RFR (S) 8075030: JvmtiEnv::GetObjectSize reports incorrect java.lang.Class instance size

[Aleksey Shipilev]

On 06/01/2016 09:41 AM, serguei.spitsyn at oracle.com wrote:
> I guess, at least, this needs a CCC request.

Why? This changes behavior within the spec.

> I'm uncomfortable to support the change without knowing the history 
> behind the original implementation. Does anyone know the reasons why
> current implementation was chosen in the first place?

Stefan Karlsson forwarded me a discussion back from 2011, which says:

--------- 8< -----------------------------------------------------------

> For the permgen removal project I'm reworking how the sizes of =
metadata are calculated, and stumbled upon this code.
> For "normal" objects it calculates the size and are done. But, for
> non-primitive java.lang.Class instances (mirrors), it uses the size of
> the Klass instead. Does anyone have a reasonable explanation why this
> is done?

My guess would be that it's attempting to provide a more abstractly =
accurate value, though it's still way off.

[...]

So we can return whatever we want but we need to be consistent.  There
are other places where the klassOop is swapped for the Class in
jvmtiTagMap.cpp and the size reported so it will need to be consistent
with whatever GetObjectSize does.  I'd maintain the current size
reporting behaviour if possible.

--------- 8< -----------------------------------------------------------

But I looked around and saw no discrepancies. The tests are also fine.

This block history predates OpenJDK. Metaspace work left it untouched
because it had bigger fish to fry. But at this point I believe we don't
have to drag this debt around anymore.

> Are there any tools that depend on the current behavior?

I don't think so. Specification itself allows for returning a size
estimate, so strictly speaking relying on the reported size is not safe.
However, I know at least one tool which acts all surprised when the
reported instance size is way off, and it looks like Class instances are
overlapping with subsequent objects.

Thanks,
-Aleksey

> Thanks,
> Serguei
> 
> On 5/31/16 00:40, Aleksey Shipilev wrote:
>> Hi,
>>
>> Please review a tiny fix in GetObjectSize:
>>    https://bugs.openjdk.java.net/browse/JDK-8075030
>>    http://cr.openjdk.java.net/~shade/8075030/webrev.01/
>>
>> I think this is a leftover from Metaspace work.
>>
>> For some reason, JvmtiEnv::GetObjectSize has a special case for
>> java.lang.Class: it returns the Klass* size, not the java.lang.Class
>> instance size. This is incorrect: Klass* is indeed referenced from
>> java.lang.Class.metadata field, but the instance size itself does not
>> depend on metadata size. This confuses Instrumentation.getObjectSize
>> users.
>>
>> Testing: new test; RBT, :hotspot_compiler, :hotspot_gc,
>> :hotspot_runtime, :hotspot_serviceability, :hotspot_misc,
>> :jdk_management, :jdk_instrument, :jdk_jmx, :jdk_jdi, :svc_tools.
>>
>> Thanks,
>> -Aleksey
>>
>