8351167: ZGC: Lazily initialize livemap

[jsikstro]

Memory for the bitmap inside the livemap of a ZPage is currently allocated upon calling its constructor, which adds a latency overhead when allocating pages. As preparation for the Mapped Cache (JDK-8350441), but also as a standalone improvement, we want to instead lazily initialize the livemap's bitmap.

This patch holds off with allocating memory for the bitmap that the livemap uses until the livemap is written to the first time (i.e. by calling ZLiveMap::set). The effect of this is that the latency impact of allocating the bitmap will only be taken by GC threads and not by mutator threads, since only GC threads mark objects before pushing them onto the mark stack. This improvement will reduce page allocation latencies somewhat.

In addition to lazily allocating the bitmap, I've converted the static C-style cast to a checked cast for ZPage::object_max_count(), which is passed as the size to the bitmaps. This is because a value not contained in 32 bits will overflow with the C-style cast and give a too small bitmap when passed to the livemap. This is not an observed issue, just more of a sanity check.

Testing:

• Tiers 1-5
• GHA

---

Progress

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Issue

• JDK-8351167: ZGC: Lazily initialize livemap (Enhancement - P4)

Reviewers

• Stefan Johansson (@kstefanj - Reviewer)
• Erik Österlund (@fisk - Reviewer)
• Thomas Schatzl (@tschatzl - Reviewer)

Reviewing

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[bridgekeeper]

👋 Welcome back jsikstro! 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]

@jsikstro 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:

8351167: ZGC: Lazily initialize livemap

Reviewed-by: sjohanss, eosterlund, tschatzl

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 98 new commits pushed to the master branch:

• 4be502e: 8350642: Interpreter: Upgrade CountBytecodes to 64 bit on 64 bit platforms
• 1fe4526: 8350194: Last 2 parameters of ReturnNode::ReturnNode are swapped in the declaration
• 1d147cc: 8351484: Race condition in max stats in MonitorList::add
• 4412c07: 8351639: Improve debuggability of test/langtools/jdk/jshell/JdiHangingListenExecutionControlTest.java test
• 1dd9cf1: 8349099: java/awt/Headless/HeadlessMalfunctionTest.java fails on CI with Compilation error
• 64464ea: 8351673: Clean up a case of if (LockingMode == LM_LIGHTWEIGHT) in a legacy-only locking mode function
• 9a49418: 8345940: Migrate security-related resources from Java classes to properties files
• e71f327: 8351045: ClassValue::remove cannot ensure computation observes up-to-date state
• cef3693: 8351656: Problemlist gc/TestAllocHumongousFragment#generational
• da2b4f0: 8351606: Use build_platform for graphviz dependency
• ... and 88 more: https://git.openjdk.org/jdk/compare/8073914af7d4ddd7bbd93d75104c7637e38a7ad9...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]

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

• hotspot-gc

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[mlbridge]

Webrevs

• 01: Full - Incremental (c70b4bda)
• 00: Full (969cb3cb)

[kstefanj]

Looks good.

[fisk]

Seems reasonable.

[jsikstro]

Thank you for the reviews!
/integrate

[openjdk]

Going to push as commit 2672c40.
Since your change was applied there have been 147 commits pushed to the master branch:

• 63bf791: 8351992: JFR: Improve robustness of the SettingControl examples
• 06289f3: 8351821: VMManagementImpl.c avoid switching off warnings
• 0460978: 8351096: Typos in Vector API doc
• 7f42804: 8350524: Some hotspot/jtreg/serviceability/dcmd/vm tier1 tests fail on static JDK
• 712a70c: 8318220: RISC-V: C2 ReverseI
• 985ca12: 8352011: RISC-V: Two IR tests fail after JDK-8351662
• 86b2433: 8352050: Problem list compiler/ciReplay/* test until JDK-8349191 is fixed
• 65c5282: 8351938: C2: Print compilation bailouts with PrintCompilation compile command
• e3c29c9: 8351556: Optimize Location.locationFor/isModuleOrientedLocation
• a7a09f6: 8349632: RISC-V: Add Zfa fminm/fmaxm
• ... and 137 more: https://git.openjdk.org/jdk/compare/8073914af7d4ddd7bbd93d75104c7637e38a7ad9...master

Your commit was automatically rebased without conflicts.

[openjdk]

@jsikstro Pushed as commit 2672c40.

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