RFR: 8011218 Kitchensink hanged, likely NMT is to blame
Zhengyu Gu
zhengyu.gu at oracle.com
Tue Apr 23 07:23:10 PDT 2013
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
>>>
>>>
>
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