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

[Daniel Fuchs]

On 7/23/13 11:45 AM, David Holmes wrote:
> On 23/07/2013 7:24 PM, Jaroslav Bachorik wrote:
>  > The result is that the offender is j.u.l.LogManager$Cleaner thread. I
>  > am attaching the profiler snapshot (can be opened in eg. jvisualvm)
>
> That doesn't quite make sense. The Cleaner thread is a shutdownhook, it
> should not be starting unless the VM is shutting down!

Hummm... Right: the javadoc says "Returns the peak live thread count 
since the Java virtual machine started or peak was reset." so the
Cleaner thread should not be counted.

If it is actually counted it might indicate a real problem in the
implementation of the ThreadMXBean.

-- daniel.


>
> David
> -----
>
>> On Tue 23 Jul 2013 11:19:13 AM CEST, David Holmes wrote:
>>> On 23/07/2013 6:29 PM, Jaroslav Bachorik wrote:
>>>> On 07/23/2013 10:19 AM, David Holmes wrote:
>>>>> Hi Jaroslav,
>>>>>
>>>>> On 22/07/2013 9:55 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).
>>>>>
>>>>> Do you know what "system threads" these are? I would not expect VM
>>>>> internal threads to be counted in getPeakThreadCount(), but even if
>>>>> they
>>>>> are I can't think of any short-lived threads that get created other
>>>>> than
>>>>> the Signal handling thread.
>>>>
>>>> Unfortunatelly I don't. Capturing the thread dump at the moment of
>>>> discovering the discrepancy seems to to be too late. I tried monitoring
>>>> the JVM under the test from external tools but it just brings more
>>>> entropy to the result.
>>>
>>> We'd need to instrument the thread creation logic to keep a separate
>>> record. Dtrace probes could probably do it - but the problem is
>>> getting the test to fail.
>>
>> Well, while responding to the previous email I thought about yet
>> another way to try to pinpoint the mysterious thread - I've tried NB
>> profiler. It filters out it's own threads and can do thread monitoring
>> at the same time as tracking the call tree.
>>
>> The result is that the offender is j.u.l.LogManager$Cleaner thread. I
>> am attaching the profiler snapshot (can be opened in eg. jvisualvm)
>>
>>>
>>>> I am completely relying on the JVM native thread accounting to be
>>>> correct and accurate - that it reports the thread count peak based on
>>>> the real data.
>>>
>>> The spec isn't clear but I would only expect these counters to apply
>>> to Java threads not VM internal threads (compiler, gc etc). So I'd
>>> really like to know what thread is messing up this count.
>>
>> I hope my previous finding makes this clearer.
>>
>> -JB-
>>
>>>
>>> David
>>>
>>>> -JB-
>>>>
>>>>>
>>>>>> 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
>>>>>
>>>>> Seems reasonable.
>>>>>
>>>>> David
>>>>> -----
>>>>>
>>>>>> Thanks,
>>>>>>
>>>>>> -JB-
>>>>>>
>>>>
>>