8328075: Shenandoah: Avoid forwarding when objects don't move in full-GC

[rkennke]

Currently, in Shenandoah's full-GC, we forward all marked objects (and preserve their headers), even if they don't move. This typically happens for a certain amount of 'sediment' that accumulates at the bottom of the heap. This results in wasted CPU cycles and memory accesses and usage. It can easily be avoided by not forwarding objects that don't move.

The fix is to simply not forward objects when they don't move.

Testing:

• hotspot_gc_shenandoah
• tier1 +UseShenandoahGC

---

Progress

• Change must be properly reviewed (1 review required, with at least 1 Reviewer)
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Issue

• JDK-8328075: Shenandoah: Avoid forwarding when objects don't move in full-GC (Enhancement - P4)

Reviewers

• William Kemper (@earthling-amzn - Committer) ⚠️ Review applies to 079fe89d
• Aleksey Shipilev (@shipilev - Reviewer) ⚠️ Review applies to 079fe89d
• Kelvin Nilsen (@kdnilsen - no project role) ⚠️ Review applies to 079fe89d
• Y. Srinivas Ramakrishna (@ysramakrishna - Reviewer)

Reviewing

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[bridgekeeper]

👋 Welcome back rkennke! 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]

@rkennke The following labels will be automatically applied to this pull request:

• hotspot-gc
• shenandoah

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[openjdk]

@rkennke 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:

8328075: Shenandoah: Avoid forwarding when objects don't move in full-GC

Reviewed-by: wkemper, shade, kdnilsen, ysr

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 54 new commits pushed to the master branch:

• e8d1ba3: 8328228: Missing comma in copyright year for a few JColorChooser tests
• be1dd27: 8328130: Remove applet usage from JColorChooser tests Test4759934
• fab0edc: 8328121: Remove applet usage from JColorChooser tests Test4759306
• 7c6fd33: 8327857: Remove applet usage from JColorChooser tests Test4222508
• 89fb0f8: 8327859: Remove applet usage from JColorChooser tests Test4319113
• df909e2: 8328011: Convert java/awt/Frame/GetBoundsResizeTest/GetBoundsResizeTest.java applet test to main
• 87bd6ca: 8328021: Convert applet test java/awt/List/SetFontTest/SetFontTest.html to main program
• 2482a50: 8326898: NSK tests should listen on loopback addresses only
• e6a8fdd: 8328135: javax/management/remote/mandatory/loading/MissingClassTest.java fails on libgraal
• 7cc1965: 8328124: Convert java/awt/Frame/ShownOnPack/ShownOnPack.html applet test to main
• ... and 44 more: https://git.openjdk.org/jdk/compare/a4a5196351a3c8ce45e2e36d27842194cbfcc5ff...master

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[mlbridge]

Webrevs

• 01: Full - Incremental (69493333)
• 00: Full (079fe89d)

[earthling-amzn]

LGTM

[shipilev]

A few nits. I think we do the same thing for humongous objects already (where it is really painful), no particular reason not to do the same for regular ones. Any performance tests for Full GC improve?

[kdnilsen]

Looks good after Aleksey's assertion comment fix. (FWIW, i've seen many back-to-back Full GCs that do not relocate any objects on some of the jtreg tests. Of course, these back-to-back Full GCs hopefully don't represent typical behavior of real workloads.)

[rkennke]

Thanks!
/integrate

[openjdk]

Going to push as commit 80ccc98.
Since your change was applied there have been 63 commits pushed to the master branch:

• 5249cc0: 8328087: Automate javax/swing/JTable/TAB/TAB.java applet test
• c05f8c7: 8316388: Opensource five Swing component related regression tests
• b8dfeaf: 8328158: Convert java/awt/Choice/NonFocusablePopupMenuTest to automatic main test
• d57bdd8: 8328165: improve assert(idx < _maxlrg) failed: oob
• 128e60a: 8328037: Test java/util/Formatter/Padding.java has unnecessary high heap requirement after JDK-8326718
• 06f4b7f: 8327756: Convert javax/swing/JSlider/4987336/bug4987336.java applet to main
• 605800e: 8203867: Delete test java/awt/TrayIcon/DblClickActionEventTest/DblClickActionEventTest.html
• 0943937: 8328218: Delete test java/awt/Window/FindOwner/FindOwner.html
• f6390e5: 8328089: Automate javax/swing/JTable/4222153/bug4222153.java applet test
• e8d1ba3: 8328228: Missing comma in copyright year for a few JColorChooser tests
• ... and 53 more: https://git.openjdk.org/jdk/compare/a4a5196351a3c8ce45e2e36d27842194cbfcc5ff...master

Your commit was automatically rebased without conflicts.

[openjdk]

@rkennke Pushed as commit 80ccc98.

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