Request Review: 6902182: Starting with jdwp agent should not incur performance penalty

Deneau, Tom tom.deneau at
Fri Dec 18 16:49:21 PST 2009

Dan --

Thanks for the very thorough review.  Just to make sure I understand the major points of your new recommendations...

   * Definitely need to expand should_post_on_exceptions to take into account the other two events, agreed.

   * Make it all fit into the should_xxx framework already in jvmti.

   * But add the additional features of
      * a second flavor of should_post_on_exceptions will take a JavaThread argument
      * the result from should_post_on_exceptions will be cached in a flag in JavaThread

If you feel that the cached flag should more cleanly be in jvmtiThreadState rather than in JavaThread itself, I don't think the extra overhead of one more dereference in the JITted code should be worried about.  Anyway, let me know of your decision.

To cache the value in  either JavaThread or jvmtiThreadState, I wanted to make sure to catch all the places where the cached value might have to change.  The original code had
   * in  recompute_thread_enabled
   * when jvmtiThreadState gets changed in JavaThread

I think you're saying that only the first of these is really needed. Assuming we initialize the cached flag to false.  Is that correct?

I think you're recommending we have both of the following:
   * a "global" should_post_on_exceptions() which would be true if one of the three events was globally enabled or enabled on any thread.

   * a thread-specific should_post_on_exceptions(JavaThread *) that just checks the specific thread (similar to the way the must_post_exception_events worked in the old webrev, except that it will check all three events)

Do we think the global should_post_on_exceptions() would be used anywhere?  I can't think of a place offhand.  Do you think we should implement this global function anyway?

-- Tom  D.

-----Original Message-----
From: serviceability-dev-bounces at [mailto:serviceability-dev-bounces at] On Behalf Of Daniel D. Daugherty
Sent: Friday, December 18, 2009 11:54 AM
To: serviceability-dev at; hotspot-compiler-dev at
Subject: Re: Request Review: 6902182: Starting with jdwp agent should not incur performance penalty

Cross posting to serviceability-dev at
since this review request also touches JVM/TI code...

This is the restored/revamped version of my comments. Sorry for
any confusion that I caused in my first posting!

> From: "Deneau, Tom" <tom.deneau at>
> Date: December 14, 2009 1:26:09 PM PST
> To: "hotspot-compiler-dev at" <hotspot-compiler-dev at>
> Subject: RE: Request Review: 6902182: Starting with jdwp agent should not	incur performance penalty
> New webrev is at
> This rev changes
>   * two places in the compiler where code for exception throws is
>     being JITted (see parse2.cpp, graphKit.cpp).  I was thinking the
>     common code in these two should be extracted to one place but I
>     wasn't sure whether that belonged in graphKit.cpp or in
>     macro.cpp.
>   * trace_exception in opto/runtime.cpp
>   * exception_handler_for_pc_helper in c1/c1_Runtime1.cpp
> Previously these places checked jvmti_can_post_exceptions() which only
> looked at whether the jvmti capabilities for exceptions were enabled,
> taking a slow path if true.
> Now they check a new flag JavaThread::_must_post_exception_events.
> This new flag is updated by calling jvmtiExport::must_post_exception_events
> whenever the jvmti situation for a thread might have changed.
> jvmtiExport::must_post_exception_events uses logic similar to that
> used by jvmtiExport::post_exception_throw and returns false if
> jvmtiExport::post_exception_throw wouldn't have done anything.
> I would appreciate it if someone familiar with the jvmti codepaths
> could review this to make sure that the must_post_exception_events
> flag is being updated in all the necessary places.  Right now, it gets
> updated in
>   * JavaThread::set_jvmti_thread_state
>   * JvmtiEventControllerPrivate::recompute_enabled


     This query method is a bundling of three different but related
     capabilities. If one of the following capabilities is enabled:


     then the agent is indicating that it may be interested in
     using JVM/TI events related to exceptions. I say "may be
     interested" because until the agent actually enables an event
     and specifies an event handler, there is no real interest.

     This function is like a "hold the date" e-mail for an upcoming
     gathering. No specifics, but a just a notice that you might
     need to block out some time on your schedule, etc.

In the current system, C1's exception_handler_for_pc_helper()
and C2's OptoRuntime::handle_exception_C_helper()
call JvmtiExport::can_post_exceptions() directly.
C2's GraphKit::builtin_throw() and Parse::do_one_bytecode()
call env()->jvmti_can_post_exceptions() which uses a
cached value from JvmtiExport::can_post_exceptions().

In the new code, must_post_exception_events_flag() is called
for the current JavaThread and that translates into a query
of the new JavaThread field where the state of needing to
post exception events is cached. This new cached field is set
to true when:

- JVMTI_EVENT_EXCEPTION is enabled in any environment
- or when JVMTI_EVENT_EXCEPTION tracing is enabled (I'm not
   sure that this check is needed, but I'd have to do more

Only the JVMTI_EVENT_EXCEPTION event is checked here. Frame
pop events and method exit event settings are not checked so
it seems like we're missing exception support when the agent
is interested in frame pop events or method exit events but
has not expressed an interest in all exception events.
Perhaps I missed it, but, since I'm going to recommend a
different way of doing this, the point is fairly moot.

I think adding the new field to the JavaThread is a bit
misplaced. This is JVM/TI state information and that belongs
in JvmtiThreadState. However, I'm guessing that you're trying
to avoid fetching the JavaThread, fetching the JvmtiThreadState
and finally adding the offset to the new field in the
JvmtiThreadState. For simplicity in the generated code, I can
see why adding to the JavaThread is easier. (Note: I tried to
remember this paragraph as I originally wrote it. Not sure how
I did, but I think I got the gist :-)).

Also, there are several other instances of JVM/TI'ness in the
JavaThread outside of the JvmtiThreadState pointer. That's an
implementation architecture problem outside of the scope of
this change.

Taking a step back, it certainly looks like this should be
done as a pair of functions:


The "can_we_do_..." function answers the question of whether
the agent "may be interested" in "foo" and maybe we need to
do some prep work. The "should_we_do_..." function answers
the question of whether one or more threads need to do "foo"
related work.

A good example of this distinction is "can_post_field_access()"
and "should_post_field_access()". The can_post_field_access()
function is called to determine if fast versions of the JNI
Get<Primitive>Field() functions should be generated. In this
particular case, the can_... function tells us to skip the work
of generating the fast versions. The should_post_field_access()
function is called by the various JNI Get... functions to
determine if any threads are interested in field access events.
The event posting code itself determines the threads to which
the events are posted.

All of the existing should_post_...() functions answer the query
in an aggregate sense, i.e., at least one thread needs to post
the event in question. Since exception posting is very expensive
due to deoptimization, I think we need an additional form of the
should_post_exceptions() query that takes a JavaThread param.
The new query will answer the question relative to the specified

We already have JvmtiExport::can_post_exceptions() so we need
to add JvmtiExport::should_post_exceptions(); the new query
will answer the question of whether any of the exception related
events are enabled globally, i.e., in any environment or any
thread. We're also going to need a new bit combination value


JvmtiEventControllerPrivate::recompute_enabled() will have to
be modified to set the new should_post_exceptions flag based on

We also need to add the new
JvmtiExport::should_post_exceptions(JavaThread *) that:

- gets the JvmtiThreadState from the JavaThread *
- if state == NULL, return false
- iterate the JvmtiEnvThreadState objects and if one of
   the interesting events is enabled, then return true
- otherwise return false

To make life easier for the compilers, we should also export
the should_post_exceptions(JavaThread *) state to the related

     Use JvmtiExport::should_post_exceptions(thread) instead of
     can_post_exceptions(). BTW, I just noticed that original
     proposed change called JavaThread::current() instead of
     using the 'thread' variable. Why?

     No comments.

     No comments.

     Use JvmtiExport::should_post_exceptions(thread) instead of
     can_post_exceptions(). BTW, I just noticed that original
     proposed change called JavaThread::current() instead of
     using the 'thread' variable. Why?

     No comments; this webrev shows no diffs for this file.

     Don't add new lines 570-572. This should be done in
     recompute_thread_enabled() itself. However, it should be done
     similar to the way the interp_only_mode field on the JavaThread
     is done.

     You'll need:
         - to check three bits instead of one
         - add enable_must_post_exception_events_flag and
           disable_must_post_exception_events_flag to
           to JvmtiThreadState; these methods will fetch the
           JavaThread from the JvmtiThreadState and call the
           right JavaThread method for setting the state.
         - call state->enable_must_post_exception_events_flag()
           or state->disable_must_post_exception_events_flag()
           as needed.

     Add new CAN_POST_EXCEPTION_EVENTS value that combines

     Add call to new JvmtiExport::set_should_post_exceptions()
     in the "if (delta != 0)" block. Use the new
     CAN_POST_EXCEPTION_EVENTS value for the comparison.

     Don't need the new JvmtiExport::must_post_exception_events()

     Don't need the new JvmtiExport::must_post_exception_events()

     Add a new JVMTI_SUPPORT_FLAG() macro call for the new
     should_post_exceptions flag.

     You'll need new methods here:

     Don't need to change set_jvmti_thread_state().
     I don't think you need update_must_post_exception_events_flag().

     Don't need to change set_jvmti_thread_state().
     I don't think you need update_must_post_exception_events_flag().

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