Setting JVMTI Capabilities when VM is in "Live Phase"

[Daniel D. Daugherty]

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 <mailto: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.html#jvmtiCapabilities.can_generate_compiled_method_load_events
>     <http://docs.oracle.com/javase/8/docs/platform/jvmti/jvmti.html#jvmtiCapabilities.can_generate_compiled_method_load_events>
>
>     and this event:
>
>     http://docs.oracle.com/javase/8/docs/platform/jvmti/jvmti.html#CompiledMethodLoad
>     <http://docs.oracle.com/javase/8/docs/platform/jvmti/jvmti.html#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.html#AddCapabilities
>     <http://docs.oracle.com/javase/8/docs/platform/jvmti/jvmti.html#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.html#GenerateEvents
>     <http://docs.oracle.com/javase/8/docs/platform/jvmti/jvmti.html#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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