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From "Matt Byrd (JIRA)" <j...@apache.org>
Subject [jira] [Created] (CASSANDRA-5345) Potential problem with GarbageCollectorMXBean
Date Thu, 14 Mar 2013 13:46:13 GMT
Matt Byrd created CASSANDRA-5345:
------------------------------------

             Summary: Potential problem with GarbageCollectorMXBean
                 Key: CASSANDRA-5345
                 URL: https://issues.apache.org/jira/browse/CASSANDRA-5345
             Project: Cassandra
          Issue Type: Bug
          Components: Core
    Affects Versions: 1.0.7
         Environment: JVM:JVM vendor/version: Java HotSpot(TM) 64-Bit Server VM/1.6.0_30 
typical 6 node 2 availability zone Mutli DC cluster on linux vms with
and mx4j-tools.jar and jna.jar both on path. Default configuration bar token setup(equispaced),
sensible cassandra-topology.properties file and use of said snitch.
            Reporter: Matt Byrd
            Priority: Trivial


I am not certain this is definitely a bug, but I thought it might be worth posting to see
if someone with more JVM//JMX knowledge could disprove my reasoning. Apologies if I've failed
to understand something.

We've seen an intermittent problem where there is an uncaught exception in the scheduled task
of logging gc results in GcInspector.java:

{code}
...
 ERROR [ScheduledTasks:1] 2013-03-08 01:09:06,335 AbstractCassandraDaemon.java (line 139)
Fatal exception in thread Thread[ScheduledTasks:1,5,main]
java.lang.reflect.UndeclaredThrowableException
        at $Proxy0.getName(Unknown Source)
        at org.apache.cassandra.service.GCInspector.logGCResults(GCInspector.java:95)
        at org.apache.cassandra.service.GCInspector.access$000(GCInspector.java:41)
        at org.apache.cassandra.service.GCInspector$1.run(GCInspector.java:85)
        at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:441)
        at java.util.concurrent.FutureTask$Sync.innerRunAndReset(FutureTask.java:317)
        at java.util.concurrent.FutureTask.runAndReset(FutureTask.java:150)
        at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$101(ScheduledThreadPoolExecutor.java:98)
        at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.runPeriodic(ScheduledThreadPoolExecutor.java:180)
        at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:204)
        at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
        at java.lang.Thread.run(Thread.java:662)
Caused by: javax.management.InstanceNotFoundException: java.lang:name=ParNew,type=GarbageCollector
        at com.sun.jmx.interceptor.DefaultMBeanServerInterceptor.getMBean(DefaultMBeanServerInterceptor.java:1094)
        at com.sun.jmx.interceptor.DefaultMBeanServerInterceptor.getAttribute(DefaultMBeanServerInterceptor.java:662)
        at com.sun.jmx.mbeanserver.JmxMBeanServer.getAttribute(JmxMBeanServer.java:638)
        at com.sun.jmx.mbeanserver.MXBeanProxy$GetHandler.invoke(MXBeanProxy.java:106)
        at com.sun.jmx.mbeanserver.MXBeanProxy.invoke(MXBeanProxy.java:148)
        at javax.management.MBeanServerInvocationHandler.invoke(MBeanServerInvocationHandler.java:248)
        ... 13 more
...
{code}

I think the problem, may be caused by the following reasoning:

In GcInspector we populate a list of mxbeans when the GcInspector instance is instantiated:

{code}
...
List<GarbageCollectorMXBean> beans = new ArrayList<GarbageCollectorMXBean>();
        MBeanServer server = ManagementFactory.getPlatformMBeanServer();
        try
        {
            ObjectName gcName = new ObjectName(ManagementFactory.GARBAGE_COLLECTOR_MXBEAN_DOMAIN_TYPE
+ ",*");
            for (ObjectName name : server.queryNames(gcName, null))
            {
                GarbageCollectorMXBean gc = ManagementFactory.newPlatformMXBeanProxy(server,
name.getCanonicalName(), GarbageCollectorMXBean.class);
                beans.add(gc);
            }
        }
        catch (Exception e)
        {
            throw new RuntimeException(e);
        }
...
{code}

Cassandra then periodically calls:

{code}
...
    private void logGCResults()
    {
        for (GarbageCollectorMXBean gc : beans)
        {
            Long previousTotal = gctimes.get(gc.getName());
...
{code}

In the oracle javadocs, they seem to suggest that these beans could disappear at any time.(I'm
not sure why when or how this might happen)
http://docs.oracle.com/javase/6/docs/api/
See: getGarbageCollectorMXBeans

{code}
...
public static List<GarbageCollectorMXBean> getGarbageCollectorMXBeans()
Returns a list of GarbageCollectorMXBean objects in the Java virtual machine. The Java virtual
machine may have one or more GarbageCollectorMXBean objects. It may add or remove GarbageCollectorMXBean
during execution.
Returns:
a list of GarbageCollectorMXBean objects.
...
{code}

Correct me if I'm wrong, but do you think this might be causing the problem? That somehow
the JVM decides to remove the GarbageCollectorMXBean temporarily or permanently (causing said
exception) and if this is expected behaviour, should it be handled in some way?
Also I'd like to point out that this may be an issue on other versions as well as I don't
believe this code has changed in quite a long time.
Unfortunately I haven't been able to reproduce this outside of the production environment,
if you have any tips, questions or are able to explain//disprove my concerns, I'd be very
grateful.

Thanks,
Matt

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