[HDFS-1194] Secondary namenode fails to fetch the image from the primary

• Type: Bug
• Status: Resolved
• Priority: Major
• Resolution: Duplicate
• Affects Version/s: 0.20.1, 0.20.2, 0.21.0, 0.22.0

Environment

Java(TM) SE Runtime Environment (build 1.6.0_14-b08)
Java HotSpot(TM) 64-Bit Server VM (build 14.0-b16, mixed mode)

CentOS 5

• Assignee: Dmytro Molkov
• Reporter: Dmytro Molkov
• Created: 08/Jun/10 23:20
• Updated: 17/Sep/13 18:21
• Resolved: 17/Sep/13 18:21

Description

We just hit the problem described in HDFS-1024 again.
After more investigation of the underlying problems with CancelledKeyException there are some findings:

One of the symptoms: the transfer becomes really slow (it does 700 kb/s) when I am doing the fetch using wget. At the same time disk and network are OK since I can copy at 50 mb/s using scp.
I was taking jstacks of the namenode while the transfer is in process and we found that every stack trace has one thread of jetty sitting in this place:

   java.lang.Thread.State: TIMED_WAITING (sleeping)
	at java.lang.Thread.sleep(Native Method)
	at org.mortbay.io.nio.SelectorManager$SelectSet.doSelect(SelectorManager.java:452)
	at org.mortbay.io.nio.SelectorManager.doSelect(SelectorManager.java:185)
	at org.mortbay.jetty.nio.SelectChannelConnector.accept(SelectChannelConnector.java:124)
	at org.mortbay.jetty.AbstractConnector$Acceptor.run(AbstractConnector.java:707)
	at org.mortbay.thread.QueuedThreadPool$PoolThread.run(QueuedThreadPool.java:522)

Here is a jetty code that corresponds to this:

// Look for JVM bug 
                    if (selected==0 && wait>0 && (now-before)<wait/2 && _selector.selectedKeys().size()==0)
                    {
                        if (_jvmBug++>5)  // TODO tune or configure this
                        {
                            // Probably JVM BUG!
                            
                            Iterator iter = _selector.keys().iterator();
                            while(iter.hasNext())
                            {
                                key = (SelectionKey) iter.next();
                                if (key.isValid()&&key.interestOps()==0)
                                {
                                    key.cancel();
                                }
                            }
                            try
                            {
                                Thread.sleep(20);  // tune or configure this
                            }
                            catch (InterruptedException e)
                            {
                                Log.ignore(e);
                            }
                        } 
                    }

Based on this it is obvious we are hitting a jetty workaround for a JVM bug that doesn't handle select() properly.

There is a jetty JIRA for this http://jira.codehaus.org/browse/JETTY-937 (it actually introduces the workaround for the JVM bug that we are hitting)

They say that the problem was fixed in 6.1.22, there is a person on that JIRA also saying that switching to using SocketConnector instead of SelectChannelConnector helped in their case.
Since we are hitting the same bug in our world we should either adopt the newer Jetty version where there is a better workaround, but it might not help if we are still hitting that bug constantly, the workaround might be better though.
Another approach is to switch to using SocketConnector which will eliminate the problem completely, although I am not sure what problems that will bring.

The java version we are running is in Environment

Any thoughts

Issue Links

• incorporates: HDFS-1024 SecondaryNamenode fails to checkpoint because namenode fails with CancelledKeyException
• is duplicated by: HADOOP-7450 Bump jetty to 6.1.26