RFR: 8311178: JMH tests don't scale well when sharing output buffers

[Hamlin Li]

On Sat, 1 Jul 2023 07:53:17 GMT, Swati Sharma <duke at openjdk.org> wrote:

> The below benchmark files have scaling issues due to cache contention and leads to poor scaling when run on multiple threads. The patch sets the scope from benchmark level to thread level to fix the issue:
> - org/openjdk/bench/java/io/DataOutputStreamTest.java
> - org/openjdk/bench/java/lang/ArrayCopyObject.java
> - org/openjdk/bench/java/lang/ArrayFiddle.java
> - org/openjdk/bench/java/time/format/DateTimeFormatterBench.java
> - org/openjdk/bench/jdk/incubator/vector/IndexInRangeBenchmark.java
> - org/openjdk/bench/jdk/incubator/vector/MemorySegmentVectorAccess.java
> - org/openjdk/bench/jdk/incubator/vector/StoreMaskedBenchmark.java
> - org/openjdk/bench/jdk/incubator/vector/StoreMaskedIOOBEBenchmark.java
> - org/openjdk/bench/vm/compiler/ArrayFill.java
> - org/openjdk/bench/vm/compiler/IndexVector.java
> 
> Also removing the static scope for variables in org/openjdk/bench/jdk/incubator/vector/VectorFPtoIntCastOperations.java for better scaling.
> 
> Please review and share your feedback.
> 
> Thanks,
> Swati

Hi,
I'm not sure if I understand this improvement correctly.
I'm not quite familiar with JMH and it's annotations, but seems to me, the change from `@State(Scope.Benchmark)` to `@State(Scope.Thread)` should not improve the performance by reducing cache contention, as in the jmh doc it says "State objects are usually injected into Benchmark methods as ***arguments***, and JMH takes care of their instantiation and sharing.", this seems mean that @State only matters when the annotated class is used as a parameter of a @Benchmark method, but in the tests you modifed, seems there is no such use case.
Please also check the sample usages at https://github.com/openjdk/jmh/blob/master/jmh-samples/src/main/java/org/openjdk/jmh/samples/JMHSample_03_States.java.

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PR Comment: https://git.openjdk.org/jdk/pull/14746#issuecomment-1625030990