RFR: 8350314: Shenandoah: Capture thread state sync times in GC timings [v2]

[William Kemper]

On Tue, 25 Feb 2025 23:27:22 GMT, Xiaolong Peng <xpeng at openjdk.org> wrote:

>> The change is to improve the observability of  Shenandoah GC, basically there are two changes for  Shenandoah GC timings in this PR:
>> 
>> 1. Net GC pause timings include the time to propagate GC state to Java threads
>> 2. Add new timing "Propagate GC state" in Shenandoah GC timing logs
>> 
>> With the change, the new GC timing log will be like:
>> 
>> [11.056s][info][gc,stats    ] Concurrent Reset                     89 us
>> [11.056s][info][gc,stats    ] Pause Init Mark (G)                 257 us
>> [11.056s][info][gc,stats    ] Pause Init Mark (N)                  17 us
>> [11.056s][info][gc,stats    ]   Update Region States                3 us
>> [11.056s][info][gc,stats    ]   Propagate GC state                  1 us
>> [11.056s][info][gc,stats    ] Concurrent Mark Roots               232 us, parallelism: 1.96x
>> [11.056s][info][gc,stats    ]   CMR: <total>                      456 us
>> [11.056s][info][gc,stats    ]   CMR: Thread Roots                 429 us, workers (us): 139, 148, 142, ---, ---, ---, 
>> [11.057s][info][gc,stats    ]   CMR: VM Strong Roots               11 us, workers (us):   8,   3,   0, ---, ---, ---, 
>> [11.057s][info][gc,stats    ]   CMR: CLDG Roots                    16 us, workers (us):  16, ---, ---, ---, ---, ---, 
>> [11.057s][info][gc,stats    ] Concurrent Marking                 1304 us, parallelism: 2.33x
>> [11.057s][info][gc,stats    ]   CM: <total>                      3043 us
>> [11.057s][info][gc,stats    ]   CM: Parallel Mark                3043 us, workers (us): 1023, 1017, 1003, ---, ---, ---, 
>> [11.057s][info][gc,stats    ]   Flush SATB                        204 us
>> [11.057s][info][gc,stats    ] Pause Final Mark (G)                865 us
>> [11.057s][info][gc,stats    ] Pause Final Mark (N)                234 us
>> [11.057s][info][gc,stats    ]   Finish Mark                       129 us, parallelism: 0.01x
>> [11.057s][info][gc,stats    ]   Propagate GC state                  2 us
>> [11.057s][info][gc,stats    ]   Update Region States               12 us
>> [11.057s][info][gc,stats    ]   Choose Collection Set              25 us
>> [11.057s][info][gc,stats    ]   Rebuild Free Set                   29 us
>> [11.057s][info][gc,stats    ] Concurrent Weak References           67 us, parallelism: 0.25x
>> [11.057s][info][gc,stats    ]   CWRF: <total>                      17 us
>> [11.057s][info][gc,stats    ]   CWRF: Weak References              17 us, workers (us):  15,   1,   0, ---, ---, ---, 
>> [11.057s][info][gc,stats    ] Concu...
>
> Xiaolong Peng has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Address review comments

Propagating gc state for init update refs is vestigial.

src/hotspot/share/gc/shenandoah/shenandoahConcurrentGC.cpp line 1081:

> 1079:     heap->pacer()->setup_for_update_refs();
> 1080:   }
> 1081:   {

This one isn't necessary. This safepoint only happens when the pacer or the verifier is enabled. We moved init update ref gc state propagation into a handshake (see `ShenandoahHeap::concurrent_prepare_for_update_refs`). Sorry, I should have caught this in my first review.

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

Changes requested by wkemper (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/23759#pullrequestreview-2642707574
PR Review Comment: https://git.openjdk.org/jdk/pull/23759#discussion_r1970695230