8372946 - TreeMap sub-map entry spliterator is expensive by olivergillespie · Pull Request #28608 · openjdk/jdk

olivergillespie · 2025-12-02T15:31:43Z

TreeMap sub-maps use the default IteratorSpliterator implementation for TreeMap$EntrySetView which is slow for some operations, because EntrySetView.size() iterates all elements. This is most trivially shown by something like largeTreeMap.tailMap(0L, false).entrySet().limit(1).count() taking a long time. This showed up in my application, where it was trivial to mitigate by switching to a for loop, but I think the fix is easy enough.

keySet() does not have the same problem, as it provides a custom Spliterator implementation which is not Spliterator.SIZED, and returns Long.MAX_VALUE for estimateSize() (which is the recommended approach when the size is expensive to compute). I'm assuming this optimization was simply missed for the EntryIterator in the original implementation, but I don't know for sure.

This patch fixes the issue by providing a custom spliterator for EntrySetView, which is not SIZED. The implementation is copied almost exactly from the equivalent KeyIterator classes in this file (SubMapKeyIterator, DescendingSubMapKeyIterator). The only difference is in SubMapEntryIterator.getComparator, for which I copied the implementation from TreeMap$EntrySpliterator.

Basic performance test: map.tailMap(0L, false).entrySet().stream().limit(1).count() for a TreeMap with 10_000_000 entries.

Before (keySet is fast using SubMapKeyIterator, entrySet is slow using IteratorSpliterator):

class java.util.TreeMap$KeySet
    .stream().limit(1).count() took 0.046ms
    spliterator = SubMapKeyIterator, estimateSize() = 9223372036854775807
class java.util.TreeMap$AscendingSubMap$AscendingEntrySetView
    .stream().limit(1).count() took 218ms
    spliterator = IteratorSpliterator, estimateSize() = 9999999

After (entrySet is now fast, using SubMapEntryIterator):

class java.util.TreeMap$KeySet
	.stream().limit(1).count() took 0.017ms
	spliterator = SubMapKeyIterator, estimateSize() = 9223372036854775807
class java.util.TreeMap$AscendingSubMap$AscendingEntrySetView
	.stream().limit(1).count() took 0.013ms
	spliterator = SubMapEntryIterator, estimateSize() = 9223372036854775807

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-8372946: TreeMap sub-map entry spliterator is expensive (Enhancement - P4)

Reviewing

Using git

Checkout this PR locally:
$ git fetch https://git.openjdk.org/jdk.git pull/28608/head:pull/28608
$ git checkout pull/28608

Update a local copy of the PR:
$ git checkout pull/28608
$ git pull https://git.openjdk.org/jdk.git pull/28608/head

Using Skara CLI tools

Checkout this PR locally:
$ git pr checkout 28608

View PR using the GUI difftool:
$ git pr show -t 28608

Using diff file

Download this PR as a diff file:
https://git.openjdk.org/jdk/pull/28608.diff

Using Webrev

Link to Webrev Comment

bridgekeeper · 2025-12-02T15:33:00Z

👋 Welcome back ogillespie! 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 · 2025-12-02T15:34:35Z

❗ This change is not yet ready to be integrated.
See the Progress checklist in the description for automated requirements.

openjdk · 2025-12-02T15:35:27Z

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

core-libs

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

mlbridge · 2025-12-02T15:39:26Z

Webrevs

01: Full - Incremental (9e669145)
00: Full (7c7299c1)

olivergillespie · 2025-12-02T16:30:44Z

Tier2 test failure. I will investigate.

 java.util.ConcurrentModificationException
	at java.base/java.util.TreeMap$NavigableSubMap$SubMapIterator.prevEntry(TreeMap.java:2068)
	at java.base/java.util.TreeMap$NavigableSubMap$DescendingSubMapEntryIterator.next(TreeMap.java:2156)
	at java.base/java.util.TreeMap$NavigableSubMap$DescendingSubMapEntryIterator.forEachRemaining(TreeMap.java:2166)
	at java.base/java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:570)
	at java.base/java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:560)
	at java.base/java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:635)
	at java.base/java.util.stream.AbstractPipeline.evaluateToArrayNode(AbstractPipeline.java:291)
	at java.base/java.util.stream.ReferencePipeline.toArray(ReferencePipeline.java:652)
	at java.base/java.util.stream.ReferencePipeline.toArray(ReferencePipeline.java:658)
	at org.openjdk.tests.java.util.stream.CollectionAndMapModifyStreamTest.testEntrySetSizeRemove(CollectionAndMapModifyStreamTest.java:164)
	at org.openjdk.tests.java.util.stream.CollectionAndMapModifyStreamTest.testMapEntriesSizeRemove(CollectionAndMapModifyStreamTest.java:155)

olivergillespie · 2025-12-02T17:06:22Z

Fixed the test failure by skipping that case. The test intentionally modifies the backing map while holding an iterator, which is not safe in general. It got away with it before, but the new implementation reasonably throws CME.
The test only runs on EntrySets, but the equivalent test would already fail on the current map.tailMap(...).keySet() implementation, so I think it's expected and reasonable that it now fails for descendingMap.entrySet(). Appreciate any second opinions, though.

bridgekeeper · 2025-12-31T01:07:08Z

@olivergillespie This pull request has been inactive for more than 4 weeks and will be automatically closed if another 4 weeks passes without any activity. To avoid this, simply issue a /touch or /keepalive command to the pull request. Feel free to ask for assistance if you need help with progressing this pull request towards integration!

olivergillespie · 2026-01-05T09:44:25Z

/keepalive

openjdk · 2026-01-21T18:53:09Z

@olivergillespie The pull request is being re-evaluated and the inactivity timeout has been reset.

liach · 2026-01-26T16:01:11Z

src/java.base/share/classes/java/util/TreeMap.java

                return false;
            }
+
+            public abstract Spliterator<Map.Entry<K,V>> spliterator();


I don't think you need this huge a patch. I think you should just do:

Suggested change

public abstract Spliterator<Map.Entry<K,V>> spliterator();

public Spliterator<Map.Entry<K,V>> spliterator() {

return Spliterators.spliterator(iterator(), Spliterator.DISTINCT);

}

Your patch is introducing spliterator behavioral changes unrelated to the performance regression fix.

Thanks for looking.

I suppose you mean Spliterators.spliteratorUnknownSize?

Hmm - I made the change this way to be consistent with the existing SubMapKeyIterator and DescendingSubMapKeyIterator, simply adding the same functionality for the Entry versions. Do you think those are overcomplicated too, or there's a reason they're like that that doesn't apply to the Entry versions? I don't know why they were originally added, to be honest, I didn't find much useful context in the history.

I don't know Spliterator well enough to spot any subtle behavioural differences, that's one reason I chose to follow the existing patterns.

DescendingSubMapEntryIterator is SORTED but SubMapEntryIterator is not, so I'd have to account for that too.

I suppose you mean Spliterators.spliteratorUnknownSize?

Yes. Thanks for corecting me.

Hmm - I made the change this way to be consistent with the existing SubMapKeyIterator and DescendingSubMapKeyIterator, simply adding the same functionality for the Entry versions.

I made the recommendation given the starting point is to address a performance regression, instead of to enhance the sub-map entry spliterator to be on par with the DescendingSubMapKeyIterator.

From this starting point, I believe we can easily identify EntrySetView inherits Set::spliterator which is slow because the spliterator calls size() frequently. This root problem is easily fixed with using Spliterators.spliteratorUnknownSize, which also has the minimal behavioral impact.

In contrast, functional enhancement to spliterators is really a can of worms where you can never find an end - sometimes you add more flags, sometimes other splitting strategies. And in your example, you already have a test case failing due to the functional enhancements while you did make new tests to verify them.

So let's keep it simple, fix the bug, and leave the functional enhancements for another time. This also makes backporting the fix much easier.

Okay sounds good to me! Thanks for the suggestion, I'll update later this week.

I created #29485 to add test cases before making this change - that required a slight functional tweak so I didn't want to include it in this change.

Attempt to fix 8372946

7c7299c

openjdk bot added the core-libs core-libs-dev@openjdk.org label Dec 2, 2025

openjdk bot added the rfr Pull request is ready for review label Dec 2, 2025

Fix failing test

9e66914

liach suggested changes Jan 26, 2026

View reviewed changes

-            public abstract Spliterator<Map.Entry<K,V>> spliterator();
+            public Spliterator<Map.Entry<K,V>> spliterator() {
+                return Spliterators.spliterator(iterator(), Spliterator.DISTINCT);
+            }

Conversation

olivergillespie commented Dec 2, 2025 • edited by openjdk bot Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Progress

Issue

Reviewing

Uh oh!

bridgekeeper bot commented Dec 2, 2025

Uh oh!

openjdk bot commented Dec 2, 2025

Uh oh!

openjdk bot commented Dec 2, 2025 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

mlbridge bot commented Dec 2, 2025 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Webrevs

Uh oh!

olivergillespie commented Dec 2, 2025

Uh oh!

olivergillespie commented Dec 2, 2025

Uh oh!

bridgekeeper bot commented Dec 31, 2025

Uh oh!

olivergillespie commented Jan 5, 2026

Uh oh!

openjdk bot commented Jan 21, 2026

Uh oh!

liach Jan 26, 2026

Choose a reason for hiding this comment

Uh oh!

olivergillespie Jan 27, 2026

Choose a reason for hiding this comment

Uh oh!

liach Jan 27, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Choose a reason for hiding this comment

Uh oh!

olivergillespie Jan 27, 2026

Choose a reason for hiding this comment

Uh oh!

olivergillespie Jan 29, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Reviewers

Assignees

Labels

Milestone

Development

Uh oh!

2 participants

olivergillespie commented Dec 2, 2025 •

edited by openjdk bot

Loading

openjdk bot commented Dec 2, 2025 •

edited

Loading

mlbridge bot commented Dec 2, 2025 •

edited

Loading

liach Jan 27, 2026 •

edited

Loading

olivergillespie Jan 29, 2026 •

edited

Loading