RFR 8205113: Update JVMTI doc references to object allocation tracking

[David Holmes]

On 19/06/2018 4:50 AM, Jeremy Manson wrote:
> Yup!  The paragraph meanders a bit.  How about something like:

I'm not sure some of the change quite works. The original text considers 
there to be three kinds of methods that can cause allocation when executed:
- Java (bytecode) methods
- JNI methods
- VM methods

but you've turned this into three kinds of allocation: via bytecode, via 
JNI, and via the VM. You then refer to "triggering" an allocation when 
we tend to use triggering for events. You also refer to an allocation 
being "executed directly by the VM" (a phrase previously applied when 
the subject was a 'method') - but you don't really execute allocations.

IIUC the problem with the existing text is just that it considers VM 
allocation events as being undetectable other than by this "VM object 
allocation event" - but that's no longer true. So how about something 
minimally changed like this:

---
   Sent when a method causes the virtual machine to directly allocate an
   Object visible to Java programming language code.
   Generally object allocation can be detected by instrumenting
   the bytecodes of allocating methods.
   Object allocation generated in native code by JNI function
   calls can be detected using
   <internallink id="jniIntercept">JNI function interception</internallink>.
    Some methods might not have associated bytecodes and are not
    native methods, they instead are executed directly by the
    VM. These methods should send this event.
    Virtual machines which are incapable of bytecode instrumentation
    for some or all of their methods can send this event.

    Note that the <internallink
    id="SampledObjectAlloc">SampledObjectAlloc</internallink>
    event is triggered on all Java object allocations, including those
    caused by bytecode method execution, JNI method execution, and
    directly by VM methods.
---

Thanks,
David

> Sent when the virtual machine allocates an
> Object visible to Java programming language code without using a
> <code>new</code> bytecode variant or a JNI method.
> Many approaches to tracking object allocation use a combination of
> bytecode-based instrumentation and <internallink id="jniIntercept">JNI 
> function
> interception</internallink> to intercept allocations.  However, this
> approach can leave a number of allocations undetected.  Allocations that 
> are neither
> triggered by bytecode nor JNI are executed directly by the VM.
> When those allocations occur, the VM should send this event.
> Virtual machines that are incapable of bytecode instrumentation
> for some or all of their methods may also send this event.
> <p/>
> Note that the <internallink 
> id="SampledObjectAlloc">SampledObjectAlloc</internallink>
> event is triggered on all Java object allocations, including those 
> triggered by bytecode,
> JNI methods, and VM events.
> 
> 
> 
> On Mon, Jun 18, 2018 at 12:57 AM David Holmes <david.holmes at oracle.com 
> <mailto:david.holmes at oracle.com>> wrote:
> 
>     On 18/06/2018 5:01 PM, Jeremy Manson wrote:
>      > We haven't changed when a VM issues "VM object allocation" events.
>      >
>      > There were references in the docs to a requirement to use bytecode
>      > rewriting and JNI interception to track allocations.  With
>      > SampledObjectAlloc, this is no longer the case -
>     SampledObjectAlloc can
>      > track them.  This change is supposed to remove the references to
>     those
>      > requirements, and provide suitable replacement text.
>      >
>      > VM object alloc has specific language about being able to use it to
>      > track allocations that cannot be tracked with bytecode
>     instrumentation
>      > and JNI interception.  My goal in rephrasing was to make it clear
>     that,
>      > while you can still use it for this, you can also just use
>      > SampledObjectAlloc for everything.
> 
>     Okay. That doesn't come across clearly to me - sorry. So you will now
>     get both kinds of events for allocations done in the VM?
> 
>     Thanks,
>     David
> 
> 
>      > Jeremy
>      >
>      > On Sun, Jun 17, 2018 at 9:11 PM David Holmes
>     <david.holmes at oracle.com <mailto:david.holmes at oracle.com>
>      > <mailto:david.holmes at oracle.com
>     <mailto:david.holmes at oracle.com>>> wrote:
>      >
>      >     Hi Jeremy,
>      >
>      >     On 16/06/2018 2:33 AM, Jeremy Manson wrote:
>      >      > Hi all,
>      >      >
>      >      > There are a number of references in the JVMTI doc to its
>     not doing
>      >      > object allocation tracking.  Now that JEP 331 has landed,
>     these
>      >      > references are obsolete.  This patch changes those references
>      >     accordingly.
>      >      >
>      >      > While I was there, I took the liberty of fixing some
>     spelling errors.
>      >      >
>      >      > As far as I know, these are non-normative changes and
>     modify no
>      >     API, so
>      >      > they should not require a CSR.
>      >
>      >     I'm unclear on the nature of the change to "VM Object
>     Allocation". Does
>      >     the existence of SampledObjectAlloc change when a VM should
>     issue "VM
>      >     object allocation" events?
>      >
>      >     Thanks,
>      >     David
>      >
>      >      >
>      >      > Bug:
>      >      > https://bugs.openjdk.java.net/browse/JDK-8205113
>      >      >
>      >      > Webrev:
>      >      > http://cr.openjdk.java.net/~jmanson/8205113/webrev.00/
>      >      >
>      >      > Thanks!
>      >      >
>      >      > Jeremy
>      >
>