Thread Local Handshake in JVMTI functions

[Yasumasa Suenaga]

On 2020/04/09 16:19, Robbin Ehn wrote:
> Hi Yasumasa,
> 
> We have had internal requests doing GetThreadListStackTraces with
> multiple threads with handshakes. Since you can sample hundreds of times
> per second using handshakes with little interference with your
> application.
> The internal request sampled all threads ~10 times per second.
> So they very much would like the performance instead of the 'precision'.

Hmm, it seems to violate the spec of GetThreadListStackTraces():

https://docs.oracle.com/en/java/javase/13/docs/specs/jvmti.html#GetThreadListStackTraces
```
All stacks are collected simultaneously, that is, no changes will occur to the thread state or stacks between the sampling one thread and the next.
```

So I think this ticket (JDK-8242428) should just support Thread-Local Handshakes when thread_count == 1.
For other case, we should introduce other API because current spec of GetThreadListStackTraces() is useful for some case IMHO.

For example, the profiler detects some threads which have much CPU time, we can gather stack traces of them. We don't need to choose threads from all thread dump.
Of course, Both GetThreadListStackTraces() and GetAllStackTrace() use safepoint, but GetThreadListStackTraces() is useful interface for profiler developers. Thus I think the spec of GetThreadListStackTraces() should not be changed.


Thanks,

Yasumasa


> Is if it's deemed feasible to always use handshakes we should consider it.
> 
> Thanks, Robbin
> 
> On 2020-04-09 09:12, Yasumasa Suenaga wrote:
>> Hi Robbin,
>>
>> I think we can change GetThreadListStackTrace(VM_GetThreadListStackTraces) if the caller requests only 1 thread stack (thread_count == 1). It does not break JVMTI spec.
>> In other case, we should use safepoint (VM Operation) for following JVMTI spec:
>>
>> ```
>> All stacks are collected simultaneously, that is, no changes will occur to the thread state or stacks between the sampling one thread and the next. The threads need not be suspended.
>> ```
>>
>> Thus I think we don't need describe release notes about it.
>>
>>
>> Thanks,
>>
>> Yasumasa
>>
>>
>> On 2020/04/09 16:03, Robbin Ehn wrote:
>>> Hi, adding the same comment as in the bug regarding GetThreadListStackTraces.
>>>
>>> Please note that there is a semantic difference taking samples in a
>>> safepoint and in handshakes, if there are mutiple thread sampled.
>>> With a safepoint; stacktraces are taken from the same exact moment (from
>>> a Java mutation perspective).
>>>
>>> But with handshake we can have mutation between the samples, consider:
>>> Thread A enters a mutual exclusive region, and get sampled inside that
>>> region.
>>> Thread A leaves the mutual exclusive region.
>>> Thread B enter the same mutual exclusive region, and get sampled inside that region.
>>>
>>> When looking at the stack-traces, we have have two thread inside the same mutual exclusive region.
>>> If you are not aware on how the sampling mechanism works, this could be confusing.
>>>
>>> Please verify that we are still following JVM TI specs and at least have
>>> information about this change in release notes.
>>>
>>> Thanks, Robbin
>>>
>>> On 2020-03-31 16:41, Daniel D. Daugherty wrote:
>>>> Add Robbin to this thread...
>>>>
>>>>
>>>> This reminded of the following RFE that Robbin filed:
>>>>
>>>>      JDK-8201641 JVMTI: GetThreadListStackTraces should use Thread-Local Handshakes
>>>>      https://bugs.openjdk.java.net/browse/JDK-8201641
>>>>
>>>> We could update 8201641 to include everything that Yasumasa-san is requesting.
>>>> Would be a good place to track it...
>>>>
>>>> Dan
>>>>
>>>>
>>>> On 3/31/20 7:40 AM, Yasumasa Suenaga wrote:
>>>>> Hi David,
>>>>>
>>>>> On 2020/03/31 19:16, David Holmes wrote:
>>>>>> Hi Yasumasa,
>>>>>>
>>>>>> On 31/03/2020 8:06 pm, Yasumasa Suenaga wrote:
>>>>>>> Hi all,
>>>>>>>
>>>>>>> Many JVMTI functions uses VM Operation to get information. However some of them need to stop only one thread - they don't need to stop all threads.
>>>>>>> So I think we can use Thread Local Handshake as this webrev. It is example for GetOneCurrentContendedMonitor().
>>>>>>
>>>>>> True, but at the moment handshakes involve the VMThread. There is work being done to support direct thread-to-thread handshakes and once that is done this kind of conversion should be more easily done. It might be worth waiting for that.
>>>>>
>>>>> Thanks, I will be back to this topic when thread-to-thread handshake is done.
>>>>> I wondered at first why VMThread involves handshake. Its improvement is welcome for me ;)
>>>>>
>>>>>
>>>>> Cheers,
>>>>>
>>>>> Yasumasa
>>>>>
>>>>>
>>>>>>> http://cr.openjdk.java.net/~ysuenaga/jvmti-thread-local-handshake/
>>>>>>
>>>>>> An observation, it seems to me that calling_thread is not used when this is not a VMOperation.
>>>>>>
>>>>>> Cheers,
>>>>>> David
>>>>>>
>>>>>>> Also I think we can replace following VM Operations to Thread Local Handshake:
>>>>>>>
>>>>>>> class VM_GetCurrentLocation
>>>>>>> class VM_EnterInterpOnlyMode
>>>>>>> class VM_UpdateForPopTopFrame
>>>>>>> class VM_SetFramePop
>>>>>>> class VM_GetOwnedMonitorInfo
>>>>>>> class VM_GetCurrentContendedMonitor
>>>>>>> class VM_GetFrameCount
>>>>>>> class VM_GetFrameLocation
>>>>>>>
>>>>>>> What do you think?
>>>>>>> It it is acceptable, I will file it to JBS and send review request.
>>>>>>>
>>>>>>>
>>>>>>> Thanks,
>>>>>>>
>>>>>>> Yasumasa
>>>>