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

[Aleksey Shipilev]

On Tue, 25 Feb 2025 01:20:35 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    ] Concurrent Weak Roots               413 us
> [11.057s][info][gc,stats  ...

src/hotspot/share/gc/shenandoah/shenandoahConcurrentGC.cpp line 308:

> 306: 
> 307:   op_init_mark();
> 308:   ShenandoahHeap::heap()->propagate_gc_state_to_all_threads();

I would say move it downwards into `op_init_mark` and wrap with its own subtimer, e.g.  with `ShenandoahGCPhase phase(ShenandoahPhaseTimings::init_propagate_gcstate);`.

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PR Review Comment: https://git.openjdk.org/jdk/pull/23759#discussion_r1969328009