RFR: 8372946 - TreeMap sub-map entry spliterator is expensive [v2]

[Chen Liang]

On Tue, 2 Dec 2025 17:00:54 GMT, Oli Gillespie <ogillespie at openjdk.org> wrote:

>> `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
>
> Oli Gillespie has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Fix failing test

Changes requested by liach (Reviewer).

src/java.base/share/classes/java/util/TreeMap.java line 2028:

> 2026:             }
> 2027: 
> 2028:             public abstract Spliterator<Map.Entry<K,V>> spliterator();

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

            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.

-------------

PR Review: https://git.openjdk.org/jdk/pull/28608#pullrequestreview-3706750477
PR Review Comment: https://git.openjdk.org/jdk/pull/28608#discussion_r2728206435