't care if not sent.
> }
> // ...
> }{code}
> If the sendResponse operation in line 365 gets stuck, then the whole
> FSEditLogAsync thread is not able to proceed. In this case, the critical
> logSync (line 243) can’t be executed, for the incoming transactions. Then the
> namenode hangs. This is undesirable because FSEditLogAsync’s key feature is
> asynchronous edit logging that is supposed to tolerate slow I/O.
> To see why the sendResponse operation in line 365 may get stuck, here is
> the stack trace:
> {code:java}
> '(org.apache.hadoop.ipc.Server,channelWrite,3593)',
> '(org.apache.hadoop.ipc.Server,access$1700,139)',
> '(org.apache.hadoop.ipc.Server$Responder,processResponse,1657)',
> '(org.apache.hadoop.ipc.Server$Responder,doRespond,1727)',
> '(org.apache.hadoop.ipc.Server$Connection,sendResponse,2828)',
> '(org.apache.hadoop.ipc.Server$Connection,access$300,1799)',
> '(org.apache.hadoop.ipc.Server$RpcCall,doResponse,1111)',
> '(org.apache.hadoop.ipc.Server$Call,doResponse,903)',
> '(org.apache.hadoop.ipc.Server$Call,sendResponse,889)',
>
> '(org.apache.hadoop.hdfs.server.namenode.FSEditLogAsync$RpcEdit,logSyncNotify,365)',
> '(org.apache.hadoop.hdfs.server.namenode.FSEditLogAsync,run,248)',
> '(java.lang.Thread,run,748)'
> {code}
> The `channelWrite` function is defined as follows:
> {code:java}
> //hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/ipc/Server.java
> private int channelWrite(WritableByteChannel channel,
> ByteBuffer buffer) throws IOException {
>
> int count = (buffer.remaining() <= NIO_BUFFER_LIMIT) ?
> channel.write(buffer) : channelIO(null, channel, buffer);
> // line 3594
> if (count > 0) {
> rpcMetrics.incrSentBytes(count);
> }
> return count;
> }{code}
> The `channel.write(buffer)` operation in line 3594 may be slow. Although
> for this specific stack trace, the channel is initialized in the non-blocking
> mode, there is still a chance of being slow depending on native write
> implementation in the OS (e.g., a kernel issue). Furthermore, the channelIO
> invocation in line 3594 may also get stuck, since it waits until the buffer
> is drained:
> {code:java}
> //hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/ipc/Server.java
> private static int channelIO(...) throws IOException {
> //...
> while (buf.remaining() > 0) { //
> line 3637
> try {
> int ioSize = Math.min(buf.remaining(), NIO_BUFFER_LIMIT);
> buf.limit(buf.position() + ioSize);
>
> ret = (readCh == null) ? writeCh.write(buf) : readCh.read(buf); //
> line 3642
>
> if (ret < ioSize) {
> break;
> } } finally {
> buf.limit(originalLimit);
> }
> } int nBytes = initialRemaining - buf.remaining();
> return (nBytes > 0) ? nBytes : ret;
> }
> {code}
> For example, if the payload is split in two batches, the second batch
> will have to wait for the first batch to be sent out, which may encounter
> high packet loss rate and thus slow I/O.
> In summary, FSEditLogAsync is a critical service, but the potential delay
> occurring in `edit.logSyncNotify(syncEx)` can block the FSEditLogAsync
> thread. According to the comment
> ([https://github.com/apache/hadoop/blob/rel/release-3.2.2/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/FSEditLogAsync.java#L369]),
> “don't care if not sent”, FSEditLogAsync thread in fact does not really need
> to be concerned with the sync notification.
> By the way, we found that this issue is related to HDFS-15486 in the old
> version Hadoop.
> *Reproduction*
> To show that the potential delay introduced by
> `FSEditLogAsync$RpcEdit.logSyncNotify` can cause the namenode to hang, we
> provide the scripts to reproduce the bug.
> The script basically blocks the FSEditLogAsync thread when it’s invoking
> `call.sendResponse()` in `FSEditLogAsync$RpcEdit#logSyncNotify`. Our
> reproduction scripts guarantee that the delay is injected once and only once.
> The reproduction script is provided in a gist
> ([https://gist.github.com/functioner/891108ee54e24f155ac395a65e46cbfd]).
> *Fix*
> Since the `logSyncNotify` is not a critical operation for
> `FSEditLogAsync`, we propose to put the
> `FSEditLogAsync$RpcEdit.logSyncNotify` invocation to a separate thread in
> `FSEditLogAsync`. In this way, even if the notifications get stuck, they will
> not affect the edit logging.
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