> 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
Roman Kennke has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 12 commits: - Merge branch 'master' into JDK-8283710 - Move JFRBitSet typedef into its own header; Make _bitset a direct member, not dynamically allocated - Add comment describing ObjectBitSet - Refactor JVMTI usage of ObjectBitSet - Typedef ObjectBitSet<mtTracing> to JFRBitSet in JFR code - Rename BitSet to ObjectBitSet - Templatize BitSet for MEMFLAGS - Rename ObjectMarkerController -> ObjectMarker - Use (former JFR) BitSet for JVMTI object marking - Restore missing include - ... and 2 more: https://git.openjdk.java.net/jdk/compare/0a026759...9ea51d78 ------------- Changes: https://git.openjdk.java.net/jdk/pull/7964/files Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7964&range=06 Stats: 774 lines in 12 files changed: 326 ins; 427 del; 21 mod Patch: https://git.openjdk.java.net/jdk/pull/7964.diff Fetch: git fetch https://git.openjdk.java.net/jdk pull/7964/head:pull/7964 PR: https://git.openjdk.java.net/jdk/pull/7964
