8342283: CDS cannot handle a large number of classes #24877

shipilev · 2025-04-25T14:49:37Z

CDS cannot handle large number of classes, because ClassLoader data structures get too large for CDS archival. The new test captures such an occasion. We do clear()-s, but that is not enough to trim the backing storages for the affected collections.

In contrast with previous attempts to fix this (#21797 and my VM-side field overwrite), we can just reinitialize the fields using Unsafe, which bypasses normal final field write restriction. ClassLoader already does a similar thing nearby! This allows us to stay fully in Java, and makes the patch fairly straightforward.

Additional testing:

Linux x86_64 server fastdebug, new test reliably fails without the fix, passes with it
Linux x86_64 server fastdebug, runtime/cds

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-8342283: CDS cannot handle a large number of classes (Bug - P4)

Reviewers

Ioi Lam (@iklam - Reviewer)
Leonid Mesnik (@lmesnik - Reviewer) Review applies to 97ab42b6
Calvin Cheung (@calvinccheung - Reviewer) Review applies to 97ab42b6

Contributors

Ioi Lam <[email protected]>

Reviewing

Using git

Checkout this PR locally:
$ git fetch https://git.openjdk.org/jdk.git pull/24877/head:pull/24877
$ git checkout pull/24877

Update a local copy of the PR:
$ git checkout pull/24877
$ git pull https://git.openjdk.org/jdk.git pull/24877/head

Using Skara CLI tools

Checkout this PR locally:
$ git pr checkout 24877

View PR using the GUI difftool:
$ git pr show -t 24877

Using diff file

Download this PR as a diff file:
https://git.openjdk.org/jdk/pull/24877.diff

Using Webrev

Link to Webrev Comment

bridgekeeper · 2025-04-25T14:50:54Z

👋 Welcome back shade! 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 · 2025-04-25T14:52:00Z

@shipilev 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:

8342283: CDS cannot handle a large number of classes

Co-authored-by: Ioi Lam <[email protected]>
Reviewed-by: iklam, lmesnik, ccheung

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 27 new commits pushed to the master branch:

b5156bb: 8355065: ConcurrentModificationException in RichDiagnosticFormatter
2ed7ad4: 8355667: RISC-V: Add backend implementation for unsigned vector Min / Max operations
9f42ff8: 8355594: Warnings occur when building with clang and enabling ubsan
... and 24 more: https://git.openjdk.org/jdk/compare/2447b9812a9f7316a2313f70db4974534fceb9d9...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 · 2025-04-25T14:52:49Z

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

core-libs
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.

shipilev · 2025-04-25T14:55:15Z

@iklam -- I think neither of our previous solutions were as cute as this one :) No messing with final-s and @Stable-s, no messing with VM code!

mlbridge · 2025-04-25T15:01:15Z

Webrevs

01: Full - Incremental (d45ecd2f)
00: Full (97ab42b6)

iklam

LGTM. This does look a cleaner fix.

lmesnik

Test changes looks good for me.

calvinccheung

LGTM.

rose00 · 2025-04-25T20:54:21Z

src/java.base/share/classes/java/lang/ClassLoader.java

+     *
+     * @implNote This is done while the JVM is running in single-threaded mode,
+     * and at the very end of Java bytecode execution. We know that no more classes
+     * will be loaded and none of the fields modified by this method will be used again.


There is a tiny technical debt here. If future AOT code generation uses a future aggressive constant folding of object fields (cf. TrustFinalNonStaticFields) and a constant CL reference ends up in optimized code and there is a constant-folded reference (and.. and…) it is remotely possible that the old value of the field will get wrongly embedded in AOT code.

If we arrange AOT code generation to occur after all of these fixups (in Java code) are done, then the problem will not occur. It's a delicate set of invariants. Your expanded comment is a good start at calling them out, but this is a long string we are pulling on.

(I borrowed this comment from the initial @iklam's PR, so I need to credit him as contributor.)

Yes, CDS does awkward state manipulations at dump time. Resetting the states of final/@Stable objects can run into issues that you described. I think this is one of the reasons why Leyden generates AOT code with -XX:-FoldStableValues:

https://github.com/openjdk/leyden/blob/885096a8b3194371cde6b96ce5554d89f99618d7/src/hotspot/share/code/aotCodeCache.cpp#L162

I would guess we need to do the same with TrustFinalNonStaticFields. The awkward part of current trust_final_non_static_fields() code is that it implicitly trusts things in java/lang, even with -TrustFinalNonStaticFields. That sounds like something we need to rectify for Leyden AOT code.

test/hotspot/jtreg/runtime/cds/appcds/LotsOfSyntheticClasses.java

shipilev · 2025-04-28T09:15:17Z

/contributor add @iklam

openjdk · 2025-04-28T09:17:05Z

@shipilev
Contributor Ioi Lam <[email protected]> successfully added.

shipilev · 2025-04-29T14:43:03Z

Are we still good with this? Integration need a re-review :)

iklam

LGTM

shipilev · 2025-04-29T15:04:43Z

Thank you! I am running final tier{1,2,3} testing before integration, and will integrate when those turn green.

shipilev · 2025-04-29T18:06:48Z

Testing is green. I am integrating.

/integrate

openjdk · 2025-04-29T18:08:28Z

Going to push as commit ead6757.
Since your change was applied there have been 30 commits pushed to the master branch:

bd7561c: 8355441: Remove antipattern from PassFailJFrame.forcePass javadoc
5d2d1ab: 8354996: Reduce dynamic code generation for a single downcall
fa2a9d1: 8354919: Move HotSpot .editorconfig into the global .editorconfig
... and 27 more: https://git.openjdk.org/jdk/compare/2447b9812a9f7316a2313f70db4974534fceb9d9...master

Your commit was automatically rebased without conflicts.

openjdk · 2025-04-29T18:08:36Z

@shipilev Pushed as commit ead6757.

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

shipilev added 2 commits April 25, 2025 16:43

Fix

c248b77

Adjust TEST.groups

97ab42b

openjdk bot added hotspot [email protected] core-libs [email protected] labels Apr 25, 2025

shipilev marked this pull request as ready for review April 25, 2025 14:57

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

iklam approved these changes Apr 25, 2025

View reviewed changes

openjdk bot added the ready Pull request is ready to be integrated label Apr 25, 2025

lmesnik approved these changes Apr 25, 2025

View reviewed changes

calvinccheung approved these changes Apr 25, 2025

View reviewed changes

rose00 reviewed Apr 25, 2025

View reviewed changes

turbanoff reviewed Apr 27, 2025

View reviewed changes

test/hotspot/jtreg/runtime/cds/appcds/LotsOfSyntheticClasses.java Outdated Show resolved Hide resolved

shipilev added 2 commits April 28, 2025 11:00

Merge branch 'master' into JDK-8342283-cds-many-classes

a3d45bf

Minor touchup

d45ecd2

openjdk bot removed the ready Pull request is ready to be integrated label Apr 28, 2025

iklam approved these changes Apr 29, 2025

View reviewed changes

openjdk bot added the ready Pull request is ready to be integrated label Apr 29, 2025

openjdk bot added the integrated Pull request has been integrated label Apr 29, 2025

openjdk bot closed this Apr 29, 2025

openjdk bot removed ready Pull request is ready to be integrated rfr Pull request is ready for review labels Apr 29, 2025

8342283: CDS cannot handle a large number of classes #24877

8342283: CDS cannot handle a large number of classes #24877

Uh oh!

Conversation

shipilev commented Apr 25, 2025 • edited by openjdk bot Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Progress

Issue

Reviewers

Contributors

Reviewing

Uh oh!

bridgekeeper bot commented Apr 25, 2025

Uh oh!

openjdk bot commented Apr 25, 2025 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

openjdk bot commented Apr 25, 2025

Uh oh!

shipilev commented Apr 25, 2025

Uh oh!

mlbridge bot commented Apr 25, 2025 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Webrevs

Uh oh!

iklam left a comment

Choose a reason for hiding this comment

Uh oh!

lmesnik left a comment

Choose a reason for hiding this comment

Uh oh!

calvinccheung left a comment

Choose a reason for hiding this comment

Uh oh!

rose00 Apr 25, 2025

Choose a reason for hiding this comment

Uh oh!

shipilev Apr 28, 2025

Choose a reason for hiding this comment

Uh oh!

Uh oh!

shipilev commented Apr 28, 2025

Uh oh!

openjdk bot commented Apr 28, 2025

Uh oh!

shipilev commented Apr 29, 2025

Uh oh!

iklam left a comment

Choose a reason for hiding this comment

Uh oh!

shipilev commented Apr 29, 2025

Uh oh!

shipilev commented Apr 29, 2025

Uh oh!

openjdk bot commented Apr 29, 2025

Uh oh!

openjdk bot commented Apr 29, 2025

Uh oh!

Reviewers

Assignees

Labels

Milestone

Development

Uh oh!

6 participants

shipilev commented Apr 25, 2025 •

edited by openjdk bot

Loading

openjdk bot commented Apr 25, 2025 •

edited

Loading

mlbridge bot commented Apr 25, 2025 •

edited

Loading