RFR: 8366122: Shenandoah: Implement efficient support for object count after gc events

[pf0n]

On Thu, 28 Aug 2025 23:49:20 GMT, Kelvin Nilsen <kdnilsen at openjdk.org> wrote:

>> ### Summary
>> 
>> The new implementation of ObjectCountAfterGC for Shenandoah piggybacks off of the existing marking phases and records strongly marked objects in a histogram. If the event is disabled, the original marking closures are used. When enabled new mark-and-count closures are used by the worker threads. Each worker thread updates its local histogram as it marks an object. These local histograms are merged at the conclusion of the marking phase under a mutex. The event is emitted outside a safepoint. Because (most) Shenandoah's marking is done concurrently, so is the object counting work.
>> 
>> ### Performance
>> The performance test were ran using the Extremem benchmark on a default and stress workload. (will edit this section to include data after average time and test for GenShen)
>> 
>> #### Default workload:
>> ObjectCountAfterGC disabled (master branch):
>> `[807.216s][info][gc,stats    ] Pause Init Mark (G)            =    0.003 s (a =      264 us)`
>> `[807.216s][info][gc,stats    ] Pause Init Mark (N)            =    0.001 s (a =       91 us)`
>> `[807.216s][info][gc,stats    ] Concurrent Mark Roots          =    0.041 s (a =     4099 us)`
>> `[807.216s][info][gc,stats    ] Concurrent Marking             =    1.660 s (a =   166035 us)`
>> `[807.216s][info][gc,stats    ] Pause Final Mark (G)           =    0.004 s (a =      446 us) `
>> `[807.216s][info][gc,stats    ] Pause Final Mark (G)           =    0.004 s (a =      446 us) `
>> `[807.216s][info][gc,stats    ] Pause Final Mark (N)           =    0.004 s (a =      357 us)`
>> 
>> ObjectCountAfterGC disabled (feature branch):
>> `[807.104s][info][gc,stats    ] Pause Init Mark (G)            =    0.003 s (a =      302 us)`
>> `[807.104s][info][gc,stats    ] Pause Init Mark (N)            =    0.001 s (a =       92 us) `
>> `[807.104s][info][gc,stats    ] Concurrent Mark Roots          =    0.048 s (a =     4827 us)`
>> `[807.104s][info][gc,stats    ] Concurrent Marking             =    1.666 s (a =   166638 us) `
>> `[807.104s][info][gc,stats    ] Pause Final Mark (G)           =    0.006 s (a =      603 us)`
>> `[807.104s][info][gc,stats    ] Pause Final Mark (N)           =    0.005 s (a =      516 us)`
>> 
>> ObjectCountAfterGC enabled (feature branch)
>> `[807.299s][info][gc,stats    ] Pause Init Mark (G)            =    0.002 s (a =      227 us)`
>> `[807.299s][info][gc,stats    ] Pause Init Mark (N)            =    0.001 s (a =       89 us) `
>> `[807.299s][info][gc,stats    ] Concurrent Mark Roots       ...
>
> src/hotspot/share/gc/shared/gcTrace.inline.hpp line 12:
> 
>> 10: 
>> 11: // The ObjectCountEventSenderClosure will determine if only the ObjectCount
>> 12: // event will be emitted instead of ObjectCountAfterGC. If false, then both
> 
> If "what" is false?  This comment is not clear.  Are you speaking of the SeparateEventEmission template parameter?
> 
> I think the use of future-tense "will" also makes this comment confusing.  Can you write this in present tense?

I'll remove this template parameter, since it won't be needed for the ObjectCountAfterGC PR.

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PR Review Comment: https://git.openjdk.org/jdk/pull/26977#discussion_r2312857523