Why would a Java prime sieve run at only half its speed _some_ of the times?

• ghost-chase-condition

  5 2 4.7 Java

  Chasing a performance-eating ghost down the JVM rabbit hole

Time: 14010, Passes: 10000, count: 78498, Valid: true Time: 14208, Passes: 10000, count: 78498, Valid: true Time: 14111, Passes: 10000, count: 78498, Valid: true Time: 13869, Passes: 10000, count: 78498, Valid: true Time: 14119, Passes: 10000, count: 78498, Valid: true Time: 13916, Passes: 10000, count: 78498, Valid: true Time: 13903, Passes: 10000, count: 78498, Valid: true Time: 13890, Passes: 10000, count: 78498, Valid: true Time: 14024, Passes: 10000, count: 78498, Valid: true Time: 13924, Passes: 10000, count: 78498, Valid: true Time: 14180, Passes: 10000, count: 78498, Valid: true Time: 13980, Passes: 10000, count: 78498, Valid: true Time: 13825, Passes: 10000, count: 78498, Valid: true Time: 13800, Passes: 10000, count: 78498, Valid: true Time: 14041, Passes: 10000, count: 78498, Valid: true Time: 13941, Passes: 10000, count: 78498, Valid: true

Running on: JDK 11.0.8 on Windows, with somewhat old Intel Core i7-3740QM

[1] https://github.com/PEZ/ghost-chase-condition/tree/master/tes...

• async-profiler

  8 5,883 8.4 C++

  Discontinued Sampling CPU and HEAP profiler for Java featuring AsyncGetCallTrace + perf_events [Moved to: https://github.com/async-profiler/async-profiler] (by jvm-profiling-tools)

  

Also, running it under a profiler (I recommend async-profiler[1]) should give you a good idea of where the slowdown occurs which might help you pin it down further.

[1] https://github.com/jvm-profiling-tools/async-profiler

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• for-linux

  34 744 0.0

  Docker Engine for Linux

  

• array

  6 45 0.8 Kotlin

  Simple array language written in kotlin (by lokedhs)

This issue isn't directly related to BitSet. I have observed the same thing in my programming language interpreter that runs on the JVM (well, it's written in Kotlin multiplatform so it runs on JS and Natively as well).

I start the interpreter and measue the time it takes to all all then numbers below 1000000000.

The first time I run it after starting the interpreter it always takes 1.4 seconds (within 0.1 second precision). The second time I measure the time it takes 1.7, and for every invocation following that it takes 2 seconds.

If I stop the interpreter and try again, I get exactly the same result.

I have not been able to explain this behaviour. This is on OpenJDK 11 by the way.

If anyone wants to test this, just run the interpreter from here: https://github.com/lokedhs/array

To run the benchmark, type the following command in the UI:

    time:measureTime { +/⍳1000000000 }

• .NET Runtime

  607 14,091 10.0 C#

  .NET is a cross-platform runtime for cloud, mobile, desktop, and IoT apps.

  

To maximize I$ hit rate you'd actually want to disable loop alignment to not inflate code size with padding.

Here's an example of unstable benchmark performance caused by the lack of loop alignment in .NET's JIT: https://github.com/dotnet/runtime/issues/43227

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