RFR: 8011218 Kitchensink hanged, likely NMT is to blame

[Karen Kinnear]

Reviewed.

I really like this approach. And if we run into any additional delay issues, you have the mechanism
in place to add any needed check_safepoint calls. Thank you for doing it this way.

thanks,
Karen

On Apr 24, 2013, at 3:00 PM, Coleen Phillimore wrote:

> 
> Reviewed.
> Coleen
> 
> On 4/24/2013 12:21 AM, David Holmes wrote:
>> Thanks Zhengyu, this looks okay to me.
>> 
>> David
>> 
>> On 24/04/2013 12:23 AM, Zhengyu Gu wrote:
>>> Round 2:
>>> 
>>> Based on feedback from David, Dan and Karen, transition thread state to _thread_in_native is dangerous. This fix keeps query thread state in _thread_in_vm, but make locks safepoint aware and explicitly check for safepoint during lengthy baseline operation.
>>> 
>>> Webrev: http://cr.openjdk.java.net/~zgu/8011218/webrev.01/
>>> 
>>> 
>>> Test:
>>>   - Manual test, ran kitchensink stress tests with additional NMT queries to stress query threads, monitored gc.log.
>>>    After 17 hours, found no long pause between gc messages.
>>> 
>>> Thanks,
>>> 
>>> -Zhengyu
>>> 
>>> 
>>> 
>>> On Apr 18, 2013, at 9:46 AM, Zhengyu Gu wrote:
>>> 
>>>> Hi David,
>>>> 
>>>> On 4/17/2013 10:59 PM, David Holmes wrote:
>>>>> On 18/04/2013 7:33 AM, Zhengyu Gu wrote:
>>>>>> The thread that runs NMT query can be blocked on NMT snapshot lock,
>>>>>> which prevents JVM to reach safepoints.
>>>>> 
>>>>> How? I don't understand this part.
>>>>> 
>>>> Currently, NMT acquires locks without safepoint checks (I assumed that query is coming from _thread_in_native, obviously a mistake). Since NMT query can be lengthy, I still want it to run in _thread_in_native state.
>>>> 
>>>>>> NMT query does not change Java heap, it does not have to stop for
>>>>>> safepoints. So the solution is to transition to _vm_in_native state,
>>>>>> before executing NMT query.
>>>>> 
>>>>> To add to Dan's comments you don't need the assertion:
>>>>> 
>>>>>  81   assert(((JavaThread*)thr)->thread_state() == _thread_in_vm,
>>>>>  82     "Just check");
>>>>> 
>>>>> because this will be checked by ThreadToNativeFromVM.
>>>>> 
>>>>> However I am far from convinced that it is valid to make this state change. You need to be sure that nothing you call from this method - including debug/tracing code that might only be enabled under certain flags - will assume/expect that you are _thread_in_vm.
>>>>> 
>>>> NMT query code acquires two locks, both with no_safepoint_check. Besides assertion and tty->print(), I don't recall it uses any vm specific methods, but is there a way to ensure that?
>>>> 
>>>> Thanks,
>>>> 
>>>> -Zhengyu
>>>> 
>>>> 
>>>>> Thanks,
>>>>> David
>>>>> -----
>>>>> 
>>>>> 
>>>>>> 
>>>>>> JBS: https://jbs.oracle.com/bugs/browse/JDK-8011218
>>>>>> Public bug: not available
>>>>>> Webrev: http://cr.openjdk.java.net/~zgu/8011218/webrev.00/
>>>>>> <http://cr.openjdk.java.net/%7Ezgu/8011218/webrev.00/>
>>>>>> 
>>>>>> 
>>>>>> Test:
>>>>>>   - Run Kitchensink stress test on low power machine Linux x86 machine
>>>>>> with GC trace on, stress out NMT query thread by submitting NMT query in
>>>>>> a loop, NMT query executions become very slow, but GC trace continues
>>>>>> generating message, and does not show long pause.
>>>>>> 
>>>>>> Thanks,
>>>>>> 
>>>>>> -Zhengyu
>>>>>> 
>>>>>> 
>>>> 
>>> 
>