8276055: ZGC: Defragment address space

[pliden]

In ZGC, small pages are allocated from the beginning of the address space (low addresses), while medium and large pages are allocated from the end of the address space (high addresses). However, small pages can sometimes end up on a high address if a medium or a large page was split into small pages. Over time this can cause the address space to become fragmented, with the risk of eventually running out of address space for new large allocations.

To mitigate this, we can defragment the address space by letting small pages at high addresses be remapped to lower addresses. That's what this patch does.

Testing:

• Tier 1-7 with ZGC on Linux/x86_64
• SPECjbb2015 shows no regression
• Manually ran test that tries to fragment the address space

---

Progress

• Change must not contain extraneous whitespace
• Commit message must refer to an issue
• Change must be properly reviewed

Issue

• JDK-8276055: ZGC: Defragment address space

Reviewers

• Erik Österlund (@fisk - Reviewer)
• Stefan Karlsson (@stefank - Reviewer)

Reviewing

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[bridgekeeper]

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[openjdk]

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[mlbridge]

Webrevs

• 01: Full - Incremental (5b7b8e96)
• 00: Full (cbd5b6b3)

[mlbridge]

Mailing list message from Stefan Karlsson on hotspot-gc-dev:

Looks good!

StefanK

On 2021-10-27 14:19, Per Liden wrote:

[fisk]

Looks good.

[openjdk]

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

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

8276055: ZGC: Defragment address space

Reviewed-by: eosterlund, stefank

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 11 new commits pushed to the master branch:

• e6fa5fa: 8276063: ProblemList gtest dll_address_to_function_and_library_name on macosx-generic
• 809488b: 8276046: codestrings.validate_vm gtest fails on ppc64, s390
• 93be099: 4718400: Many quantities are held as signed that should be unsigned
• 168081e: 8270490: Charset.forName() taking fallback default value
• a292733: 8275975: Remove dead code in ciInstanceKlass
• 4060602: 8275800: Redefinition leaks MethodData::_extra_data_lock
• 485d658: 8275851: Deproblemlist open/test/jdk/javax/swing/JComponent/6683775/bug6683775.java
• b3f45f8: 8275689: [TESTBUG] Use color tolerance only for XRender in BlitRotateClippedArea test
• 6c05cc9: 8275863: Use encodeASCII for ASCII-compatible DoubleByte encodings
• 2f979ec: 8213120: java/awt/TextArea/AutoScrollOnSelectAndAppend/AutoScrollOnSelectAndAppend.java fails on mac10.13
• ... and 1 more: https://git.openjdk.java.net/jdk/compare/9f75d5ce500886b32175cc541939b7f0eee190ca...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.

[pliden]

Renamed a few functions after off-line discussion with @stefank and @fisk.

[pliden]

Thanks for reviewing!

/integrate

[openjdk]

Going to push as commit 1750a6e.
Since your change was applied there have been 13 commits pushed to the master branch:

• d9b0138: 8275704: Metaspace::contains() should be threadsafe
• 9a3e954: 8274879: Replace uses of StringBuffer with StringBuilder within java.base classes
• e6fa5fa: 8276063: ProblemList gtest dll_address_to_function_and_library_name on macosx-generic
• 809488b: 8276046: codestrings.validate_vm gtest fails on ppc64, s390
• 93be099: 4718400: Many quantities are held as signed that should be unsigned
• 168081e: 8270490: Charset.forName() taking fallback default value
• a292733: 8275975: Remove dead code in ciInstanceKlass
• 4060602: 8275800: Redefinition leaks MethodData::_extra_data_lock
• 485d658: 8275851: Deproblemlist open/test/jdk/javax/swing/JComponent/6683775/bug6683775.java
• b3f45f8: 8275689: [TESTBUG] Use color tolerance only for XRender in BlitRotateClippedArea test
• ... and 3 more: https://git.openjdk.java.net/jdk/compare/9f75d5ce500886b32175cc541939b7f0eee190ca...master

Your commit was automatically rebased without conflicts.

[openjdk]

@pliden Pushed as commit 1750a6e.

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