RFR: 8373595: A new ObjectMonitorTable implementation

[David Holmes]

On Fri, 23 Jan 2026 10:36:26 GMT, Fredrik Bredberg <fbredberg at openjdk.org> wrote:

> Me and Anton Artemov (@toxaart) investigated a quite large regression that occurred in Pet Clinic that happened if you turned on Compact Object Headers. It was found that a large portion of that regression could be attributed to not finding the monitor in the Object Monitor Cache, and because we couldn't access the Object Monitor Table from C2 generated code, we often had to take the slow path.
> 
> By making the object monitor cache larger and make it use the object's hash value as a key, we managed to mitigate the regression.
> 
> Erik Österlund (@fisk) took that idea and elevated it to the next level, which means that he rewrote the object monitor table code so that we can now search for an object in the global object monitor table from C2 generated code. I.e. from `C2_MacroAssembler::fast_lock()`. As in my and Anton's version, the key is the hash value from the object.
> 
> Erik also provided new barrier code needed for ZGC for x86 and aarch64. Roman Kennke (@rkennke) provided the same for Shenandoah, also for x86 and aarch64.
> 
> We decided to keep the Object Monitor Cache, since we found that for most programs (but not Pet Clinic) the monitor you are looking for is likely found in the first positions of the cache (it's sorted in most recently used order). However we decresed the size from 8 to 2 elements.
> 
> After running extensive performance tests we can say that this has improved the performance in many of them, not only mitigated the regression in Pet Clinic.
> 
> Tests are running okay tier1-7 on supported platforms.
> 
> The rest of the platforms (`ppc`, `riscv` and `s390`) have been smoke tested using QEMU.
> I mainly used this test for smoke testing with QEMU: `-XX:+UnlockDiagnosticVMOptions -XX:+UseObjectMonitorTable ./test/hotspot/jtreg/runtime/Monitor/UseObjectMonitorTableTest.java `

Not a review, just a couple of drive-by comments.

There is no high-level design doc description for the OMT itself - nothing to describe what "tombstones" and "relocations" are. Maybe such things are well known if you write CHTs ...

src/hotspot/share/runtime/objectMonitorTable.cpp line 417:

> 415:     if (result != curr) {
> 416:       // Somebody else racingly started rehashing; try again.
> 417:       new_table = result;

First, how is this "trying again"? I see no loop or repeated action.

Second, you seem to be leaking the table you just allocated.

src/hotspot/share/runtime/synchronizer.cpp line 1204:

> 1202:       }
> 1203:     }
> 1204: #endif

I find it very hard to see how rebuilding relates to the deleted debug code? Maybe this is just a quirk of the diff: we don't need the old debug code; we do need the new rebuild code - they aren't actually related in any way. ?

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PR Review: https://git.openjdk.org/jdk/pull/29383#pullrequestreview-3725612723
PR Review Comment: https://git.openjdk.org/jdk/pull/29383#discussion_r2744053892
PR Review Comment: https://git.openjdk.org/jdk/pull/29383#discussion_r2744045743