On Mon, 3 Jun 2024 16:47:20 GMT, jengebr <d...@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
The jmh benchmark checks only the empty case, you need to also show lack of impact on non-empty cases. Assuming you demonstrate this, it seems basically OK, (Deja vu previous cases including hash maps). It is only a small band-aid -- programs generating lots of them still have to allocate the COWAL object, so the savings are small compared to not generating them at all unless needed. ------------- PR Comment: https://git.openjdk.org/jdk/pull/19527#issuecomment-2145896233
