RFR: 8338737: Shenandoah: Reset marking bitmaps after the cycle

[Xiaolong Peng]

On Fri, 20 Dec 2024 18:44:45 GMT, William Kemper <wkemper at openjdk.org> wrote:

>> Reset marking bitmaps after collection cycle; for GenShen only do this for young generation, also choose not do this for Degen and full GC since both are running at safepoint, we should leave safepoint as ASAP.
>> 
>> I have run same workload for 30s with Shenandoah in generational mode and classic mode, average average time of concurrent reset dropped significantly since in most case bitmap for young gen should have been reset after pervious concurrent cycle finishes if there is no need to preserve bitmap states.
>> 
>> GenShen:
>> Before:
>> 
>> [33.342s][info][gc,stats    ] Concurrent Reset               =    0.023 s (a =     1921 us) (n =    12) (lvls, us =      133,      385,     1191,     1836,     8878)
>> 
>> 
>> After:
>> 
>> [33.597s][info][gc,stats    ] Concurrent Reset               =    0.004 s (a =      317 us) (n =    13) (lvls, us =       58,      119,      217,      410,      670)
>> [33.597s][info][gc,stats    ] Concurrent Reset After Collect =    0.018 s (a =     1365 us) (n =    13) (lvls, us =       91,      186,      818,     1836,     3872)
>> 
>> 
>> Shenandoah:
>> Before:
>> 
>> [33.144s][info][gc,stats    ] Concurrent Reset               =    0.014 s (a =     1067 us) (n =    13) (lvls, us =      139,      277,      898,     1328,     2118)
>> 
>> After:
>> 
>> [33.128s][info][gc,stats    ] Concurrent Reset               =    0.003 s (a =      225 us) (n =    13) (lvls, us =       32,       92,      137,      295,      542)
>> [33.128s][info][gc,stats    ] Concurrent Reset After Collect =    0.009 s (a =      661 us) (n =    13) (lvls, us =       92,      160,      594,      896,     1661)
>> 
>> 
>> Additional changes:
>> * Remove `ShenandoahResetBitmapClosure` and  `ShenandoahPrepareForMarkClosure`, merge the code with `ShenandoahResetBitmapClosure`, saving one iteration over all the regions. 
>> * Use API `ShenandoahGeneration::parallel_heap_region_iterate_free` to iterate the regions, two benefits from this:
>>   -  Underneath it calls `ShenandoahHeap::parallel_heap_region_iterate`, which is faster for very light tasks, see https://bugs.openjdk.org/browse/JDK-8337154
>>   -  `ShenandoahGeneration::parallel_heap_region_iterate_free` decorate the closure with `ShenandoahExcludeRegionClosure`, which simplifies the code in closure.
>> * When `_do_old_gc_bootstrap is true`, instead of reset mark bitmap for old gen separately, simply reset the global generations, so we don't need walk the all regions twice. 
>> * Clean up FullGC code, remove duplicate code.
>> 
>> ...
>
> src/hotspot/share/gc/shenandoah/shenandoahGeneration.cpp line 58:
> 
>> 56:     if (PREPARE_FOR_CURRENT_CYCLE) {
>> 57:       if (region->need_bitmap_reset() && _heap->is_bitmap_slice_committed(region)) {
>> 58:         _ctx->clear_bitmap(region);
> 
> Should this also `region->unset_need_bitmap_reset()`?

It is for current cycles, bitmaps will be dirty anyway, for next cycle reset will be needed, so the flag can't be unset here

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PR Review Comment: https://git.openjdk.org/jdk/pull/22778#discussion_r1894511194