RFR: JDK-8212186: JVMTI lacks a few GC barriers/hooks

Aleksey Shipilev shade at redhat.com
Thu Oct 18 07:46:26 UTC 2018


On 10/18/2018 09:38 AM, Per Liden wrote:
> On 10/18/2018 09:24 AM, Aleksey Shipilev wrote:
>> On 10/18/2018 09:13 AM, Per Liden wrote:
>>>> Tracing. Scanning would require to carry around complete marking info
>>>> (to be able to avoid holes of dead objects with recycled Klass*), which
>>>> we don't.
>>>
>>> So that makes me wonder why you needed JDK-8211955 (GC abstraction for LAB reserve) and why you
>>> insert filler objects at all? Am I missing something?
>>
>> I think Roman meant that Shenandoah does tracing for _external_ heap walk, e.g. for heap dumps. The
>> internal heap walks, e.g. for evac/update-refs, are still pretty much scanning the self-parsable
>> heap, which requires us doing fillers right.
> 
> Don't you have the problem of potentially stepping on an object with a stale klass pointer in that
> case too?

No, we do not. Objects below TAMS would be walked using marking bitmap data, which would bypass dead
objects, and object above TAMS would be walked as self-parsable heap, but those objects (and their
klasses) are guaranteed to be alive.

Shenandoah does tracing for external heap walks, on the off chance external walk is requested
mid-marking. There are other ways to do this, e.g. wait for marking to complete before doing
external walk, which gets us back to heap parsability :)

-Aleksey

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