Setting JVMTI Capabilities when VM is in "Live Phase"

[Leela Mohan]

Your thoughts on this will be helpful.

Thanks,
Leela

[ Removed "serviceability-dev at openjdk.net" mailing alias added
"hotspot-dev" since i am getting failed mail delivery notification ]

On Fri, Aug 12, 2016 at 4:33 PM, Daniel D. Daugherty <
daniel.daugherty at oracle.com> wrote:

> On 8/12/16 5:27 PM, Leela Mohan wrote:
>
> Hi Daniel,
>
> Actually,  I was thinking about the case where compiler choose not to have
> complete "de-opt" state. For ex: Local pruning. I can also think of other
> cases which need callback events like, posting exceptions to the agent.
>
> JVMTI requests for examining/changing the stack frame would conservatively
> de-optimize the compile methods but not all de-optimizable locations can
> restore the java state user expect.
>
> What are the expectations for VM for these cases ?
>
>
> I think we'll have to wait for someone more current in how the compilers
> interact with JVM/TI to chime in here. I stopped actively working on
> JVM/TI back in 2010 or so... :-)
>
> Dan
>
>
>
> Thanks,
> Leela
>
> On Fri, Aug 12, 2016 at 4:04 PM, Daniel D. Daugherty <
> daniel.daugherty at oracle.com> wrote:
>
>> On 8/12/16 4:21 PM, Leela Mohan wrote:
>>
>>> Hi experts,
>>>
>>> It looks like, we don't disallow setting capabilities when VM is in
>>> "JVMTI_PHASE_LIVE". And, I notice, for every new compilation of method,
>>> ciEnv caches the JVMTI state and expects those assumptions to be true
>>> during the compilation. Otherwise, dump the compiled method.
>>>
>>> However, we don't seem to do anything with the methods which were
>>> compiled
>>> before setting the capability.
>>>
>>> What is the understanding?
>>>
>>> Thanks,
>>> Leela
>>>
>>
>> Hi Leela,
>>
>> I'm guessing that you are talking about this capability:
>>
>> http://docs.oracle.com/javase/8/docs/platform/jvmti/jvmti.ht
>> ml#jvmtiCapabilities.can_generate_compiled_method_load_events
>>
>> and this event:
>>
>> http://docs.oracle.com/javase/8/docs/platform/jvmti/jvmti.ht
>> ml#CompiledMethodLoad
>>
>>
>> The can_generate_compiled_method_load_events capability needs to be added
>> in order to generate CompiledMethodLoad events. Capabilities are added via
>> http://docs.oracle.com/javase/8/docs/platform/jvmti/jvmti.ht
>> ml#AddCapabilities
>> which can be called from different JVM/TI phases. Different VM
>> implementations
>> can require that certain capabilities can only be added in certain JVM/TI
>> phases.
>> However, if AddCapabilities() does not return a JVM/TI error when a
>> capability
>> is added in a phase, e.g., the live phase, then you can safely assume that
>> the capability has been added.
>>
>>
>> In your example, it sounds like the capability is added in the live phase
>> because you are seeing events generated for newly compiled methods. In
>> order
>> to see synthetic events for methods that were compiled before you added
>> the
>> capability, your agent needs to use a different function:
>>
>> http://docs.oracle.com/javase/8/docs/platform/jvmti/jvmti.ht
>> ml#GenerateEvents
>>
>> The documentation for Compiled Method Load has this line:
>>
>> > These events can be sent after their initial occurrence with
>> GenerateEvents.
>>
>> and that sounds just like your situation.
>>
>> Hope this helps.
>>
>> Dan
>>
>>
>
>
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