RFR: 8373595: A new ObjectMonitorTable implementation [v9]

[Coleen Phillimore]

On Fri, 20 Feb 2026 12:25:35 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 `
>
> Fredrik Bredberg has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Updated for Erik

I have a couple of drive-by comments.

src/hotspot/share/runtime/objectMonitorTable.cpp line 34:

> 32: #include "runtime/timerTrace.hpp"
> 33: #include "runtime/trimNativeHeap.hpp"
> 34: #include "utilities/concurrentHashTableTasks.inline.hpp"

I think you don't need to include this anymore.

src/hotspot/share/runtime/objectMonitorTable.cpp line 120:

> 118: 
> 119: // -----------------------------------------------------------------------------
> 120: // ConcurrentHashTable storing links from objects to ObjectMonitors

This ConcurrentHashTable is now not used , so this comment should be removed or rewritten to not have the name ConcurrentHashTable.

You say that it's concurrent in the big paragraph above so this comment is extra.

src/hotspot/share/runtime/synchronizer.hpp line 33:

> 31: #include "runtime/handles.hpp"
> 32: #include "runtime/javaThread.hpp"
> 33: #include "runtime/objectMonitorTable.hpp"

Small comment.  Why do you have to include objectMonitorTable.hpp here?

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PR Review: https://git.openjdk.org/jdk/pull/29383#pullrequestreview-3831852862
PR Review Comment: https://git.openjdk.org/jdk/pull/29383#discussion_r2833036871
PR Review Comment: https://git.openjdk.org/jdk/pull/29383#discussion_r2833053305
PR Review Comment: https://git.openjdk.org/jdk/pull/29383#discussion_r2833031754