RFR: 8332842: Optimize empty CopyOnWriteArrayList allocations

[Doug Lea]

On Tue, 4 Jun 2024 13:03:45 GMT, Viktor Klang <vklang at openjdk.org> wrote:

>> Improve `java/util/concurrent/CopyOnWriteArrayList` by eliminating needless cloning of Object[0] instances. This cloning is intended to prevent callers from changing array contents, but many `CopyOnWriteArrayList`s are allocated to size zero, or are otherwise maintained empty, so cloning is unnecessary.
>> 
>> Results from the included JMH benchmark:
>> Before:
>> 
>> Benchmark                                                    Mode  Cnt   Score   Error  Units
>> CopyOnWriteArrayListBenchmark.clear                          avgt    5  74.487 ± 1.793  ns/op
>> CopyOnWriteArrayListBenchmark.clearEmpty                     avgt    5  27.918 ± 0.759  ns/op
>> CopyOnWriteArrayListBenchmark.createInstanceArray            avgt    5  16.656 ± 0.375  ns/op
>> CopyOnWriteArrayListBenchmark.createInstanceArrayEmpty       avgt    5  15.415 ± 0.489  ns/op
>> CopyOnWriteArrayListBenchmark.createInstanceCollection       avgt    5  21.608 ± 0.363  ns/op
>> CopyOnWriteArrayListBenchmark.createInstanceCollectionEmpty  avgt    5  15.374 ± 0.260  ns/op
>> CopyOnWriteArrayListBenchmark.createInstanceDefault          avgt    5  15.688 ± 0.350  ns/op
>> 
>> 
>> After:
>> 
>> Benchmark                                                    Mode  Cnt   Score   Error  Units
>> CopyOnWriteArrayListBenchmark.clear                          avgt    5  75.365 ± 2.092  ns/op
>> CopyOnWriteArrayListBenchmark.clearEmpty                     avgt    5  20.803 ± 0.539  ns/op
>> CopyOnWriteArrayListBenchmark.createInstanceArray            avgt    5  16.808 ± 0.582  ns/op
>> CopyOnWriteArrayListBenchmark.createInstanceArrayEmpty       avgt    5  12.980 ± 0.418  ns/op
>> CopyOnWriteArrayListBenchmark.createInstanceCollection       avgt    5  21.627 ± 0.173  ns/op
>> CopyOnWriteArrayListBenchmark.createInstanceCollectionEmpty  avgt    5  12.864 ± 0.408  ns/op
>> CopyOnWriteArrayListBenchmark.createInstanceDefault          avgt    5  12.931 ± 0.255  ns/op
>
> src/java.base/share/classes/java/util/concurrent/CopyOnWriteArrayList.java line 103:
> 
>> 101:     private static final long serialVersionUID = 8673264195747942595L;
>> 102: 
>> 103:     private static final Object[] EMPTY_ELEMENTDATA = {};
> 
> Looks like readObject() ought to also be changed to use the EMPTY_ELEMENTDATA if the length of the read array is 0? And perhaps `clone()`.

Agreed. You might as well be consistent with this so that no one will need to re-address if some application does zillions of clones of empty lists.

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PR Review Comment: https://git.openjdk.org/jdk/pull/19527#discussion_r1626520838