Integrated: 8283710: JVMTI: Use BitSet for object marking

[Roman Kennke]

On Fri, 25 Mar 2022 20:10:32 GMT, Roman Kennke <rkennke at openjdk.org> wrote:

> JVMTI heap walking marks objects in order to track which have been visited already. In order to do that, it uses bits in the object header. Those are the same bits that are also used by some GCs to mark objects (the lowest two bits, also used by locking code). Some GCs also use the bits in order to indicate 'forwarded' objects, where the upper bits of the header represent the forward-pointer. In the case of Shenandoah, it's even more problematic because this happens concurrently, even while JVMTI heap walks can intercept. So far we carefully worked around that problem, but it becomes very problematic in Lilliput, where accesses to the Klass* also requires to decode the header, and figure out what bits means what.
> 
> In addition to that, marking objects in their header requires that the original header gets saved and restored. We only do that for 'interesting' headers, that is headers that have a stack-lock, monitor or hash-code. All other headers are reset to their default value. This means we are losing object's GC age. This is not catastrophic, but nontheless interferes with GC. 
> 
> JFR already has a datastructure called BitSet to support object marking without messing with object's headers. We can use that in JVMTI too.
> 
> Testing:
>  - [x] tier1
>  - [x] tier2
>  - [x] tier3
>  - [x] serviceability/jvmti
>  - [x] vmTestbase/nsk/jvmti

This pull request has now been integrated.

Changeset: abfd2f98
Author:    Roman Kennke <rkennke at openjdk.org>
URL:       https://git.openjdk.java.net/jdk/commit/abfd2f98dcbe3e96efe52b1d66e4c2efb3542955
Stats:     840 lines in 13 files changed: 392 ins; 427 del; 21 mod

8283710: JVMTI: Use BitSet for object marking

Reviewed-by: stuefe, coleenp

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PR: https://git.openjdk.java.net/jdk/pull/7964