RFR: 8320318: ObjectMonitor Responsible thread [v4]

[David Holmes]

On Wed, 25 Sep 2024 13:54:07 GMT, Fredrik Bredberg <fbredberg at openjdk.org> wrote:

>> Removed the concept of an ObjectMonitor Responsible thread.
>> 
>> The reason to have an ObjectMonitor Responsible thread was to avoid threads getting stranded due to a hole in the successor protocol. This hole was there because adding the necessary memory barrier was considered too expensive some 20 years ago.
>> 
>> The ObjectMonitor Responsible thread code adds complexity, and doing timed parks just to avoid getting stranded is not the way forward. More info about the problems with the ObjectMonitor responsible thread can be found in [JDK-8320318](https://bugs.openjdk.org/browse/JDK-8320318).
>> 
>> After removing the ObjectMonitor Responsible thread we see increased performance on all supported platforms except Windows. [JDK-8339730](https://bugs.openjdk.org/browse/JDK-8339730) has been created to handle this.
>> 
>> Passes tier1-tier7 on supported platforms.
>> x64, AArch64, Riscv64, ppc64le and s390x passes ok on the test/micro/org/openjdk/bench/vm/lang/LockUnlock.java test.
>> Arm32 and Zero doesn't need any changes as far as I can tell.
>
> Fredrik Bredberg has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 14 commits:
> 
>  - Update three, after the review
>  - Merge branch 'master' into 8320318_objectmon_responsible_thread
>  - Update two, after the review
>  - Update one, after the review
>  - Small fixes before the review
>  - Merge branch 'master' into 8320318_objectmon_responsible_thread
>  - Merge branch 'master' into 8320318_objectmon_responsible_thread
>  - Removed _Responsible
>  - Fixed s390
>  - Fixed legacy locking
>  - ... and 4 more: https://git.openjdk.org/jdk/compare/0f253d11...8140570f

src/hotspot/share/runtime/objectMonitor.cpp line 312:

> 310:   // The monitor is private to or already owned by locking_thread which must be suspended.
> 311:   // So this code may only contend with deflation.
> 312:   assert(locking_thread == Thread::current() || locking_thread->is_obj_deopt_suspend(), "must be");

These comments and asserts seem to belong to `enter_for_with_contention_mark`. ??

src/hotspot/share/runtime/objectMonitor.cpp line 336:

> 334:       // decrement occurs when the contention_mark goes out of
> 335:       // scope. ObjectMonitor::deflate_monitor() which is called by
> 336:       // the deflater thread who will decrement contentions after it

Suggestion:

      // scope. ObjectMonitor::deflate_monitor() (which is called by
      // the deflater thread) will decrement contentions after it

-------------

PR Review Comment: https://git.openjdk.org/jdk/pull/19454#discussion_r1778045723
PR Review Comment: https://git.openjdk.org/jdk/pull/19454#discussion_r1778038127