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List:       openjdk-serviceability-dev
Subject:    RE: RFR(S) 8230956: Should disable Escape Analysis when JVMTI capability can_tag_objects is taken
From:       "Reingruber, Richard" <richard.reingruber () sap ! com>
Date:       2019-09-25 13:43:00
Message-ID: DB7PR02MB482290224B170D6860C919AB9B870 () DB7PR02MB4822 ! eurprd02 ! prod ! outlook ! com
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Hello Vladimir,

thanks for looking at this.

  > can_tag_objects is "always" capability.

That's correct.

  > If it is true then EA will be disabled in all cases when JVMTI agent is used. It \
is too broad.  >
  > Am I missing something?

No that's correct too. If you include jvmti as hotspot feature in your build then you \
should disable escape analysis by default.

That's really too hard. My proposed improvement has the effect that escape analysis \
is disabled if any agent calls AddCapabilities() during the OnLoad phase. Because \
calling AddCapabilities(), even with an empty set of capabilities, has the effect \
that JvmtiExport::can_walk_any_space() will return always true after that. Not sure \
if this is a bug or feature though. But only disabling EA by default or JDK-8227745 \
would be a real fix.

  > It is also not clear to me that it is bug. Based on all description this \
functionality is used to   > catch leaks in Java heap. But scalar replaced objects do \
not exists. JVMTI should not even see them.

I do think, JVMTI heap functions must report scalar replaced objects for formal and \
for practical reasons.

JVM spec defines for the new bytecode [1]

  Operation

    Create new object

  Description

    [...] Memory for a new instance of that class is allocated from the \
garbage-collected heap, [...]

So I think JVMTI heap functions have to report that new instance on the virtual heap.

If they don't do it you can get contradicting data, too, from agents that employ \
bytecode instrumentation to count instances.

Also there can be a leak rooted at a chain of scalar replaced objects. This is \
currently not reported either. If an agent traverses references it won't arrive at \
the leaked objects. If it itereates objects on the heap then the leaking objects are \
found, but it will remain unknown what's keeping them alive.

Thanks, Richard.

[1] https://docs.oracle.com/javase/specs/jvms/se13/html/jvms-6.html#jvms-6.5.new

-----Original Message-----
From: Vladimir Kozlov <vladimir.kozlov@oracle.com> 
Sent: Mittwoch, 25. September 2019 03:04
To: Reingruber, Richard <richard.reingruber@sap.com>; \
hotspot-compiler-dev@openjdk.java.net; OpenJDK Serviceability \
                <serviceability-dev@openjdk.java.net>
Subject: Re: RFR(S) 8230956: Should disable Escape Analysis when JVMTI capability \
can_tag_objects is taken

It is also not clear to me that it is bug. Based on all description this \
functionality is used to  catch leaks in Java heap. But scalar replaced objects do \
not exists. JVMTI should not even see them.

Thanks,
Vladimir

On 9/24/19 3:37 PM, Vladimir Kozlov wrote:
> can_tag_objects is "always" capability.
> 
> If it is true then EA will be disabled in all cases when JVMTI agent is used. It is \
> too broad. 
> Am I missing something?
> 
> Thanks,
> Vladimir
> 
> On 9/13/19 7:12 AM, Reingruber, Richard wrote:
> > Hi,
> > 
> > could I please get reviews for
> > 
> > Webrev: http://cr.openjdk.java.net/~rrich/webrevs/2019/8230956/webrev.0/
> > Bug:    https://bugs.openjdk.java.net/browse/JDK-8230956
> > 
> > JVMTI provides functions to follow references beginning at the roots of the \
> > object graph and it provides functions to iterate all objects on the heap[1][2]. \
> > These functions are means to access objects which are otherwise local to a Java \
> > thread. In terms of escape analysis these local objects escape through these \
> > JVMTI functions invalidating optimizations based on escape analysis. 
> > Example:
> > 
> > - Let J be a JavaThread that calls a compiled method M with a NoEscape instance I \
> > of class C that is scalar replaced.
> > 
> > - JVMTI agent A uses JVMTI FollowReferences() to iterate the objects in the \
> > object graph tagging all instances of C.
> > 
> > - A uses GetObjectsWithTags() to retrieve the tagged instances of C.
> > 
> > - Error: I is missing because its allocation was eliminated / scalar replaced.
> > 
> > Agents are required to possess the capability can_tag_objects in order to call \
> > the JVMTI heap functions that let objects escape.  Currently it is not possible \
> > to revert EA based optimizations just before objects escape through JVMTI \
> > therefore escape analysis should be disabled as soon as the JVMTI capability \
> > can_tag_objects is taken. 
> > But this is not sufficient, because there may be compiled frames on stack with EA \
> > based optimizations when a JVMTI agent takes can_tag_objects (see included \
> > exclusive test cases), and then it does not help to disable escape analysis or \
> > invalidate compiled methods with ea based optimizations. In general it is still \
> > an improvement to do so. JDK-8227745 would be a complete solution to the issue.
> > 
> > An further improvement could be to invalidate methods compiled by c2 when \
> > can_tag_objects gets added, but I'd rather suggest to integrated the \
> > implementation for JDK-8227745. Note also that after calling JVMTI \
> > AddCapabilities(), even with an empty set of capabilities, \
> > JvmtiExport::can_walk_any_space() will return true. 
> > I've run tier1 tests.
> > 
> > Thanks, Richard.
> > 
> > [1] https://docs.oracle.com/en/java/javase/11/docs/specs/jvmti.html#Heap
> > [2] https://docs.oracle.com/en/java/javase/11/docs/specs/jvmti.html#Heap_1_0
> > 


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