8352765: G1CollectedHeap::expand_and_allocate() may fail to allocate even after heap expansion succeeds #24257

walulyai · 2025-03-26T15:05:46Z

Hi,

Please review this change to ensure that G1 provisions for at least one Eden region after a GC when computing the young length target.

The issue reported in the CR occurs at the end of a GC, after successfully expanding the heap, an allocation fails because policy()->should_allocate_mutator_region() returns false. This happens because the computation did not properly account for young regions already allocated as survivor regions, leading to an Eden region target of zero.

With this change, we factor in the young regions that have already been allocated as survivor regions and ensure that at least one region is targeted for Eden.

Testing: Tier 1-3
Reproducer in the CR.

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Issue

JDK-8352765: G1CollectedHeap::expand_and_allocate() may fail to allocate even after heap expansion succeeds (Bug - P4)

Reviewers

Thomas Schatzl (@tschatzl - Reviewer)
Man Cao (@caoman - Committer)

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$ git fetch https://git.openjdk.org/jdk.git pull/24257/head:pull/24257
$ git checkout pull/24257

Update a local copy of the PR:
$ git checkout pull/24257
$ git pull https://git.openjdk.org/jdk.git pull/24257/head

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$ git pr checkout 24257

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bridgekeeper · 2025-03-26T15:06:41Z

👋 Welcome back iwalulya! A progress list of the required criteria for merging this PR into master will be added to the body of your pull request. There are additional pull request commands available for use with this pull request.

openjdk · 2025-03-26T15:07:07Z

@walulyai This change now passes all automated pre-integration checks.

ℹ️ This project also has non-automated pre-integration requirements. Please see the file CONTRIBUTING.md for details.

After integration, the commit message for the final commit will be:

8352765: G1CollectedHeap::expand_and_allocate() may fail to allocate even after heap expansion succeeds

Reviewed-by: tschatzl, manc

You can use pull request commands such as /summary, /contributor and /issue to adjust it as needed.

At the time when this comment was updated there had been 109 new commits pushed to the master branch:

2592513: 8350577: Fix missing Assertion Predicates when splitting loops
59629f8: 8351040: [REDO] Protection zone for easier detection of accidental zero-nKlass use
8cbadf7: 8317976: Optimize SIMD sort for AMD Zen 4
b9d7a75: 8352879: TestPeriod.java and TestGetContentType.java run wrong test class
895aabc: 8351233: [ASAN] avx2-emu-funcs.hpp:151:20: error: ‘D.82188’ is used uninitialized
3d2c3cd: 8352970: Remove unnecessary Windows version check in Win32ShellFolderManager2
c70ad6a: 8352906: stdout/err.encoding on Windows set by incorrect Win32 call
da3bb06: 8352685: Opensource JInternalFrame tests - series2
d809033: 8341775: Duplicate manifest files are removed by jarsigner after signing
a269bef: 8350459: MontgomeryIntegerPolynomialP256 multiply intrinsic with AVX2 on x86_64
... and 99 more: https://git.openjdk.org/jdk/compare/a8757332667df3fe41a29a7eedb2a7234d23c2a0...master

As there are no conflicts, your changes will automatically be rebased on top of these commits when integrating. If you prefer to avoid this automatic rebasing, please check the documentation for the /integrate command for further details.

➡️ To integrate this PR with the above commit message to the master branch, type /integrate in a new comment.

openjdk · 2025-03-26T15:07:51Z

@walulyai The following label will be automatically applied to this pull request:

hotspot-gc

When this pull request is ready to be reviewed, an "RFR" email will be sent to the corresponding mailing list. If you would like to change these labels, use the /label pull request command.

mlbridge · 2025-03-26T15:31:46Z

Webrevs

01: Full - Incremental (041b478c)
00: Full (7eea1ee3)

tschatzl · 2025-03-27T11:49:17Z

src/hotspot/share/gc/g1/g1Policy.cpp

                                "receiving young %u receiving additional eden %u",
-                                receiving_young,
+                                receiving_eden,


the log message reads "receiving young" still.

tschatzl · 2025-03-27T11:50:10Z

src/hotspot/share/gc/g1/g1Policy.cpp

                                "receiving additional eden %u",
                                free_outside_reserve, receiving_within_reserve,
-                                receiving_young, receiving_additional_eden);
+                                receiving_eden, receiving_additional_eden);


Log message needs update.

caoman

Thank you for the quick fix!

I just have a two questions, which probably does not need to be addressed in this PR.

Is it possible for expand_and_allocate() to run into the receiving_additional_eden = 0; situation under the if (allocated_young_length >= desired_young_length) branch?
It feels like G1 should recalculate young list length parameters after a successful expand() or shrink(). Currently G1Policy::record_new_heap_size() only recalculates the min/max young lengths. Is it reasonable to run calculate_young_desired_length() and calculate_young_target_length() inside G1Policy::record_new_heap_size()?

walulyai · 2025-03-28T13:07:07Z

I just have a two questions, which probably does not need to be addressed in this PR.

1. Is it possible for `expand_and_allocate()` to run into the [`receiving_additional_eden = 0;`](https://github.com/openjdk/jdk/blob/2ea1557a0fdaf551d75365d1351bfbd73319dcfb/src/hotspot/share/gc/g1/g1Policy.cpp#L320) situation under the `if (allocated_young_length >= desired_young_length)` branch?

2. It feels like G1 should recalculate young list length parameters after a successful `expand()` or `shrink()`. Currently `G1Policy::record_new_heap_size()` only recalculates the min/max young lengths. Is it reasonable to run `calculate_young_desired_length()` and `calculate_young_target_length()` inside `G1Policy::record_new_heap_size()`?

Yes, I think we need to do the recalculation after expand as the expansion changes the number of free regions available and so should affect the young_[desired,target]_lengths. Otherwise, it is possible to expand and fail allocation. This can be addressed in a separate PR though.

No, scratch that; after GC, the desired_young_length > allocated_young_length.

walulyai · 2025-03-31T11:14:33Z

Thanks @caoman and @tschatzl for the reviews!

/integrate

openjdk · 2025-03-31T11:15:49Z

Going to push as commit 804a8fa.
Since your change was applied there have been 113 commits pushed to the master branch:

22f630c: 8352415: x86: Tighten up template interpreter method entry code
6fbaa06: 8353176: C1: x86 patching stub always calls Thread::current()
79d0c94: 8352114: New test runtime/interpreter/CountBytecodesTest.java is failing
23e3b3f: 8351156: C1: Remove FPU stack support after 32-bit x86 removal
2592513: 8350577: Fix missing Assertion Predicates when splitting loops
59629f8: 8351040: [REDO] Protection zone for easier detection of accidental zero-nKlass use
8cbadf7: 8317976: Optimize SIMD sort for AMD Zen 4
b9d7a75: 8352879: TestPeriod.java and TestGetContentType.java run wrong test class
895aabc: 8351233: [ASAN] avx2-emu-funcs.hpp:151:20: error: ‘D.82188’ is used uninitialized
3d2c3cd: 8352970: Remove unnecessary Windows version check in Win32ShellFolderManager2
... and 103 more: https://git.openjdk.org/jdk/compare/a8757332667df3fe41a29a7eedb2a7234d23c2a0...master

Your commit was automatically rebased without conflicts.

openjdk · 2025-03-31T11:15:56Z

@walulyai Pushed as commit 804a8fa.

💡 You may see a message that your pull request was closed with unmerged commits. This can be safely ignored.

walulyai added 2 commits March 24, 2025 10:38

init

2526100

init

7eea1ee

walulyai changed the title ~~8352765 : G1CollectedHeap::expand_and_allocate() may fail to allocate even after heap expansion succeeds~~ 8352765: G1CollectedHeap::expand_and_allocate() may fail to allocate even after heap expansion succeeds Mar 26, 2025

openjdk bot added the hotspot-gc hotspot-gc-dev@openjdk.org label Mar 26, 2025

walulyai marked this pull request as ready for review March 26, 2025 15:27

openjdk bot added the rfr Pull request is ready for review label Mar 26, 2025

tschatzl suggested changes Mar 27, 2025

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041b478

caoman approved these changes Mar 28, 2025

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8352765: G1CollectedHeap::expand_and_allocate() may fail to allocate even after heap expansion succeeds #24257

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