On Fri, 25 Mar 2022 20:10:32 GMT, Roman Kennke <rken...@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 <rken...@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 ------------- PR: https://git.openjdk.java.net/jdk/pull/7964
