8343840: Rewrite the ObjectMonitor lists #23421

fbredber · 2025-02-03T16:29:25Z

I've combined two ObjectMonitor's lists, EntryList and cxq, into one list. The entry_list.

This way c2 no longer has to check both EntryList and cxq in order to opt out if the "conceptual entry list" is empty, which also means that the constant question about if it's safe to first check the EntryList and then cxq will be a thing of the past.

In the current multi-queue design new threads where always added to the cxq, then ObjectMonitor::exit would choose a successor from the head of EntryList. When the EntryList was empty and cxq was not, ObjectMonitor::exit whould detached the singly linked cxq list, and add the elements to the doubly linked EntryList. The element that was first added to cxq whould be at the tail of the EntryList. This way you ended up working through the contending threads in LIFO-chunks.

The new list-design is as much a multi-queue as the current. Conceptually it can be looked upon as if the old singly linked cxq list doesn't end with a null pointer, but instead has a link that points to the head of the doubly linked entry_list.

You always add to the entry_list by Compare And Exchange to the head. The most common case is that you remove from the tail (the successor is chosen in strict FIFO order). The head is volatile, but the interior is stable.

The first contending thread that "pushes" itself onto entry_list, will be the last thread in the list. Each newly pushed thread in entry_list will be linked trough its next pointer, and have its prev pointer set to null, thus pushing new threads onto entry_list will form a singly linked list. The list is always in the right order (via the next-pointers) and is never moved to another list.

Since we choose the successor in FIFO order, the exiting thread needs to find the tail of the entry_list. This is done by walking from the entry_list head. While walking the list we assign the prev pointers of each thread, essentially forming a doubly linked list. The tail pointer is cached in entry_list_tail so that we don't need to walk from the entry_list head each time we need to find the tail (successor).

Performance wise the new design seems to be equal to the old design, even though c2 generates two less instructions per monitor unlock operation.

However the complexity of the source has been reduced by removing the TS_CXQ state and adding functions instead of inlining cmpxchg here and there, and the fact that c2 no longer has to check both EntryList and cxq makes this PR worthwhile, I think.

Tests tier1-7 passes okay as well as micro-benchmarks like vm.lang.LockUnlock.
Unsupported platforms { ppc, riscv, s390 } has been tested with QEmu.

Progress

Change must be properly reviewed (1 review required, with at least 1 Reviewer)
Change must not contain extraneous whitespace
Commit message must refer to an issue

Issue

JDK-8343840: Rewrite the ObjectMonitor lists (Enhancement - P4)

Reviewers

David Holmes (@dholmes-ora - Reviewer)
Coleen Phillimore (@coleenp - Reviewer)
Patricio Chilano Mateo (@pchilano - Reviewer)
Yudi Zheng (@mur47x111 - Committer)

Reviewing

Using git

Checkout this PR locally:
$ git fetch https://git.openjdk.org/jdk.git pull/23421/head:pull/23421
$ git checkout pull/23421

Update a local copy of the PR:
$ git checkout pull/23421
$ git pull https://git.openjdk.org/jdk.git pull/23421/head

Using Skara CLI tools

Checkout this PR locally:
$ git pr checkout 23421

View PR using the GUI difftool:
$ git pr show -t 23421

Using diff file

Download this PR as a diff file:
https://git.openjdk.org/jdk/pull/23421.diff

Using Webrev

Link to Webrev Comment

bridgekeeper · 2025-02-03T16:30:44Z

👋 Welcome back fbredberg! 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 · 2025-02-03T16:31:52Z

@fbredber 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:

8343840: Rewrite the ObjectMonitor lists

Reviewed-by: dholmes, coleenp, pchilanomate, yzheng

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 169 new commits pushed to the master branch:

4bb3d81: 8351138: Running subset of gtests gets error printing result information
107ee87: 8346954: [JMH] jdk.incubator.vector.MaskedLogicOpts fails due to IndexOutOfBoundsException
11a37c8: 8351165: Remove unused includes from vmStructs
c3b4819: 8351074: Disallow null prefix and suffix in DecimalFormat
6012e8d: 8350808: Small typos in JShell method SnippetEvent.toString()
661bd5b: 8343478: Remove unnecessary @SuppressWarnings annotations (core-libs)
61d9ab9: 8350854: Include thread counts in safepoint logging
ea9e3cf: 8281511: java/net/ipv6tests/UdpTest.java fails with checkTime failed
caaf409: 8350546: Several java/net/InetAddress tests fails UnknownHostException
062b7c7: 8351115: Test AOTClassLinkingVMOptions.java fails after JDK-8348322
... and 159 more: https://git.openjdk.org/jdk/compare/1e87ff01994df16df7de331040fc5d7a4a85f630...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 · 2025-02-03T16:32:25Z

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

graal
hotspot

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

src/hotspot/share/jvmci/vmStructs_jvmci.cpp

src/hotspot/share/runtime/objectMonitor.cpp

mlbridge · 2025-02-25T13:19:09Z

Webrevs

dholmes-ora

Disclaimer for other reviewers, I have been looking at this code for some time now.

Overall code looks good. I have quite a few comments/suggestions about comments.

I suggest renaming _vthread_cxq_head to just _vthread_head as the cxq part is no longer meaningful.

I agree that even though this seems performance neutral, the code simplification (for people reading it for the first time) will be worth it.

Thanks.

src/hotspot/share/jvmci/vmStructs_jvmci.cpp

src/hotspot/share/runtime/objectMonitor.hpp

src/hotspot/share/runtime/objectMonitor.cpp

coleenp

This looks really good - I have some small change and improvement requests.

src/hotspot/cpu/x86/c2_MacroAssembler_x86.cpp

src/hotspot/share/runtime/objectMonitor.hpp

src/hotspot/share/runtime/synchronizer.cpp

src/hotspot/share/runtime/objectMonitor.cpp

src/hotspot/share/runtime/objectMonitor.hpp

src/hotspot/share/runtime/objectMonitor.cpp

coleenp

This change looks great. Thank you!

src/hotspot/share/runtime/objectMonitor.cpp

src/hotspot/share/runtime/objectMonitor.hpp

fbredber · 2025-02-27T20:38:32Z

I've used QEMU to smoke test this PR on ppc64le, riscv64 and s390x, But it would be nice if @TheRealMDoerr, @RealFYang and @offamitkumar could check if it runs okay on real hardware as well.

fbredber · 2025-02-27T20:50:56Z

@pchilano
Since I have removed the cxq list @dholmes-ora suggested that I should also rename _vthread_cxq_head. Thereby removing the term "cxq" altogether. I chose to rename _vthread_cxq_head with _vthread_list_head. Hope that is okay.

RealFYang · 2025-02-28T05:21:34Z

I've used QEMU to smoke test this PR on ppc64le, riscv64 and s390x, But it would be nice if @TheRealMDoerr, @RealFYang and @offamitkumar could check if it runs okay on real hardware as well.

FYI: hs:tier1 - hs:tier3 test good on linux-riscv64 platform.

dholmes-ora

Okay that's good enough for me. :)

Thanks

offamitkumar · 2025-02-28T07:00:40Z

I've used QEMU to smoke test this PR on ppc64le, riscv64 and s390x, But it would be nice if @TheRealMDoerr, @RealFYang and @offamitkumar could check if it runs okay on real hardware as well.

Tier1 test passed on s390x.

TheRealMDoerr · 2025-02-28T10:47:39Z

I've used QEMU to smoke test this PR on ppc64le, riscv64 and s390x, But it would be nice if @TheRealMDoerr, @RealFYang and @offamitkumar could check if it runs okay on real hardware as well.

The PPC64 code looks correct and some quick tests have passed. I'll run larger test suites over the weekend.

TheRealMDoerr · 2025-03-03T16:28:55Z

I've used QEMU to smoke test this PR on ppc64le, riscv64 and s390x, But it would be nice if @TheRealMDoerr, @RealFYang and @offamitkumar could check if it runs okay on real hardware as well.

The PPC64 code looks correct and some quick tests have passed. I'll run larger test suites over the weekend.

Test results look good (including tier 1-4 on many platforms). I didn't see any new issue related to this.

pchilano

Changes look good to me. Just a few comments.

Thanks,
Patricio

src/hotspot/share/runtime/objectMonitor.cpp

fbredber · 2025-03-05T12:48:24Z

@mur47x111
I'm getting ready to integrate. I've seen that you have created [JDK-8349711] Adapt JDK-8343840: Rewrite the ObjectMonitor lists to handle the change on your side. Do you see any reason why I shouldn't integrate, or are you fine with me integrating this PR now?

mur47x111

JVMCI changes look go to me! We are good to go!

pchilano

Thanks, looks good.

src/hotspot/share/runtime/objectMonitor.cpp

dholmes-ora

LGTM!

Thanks

fbredber · 2025-03-06T09:08:07Z

Thanks everyone for the reviews, testing and Graal adaptation.

/integrate

openjdk · 2025-03-06T09:08:35Z

Going to push as commit 7a5acb9.
Since your change was applied there have been 174 commits pushed to the master branch:

40f150d: 8330936: [ubsan] exclude function BilinearInterp and ShapeSINextSpan in libawt java2d from ubsan checks
649ef77: 8323158: HotSpot Style Guide should specify more include ordering
5c552a9: 8349358: [JMH] Cannot access class jdk.internal.vm.ContinuationScope
e82031e: 8350756: C2 SuperWord Multiversioning: remove useless slow loop when the fast loop disappears
3626ac3: 8204868: java/util/zip/ZipFile/TestCleaner.java still fails with "cleaner failed to clean zipfile."
4bb3d81: 8351138: Running subset of gtests gets error printing result information
107ee87: 8346954: [JMH] jdk.incubator.vector.MaskedLogicOpts fails due to IndexOutOfBoundsException
11a37c8: 8351165: Remove unused includes from vmStructs
c3b4819: 8351074: Disallow null prefix and suffix in DecimalFormat
6012e8d: 8350808: Small typos in JShell method SnippetEvent.toString()
... and 164 more: https://git.openjdk.org/jdk/compare/1e87ff01994df16df7de331040fc5d7a4a85f630...master

Your commit was automatically rebased without conflicts.

openjdk · 2025-03-06T09:08:42Z

@fbredber Pushed as commit 7a5acb9.

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

8343840: Rewrite the ObjectMonitor lists

e9f447b

openjdk bot added graal [email protected] hotspot [email protected] labels Feb 3, 2025

coleenp mentioned this pull request Feb 4, 2025

Rename TryWithContentionMark to try_with_contention_mark #23450

Draft

3 tasks

coleenp reviewed Feb 7, 2025

View reviewed changes

src/hotspot/share/jvmci/vmStructs_jvmci.cpp Outdated Show resolved Hide resolved

fbredber added 2 commits February 11, 2025 14:47

Updated theory of operations comment

755e4a4

General cleanup

d9b9713

dholmes-ora reviewed Feb 17, 2025

View reviewed changes

Fixed a bug in UnlinkAfterAcquire

f8e8e10

dholmes-ora reviewed Feb 19, 2025

View reviewed changes

src/hotspot/share/runtime/objectMonitor.cpp Outdated Show resolved Hide resolved

dholmes-ora reviewed Feb 19, 2025

View reviewed changes

src/hotspot/share/runtime/objectMonitor.cpp Outdated Show resolved Hide resolved

dholmes-ora reviewed Feb 19, 2025

View reviewed changes

src/hotspot/share/runtime/objectMonitor.cpp Show resolved Hide resolved

dholmes-ora reviewed Feb 19, 2025

View reviewed changes

src/hotspot/share/runtime/objectMonitor.cpp Outdated Show resolved Hide resolved

dholmes-ora reviewed Feb 19, 2025

View reviewed changes

src/hotspot/share/runtime/objectMonitor.cpp Outdated Show resolved Hide resolved

dholmes-ora reviewed Feb 19, 2025

View reviewed changes

src/hotspot/share/runtime/objectMonitor.cpp Outdated Show resolved Hide resolved

fbredber added 3 commits February 20, 2025 09:58

Atomic hygiene

b4fb7fd

Merge branch 'master' into 8343840_rewrite_objectmonitor_lists

b4a0850

Moved set_bad_pointers() and added accessors.

e1d4fac

fbredber marked this pull request as ready for review February 25, 2025 13:14

openjdk bot added the rfr Pull request is ready for review label Feb 25, 2025

graalvmbot mentioned this pull request Feb 25, 2025

[JDK-8349711] Adapt JDK-8343840: Rewrite the ObjectMonitor lists oracle/graal#10757

Merged

dholmes-ora suggested changes Feb 26, 2025

View reviewed changes

coleenp suggested changes Feb 27, 2025

View reviewed changes

Update after review by David and Coleen.

283c243

coleenp approved these changes Feb 27, 2025

View reviewed changes

src/hotspot/share/runtime/objectMonitor.cpp Show resolved Hide resolved

src/hotspot/share/runtime/objectMonitor.cpp Show resolved Hide resolved

src/hotspot/share/runtime/objectMonitor.hpp Show resolved Hide resolved

openjdk bot added the ready Pull request is ready to be integrated label Feb 27, 2025

dholmes-ora approved these changes Feb 28, 2025

View reviewed changes

coleenp mentioned this pull request Mar 3, 2025

8350955: Fix repetitions of the word "the" in runtime component comments #23857

Closed

3 tasks

coleenp mentioned this pull request Mar 3, 2025

8348829: Remove ObjectMonitor perf counters #23326

Open

4 tasks

pchilano reviewed Mar 3, 2025

View reviewed changes

Updated comments after review by Patricio.

0d2d6c3

openjdk bot removed the ready Pull request is ready to be integrated label Mar 5, 2025

coleenp approved these changes Mar 5, 2025

View reviewed changes

openjdk bot added the ready Pull request is ready to be integrated label Mar 5, 2025

mur47x111 approved these changes Mar 5, 2025

View reviewed changes

pchilano approved these changes Mar 5, 2025

View reviewed changes

src/hotspot/share/runtime/objectMonitor.cpp Show resolved Hide resolved

dholmes-ora approved these changes Mar 6, 2025

View reviewed changes

openjdk bot added the integrated Pull request has been integrated label Mar 6, 2025

openjdk bot closed this Mar 6, 2025

openjdk bot removed ready Pull request is ready to be integrated rfr Pull request is ready for review labels Mar 6, 2025

Provide feedback

Saved searches

Use saved searches to filter your results more quickly

8343840: Rewrite the ObjectMonitor lists #23421

8343840: Rewrite the ObjectMonitor lists #23421

fbredber commented Feb 3, 2025 •

edited by openjdk bot

Loading

bridgekeeper bot commented Feb 3, 2025

openjdk bot commented Feb 3, 2025 •

edited

Loading

openjdk bot commented Feb 3, 2025

mlbridge bot commented Feb 25, 2025 •

edited

Loading

dholmes-ora left a comment

coleenp left a comment

coleenp left a comment

fbredber commented Feb 27, 2025

fbredber commented Feb 27, 2025

RealFYang commented Feb 28, 2025

dholmes-ora left a comment

offamitkumar commented Feb 28, 2025

TheRealMDoerr commented Feb 28, 2025

TheRealMDoerr commented Mar 3, 2025

pchilano left a comment

fbredber commented Mar 5, 2025

mur47x111 left a comment

pchilano left a comment

dholmes-ora left a comment

fbredber commented Mar 6, 2025

openjdk bot commented Mar 6, 2025

openjdk bot commented Mar 6, 2025

8343840: Rewrite the ObjectMonitor lists #23421

8343840: Rewrite the ObjectMonitor lists #23421

Conversation

fbredber commented Feb 3, 2025 • edited by openjdk bot Loading

Progress

Issue

Reviewers

Reviewing

bridgekeeper bot commented Feb 3, 2025

openjdk bot commented Feb 3, 2025 • edited Loading

openjdk bot commented Feb 3, 2025

mlbridge bot commented Feb 25, 2025 • edited Loading

Webrevs

dholmes-ora left a comment

Choose a reason for hiding this comment

coleenp left a comment

Choose a reason for hiding this comment

coleenp left a comment

Choose a reason for hiding this comment

fbredber commented Feb 27, 2025

fbredber commented Feb 27, 2025

RealFYang commented Feb 28, 2025

dholmes-ora left a comment

Choose a reason for hiding this comment

offamitkumar commented Feb 28, 2025

TheRealMDoerr commented Feb 28, 2025

TheRealMDoerr commented Mar 3, 2025

pchilano left a comment

Choose a reason for hiding this comment

fbredber commented Mar 5, 2025

mur47x111 left a comment

Choose a reason for hiding this comment

pchilano left a comment

Choose a reason for hiding this comment

dholmes-ora left a comment

Choose a reason for hiding this comment

fbredber commented Mar 6, 2025

openjdk bot commented Mar 6, 2025

openjdk bot commented Mar 6, 2025

fbredber commented Feb 3, 2025 •

edited by openjdk bot

Loading

openjdk bot commented Feb 3, 2025 •

edited

Loading

mlbridge bot commented Feb 25, 2025 •

edited

Loading