RFR: 8346920: Serial: Support allocation in old generation before GC

[Guoxiong Li]

On Thu, 2 Jan 2025 06:30:00 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

> This PR introduces a new strategy to determine whether an allocation should be attempted in the old generation or if a GC cycle should be initiated, based on the `GCTimeRatio`. With this change, the benchmark attached to the ticket now completes in ~13 GC, a significant improvement compared to the >1000 GC observed previously.
> 
> Test: tier1-3

Nice improvement.

src/hotspot/share/gc/serial/defNewGeneration.hpp line 53:

> 51: class DefNewGeneration: public Generation {
> 52:   friend class VMStructs;
> 53:   friend class SerialHeap;

Not really know whether it is a good way. Maybe we can export  a method `DefNewGeneration::gc_timer`.

src/hotspot/share/gc/serial/serialHeap.cpp line 334:

> 332: 
> 333:       bool first_only = !should_try_older_generation_allocation(size)
> 334:                      && is_long_enough_from_prev_gc_pause_end();

Is the `first_only` actually `young_only`? It means that the allocation is only attempted in young generation?

src/hotspot/share/gc/serial/serialHeap.cpp line 643:

> 641:   Ticks prev_gc_pause_end;
> 642:   Tickspan gc_pause;
> 643:   if (full_gc_pause_end < young_gc_pause_end ) {

A unnecessary space after `young_gc_pause_end`.

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Changes requested by gli (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/22899#pullrequestreview-2532209941
PR Review Comment: https://git.openjdk.org/jdk/pull/22899#discussion_r1904201771
PR Review Comment: https://git.openjdk.org/jdk/pull/22899#discussion_r1904191931
PR Review Comment: https://git.openjdk.org/jdk/pull/22899#discussion_r1904196305