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ASF GitHub Bot logged work on HDFS-15869: ----------------------------------------- Author: ASF GitHub Bot Created on: 19/Apr/21 19:51 Start Date: 19/Apr/21 19:51 Worklog Time Spent: 10m Work Description: functioner edited a comment on pull request #2737: URL: https://github.com/apache/hadoop/pull/2737#issuecomment-822713500 > Thanks @functioner > The detailed discussions (except the lambda argument) should have been on the Jira. > > > IMO, this is a classic Producer-Consumer problem, and it is natural idea to improve performance using parallel way. > > > So, call.sendResponse() (network service) affects FSEditLogAsync (edit log sync service). So, I would say it's a bug. > > Now, I am really even more confused about the (Bug Vs. Improvement). So, I am going to pass on reviewing. @amahussein Thanks for your feedback, and your time! Sorry for all the possible confusion I made. It's not a big deal whether it's marked as bug or improvement. One of my bug reports ([HADOOP-17552](https://issues.apache.org/jira/browse/HADOOP-17552)) is also finally marked as improvement rather than bug. The point is that the developers (in [HADOOP-17552](https://issues.apache.org/jira/browse/HADOOP-17552)) finally realized that there's a potential hanging issue as I point out, and the patch (as well as the relevant discussion) is very helpful for the developers and the users. > Ok, is the purpose of the change is to improve performance of the FSEditLogAsync.java by executing sendResponse() in parallel? > In that case, please change the title of the Jira and the description to remove references to "hanging" problems. My intention is to defend that we should not remove the references to "hanging" problems. In short, the discussion above can be summarized into 3 arguments: 1. https://github.com/apache/hadoop/pull/2737#issuecomment-822151838: this is a classic Producer-Consumer problem, and it is natural idea to improve performance using parallel way 2. https://github.com/apache/hadoop/pull/2737#issuecomment-822591028: the `call.sendResponse()` may hang due to network issue, without throwing any exception 3. https://github.com/apache/hadoop/pull/2737#issuecomment-822617097: a). the `FSEditLogAsync` thread (without this patch) directly invokes a network I/O call `call.sendResponse()`, so if this network I/O invocation hangs, the `FSEditLogAsync` thread also hangs b). in the "correct" system design, if this network I/O invocation hangs in this way, then that should be fine, because HDFS (as a fault-tolerant system) should tolerate it. c). when the system tolerates this network issue, the `FSEditLogAsync` thread should not hang, otherwise everybody can't commit the log. d). our expected behavior is that, when the system tolerates this network issue, the `FSEditLogAsync` thread should continue, so that everything still works well, despite this network issue. Both Argument 3 and Argument 1 can be resolved with this patch. In conclusion, this patch not only improves the performance, but also enhances the availability & fault-tolerance. So, I think the references to "hanging" problems should not be removed. If it keeps "Improvement" tag instead of "Bug" tag, that's fine. P.S. I will summarize our discussion with a comment in Jira after we reach a consensus. -- This is an automated message from the Apache Git Service. To respond to the message, please log on to GitHub and use the URL above to go to the specific comment. For queries about this service, please contact Infrastructure at: us...@infra.apache.org Issue Time Tracking ------------------- Worklog Id: (was: 585345) Time Spent: 5h 40m (was: 5.5h) > 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 > Time Spent: 5h 40m > 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. -- This message was sent by Atlassian Jira (v8.3.4#803005) --------------------------------------------------------------------- To unsubscribe, e-mail: hdfs-issues-unsubscr...@hadoop.apache.org For additional commands, e-mail: hdfs-issues-h...@hadoop.apache.org