jmx-dev RFR: 8020875 java/lang/management/ThreadMXBean/ResetPeakThreadCount.java fails intermittently

[Jaroslav Bachorik]

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On 07/23/2013 10:25 AM, Daniel Fuchs wrote:
> Hi Jaroslav,
> 
> This looks like a tough problem as it is altogether possible that 
> some of the VM daemon threads will terminate during the duration of
> the call - and if that's the case, the condition: new peak >= old
> peak + delta might not even be true. I am not a VM specialist so I
> don't know whether there can be such daemon threads that will be
> arbitrarily started and stopped by the VM - but if that happens I
> don't see how you could work around it.

As I wrote in my reply to David the offending thread is
j.u.l.LogManager$Cleaner which kicks in randomly.

This would confirm my observations that the discrepancy is always at
most one thread more than expected.

> 
> There seems to be something strange in the test though: line 209, 
> you catch InterruptedException just to call 
> Thread.currentThread().interrupt() and interrupt the thread
> again?? Did you mean maybe to call
> Thread.currentThread().interrupted() instead?

No, it checks whether the thread has been interrupted and cleans the
interrupted flag.

> 
> There are other places that seems to be prone to failures in this
> test too for instance:
> 
> startThreads(...) {
> 
> while(mbean.getThreadCount() < (current + count)) { ... }
> 
> }
> 
> If the VM can start and stop arbitrary threads then this condition 
> seems dubious. There's the same kind of logic in terminateThreads. 
> Not sure you can/should do anything about it though - it's just to
> point out that these steps might need to be revisited if the test
> still fails sporadically...
> 
> Also I'm not sure that using volatile for the 'live' array will 
> work - the array itself is volatile - but does it extends to its 
> elements?

No, it does not. But this code has been sitting there for some time.

- -JB-

> 
> It might be better to declare the live array as static final and 
> use a synchronization block on the array itself when accessing it:
> 
> private static final boolean live[] = new boolean[ALL_THREADS]; 
> private static boolean isAlive(int i) { synchronized(live) { return
> live[i] }; }
> 
> ...
> 
> synchronized(live) { live[i] == false; }
> 
> ...
> 
> while (isAlive[id]) { ... }
> 
> ...
> 
> best regards,
> 
> -- daniel
> 
> On 7/22/13 1:55 PM, Jaroslav Bachorik wrote:
>> The java/lang/management/ThreadMXBean/ResetPeakThreadCount.java
>> test seems to be failing intermittently.
>> 
>> The test checks the functionality of the 
>> j.l.m.ThreadMXBean.resetPeakThreadCount() method. It does so by 
>> capturing the current value of "getPeakThreadCount()", starting
>> a predefined number of the user threads, stopping them and
>> resetting the stored peak value and making sure the new peak
>> equals to the number of the actually running threads.
>> 
>> The main problem is that it is not possible to prevent JVM to
>> start/stop arbitrary system threads while executing the test.
>> This might lead to small variations of the reported peak (a
>> short-lived system thread is started while the batch of the user
>> threads is running) or the expected number of running threads
>> (again, a short-lived system thread is started at the moment the
>> test asks for the number of running threads).
>> 
>> The patch does not fix those shortcomings as it is not really
>> possible to do given the nature of the JVM threading system. It
>> rather tries to relax the conditions while still maintaining the
>> ability to detect functional problems - eg. decreasing peak
>> without explicitly resetting it and reporting false number of
>> threads.
>> 
>> The webrev is at: 
>> http://cr.openjdk.java.net/~jbachorik/8020875/webrev.00
>> 
>> Thanks,
>> 
>> -JB-
>> 
> 

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