RFR: 8338977: Parallel: Improve heap resizing heuristics [v14]

[Ivan Walulya]

On Mon, 23 Jun 2025 08:34:22 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

>> This patch refines Parallel's sizing strategy to improve overall memory management and performance.
>> 
>> The young generation layout has been reconfigured from the previous `eden-from/to` arrangement to a new `from/to-eden` order. This new layout facilitates young generation resizing, since we perform resizing after a successful young GC when all live objects are located at the beginning of the young generation. Previously, resizing was often inhibited by live objects residing in the middle of the young generation (from-space). The new layout is illustrated in `parallelScavengeHeap.hpp`.
>> 
>> `NumberSeq` is now used to track various runtime metrics, such as minor/major GC pause durations, promoted/survived bytes after a young GC, highest old generation usage, etc. This tracking primarily lives in `AdaptiveSizePolicy` and its subclass `PSAdaptiveSizePolicy`.
>> 
>> GC overhead checking, which was previously entangled with adaptive resizing logic, has been extracted and is now largely encapsulated in `ParallelScavengeHeap::is_gc_overhead_limit_reached`.
>> 
>> ## Performance evaluation
>> 
>> - SPECjvm2008-Compress shows ~8% improvement on Linux/AArch64 and Linux/x64 (restoring the regression reported in [JDK-8332485](https://bugs.openjdk.org/browse/JDK-8332485) and [JDK-8338689](https://bugs.openjdk.org/browse/JDK-8338689)).
>> - Fixes the surprising behavior when using a non-default (smaller) value of `GCTimeRatio` with Heapothesys/Hyperalloc, as discussed in [this thread](https://mail.openjdk.org/pipermail/hotspot-gc-dev/2024-November/050146.html).
>> - Performance is mostly neutral across other tested benchmarks: **DaCapo**, **SPECjbb2005**, **SPECjbb2015**, **SPECjvm2008**, and **CacheStress**. The number of young-gc sometimes goes up a bit and the total heap-size decreases a bit, because promotion-size-to-old-gen goes down with the more effective eden/survivor-space resizing.
>> 
>> PS: I have opportunistically set the obsolete/expired version to ~~25/26~~ 26/27 for now. I will update them accordingly before merging.
>> 
>> Test: tier1-8
>
> Albert Mingkun Yang has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 20 commits:
> 
>  - Merge branch 'master' into pgc-size-policy
>  - review
>  - Merge branch 'master' into pgc-size-policy
>  - merge
>  - version
>  - Merge branch 'master' into pgc-size-policy
>  - revert-aliases
>  - Merge branch 'master' into pgc-size-policy
>  - merge
>  - merge-fix
>  - ... and 10 more: https://git.openjdk.org/jdk/compare/516197f5...41027bdf

src/hotspot/share/gc/parallel/psScavenge.cpp line 539:

> 537:   if (!young_gen->to_space()->is_empty()) {
> 538:     // To-space is not empty; should run full-gc instead.
> 539:     log_debug(gc, ergo)("non-empty to-space; full-gc instead");

"To-space is not empty; should run full-gc instead" seems like a better log string if we need logging here

src/hotspot/share/gc/parallel/psVirtualspace.cpp line 66:

> 64:     _committed_high_addr += bytes;
> 65:   } else {
> 66:     log_warning(gc)("expand_by commit %zu bytes failed", bytes);

probably leftover from debugging, if not, then maybe improve the log string

-------------

PR Review Comment: https://git.openjdk.org/jdk/pull/25000#discussion_r2162115342
PR Review Comment: https://git.openjdk.org/jdk/pull/25000#discussion_r2162111028