RFR: 8236073: G1: Use SoftMaxHeapSize to guide GC heuristics [v8]

[Man Cao]

On Thu, 3 Apr 2025 07:08:19 GMT, Man Cao <manc at openjdk.org> wrote:

>> Hi all,
>> 
>> I have implemented SoftMaxHeapSize for G1 as attached. It is completely reworked compared to [previous PR](https://github.com/openjdk/jdk/pull/20783), and excludes code for `CurrentMaxHeapSize`. I believe I have addressed all direct concerns from [previous email thread](https://mail.openjdk.org/pipermail/hotspot-gc-dev/2024-November/050214.html), such as:
>> 
>> - does not respect `MinHeapSize`;
>> - being too "blunt" and does not respect other G1 heuristics and flags for resizing, such as `MinHeapFreeRatio`, `MaxHeapFreeRatio`;
>> - does not affect heuristcs to trigger a concurrent cycle;
>> 
>> [This recent thread](https://mail.openjdk.org/pipermail/hotspot-gc-dev/2025-March/051619.html) also has some context.
>
> Man Cao has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Use Atomic::load for flag

Re: concurrency issue with reading `SoftMaxHeapSize`

I updated to `Atomic::load()`, but not sure if I understand the concern correctly.

> So e.g. the assignment of `G1IHOPControl::get_conc_mark_start_threshold` to `marking_initiating_used_threshold` in that call can be inlined in `G1Policy::need_to_start_conc_mark` (called by the mutator in `G1CollectedHeap::attempt_allocation_humongous`) in multiple places, and so `SoftMaxHeapSize` re-read with multiple different values in that method.

I don't see where the re-read is. I think in any code path from `G1IHOPControl::get_conc_mark_start_threshold`,  `G1CollectedHeap::heap()->soft_max_capacity()` is called only once. `G1CollectedHeap::attempt_allocation_humongous` also appears to call `G1Policy::need_to_start_conc_mark` only once, which calls `G1IHOPControl::get_conc_mark_start_threshold` only once.

I agree it is a data race if `soft_max_capacity()` runs outside of a safepoint, so `Atomic::load()` makes sense regardless.

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PR Comment: https://git.openjdk.org/jdk/pull/24211#issuecomment-2774731515