RFR: 8011218 Kitchensink hanged, likely NMT is to blame

[David Holmes]

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
>>>>
>>>>
>>
>