thread local allocation in benchmark

[Dan Burkert]

I'm having trouble benchmarking some code that calls String.getBytes().  Behind the scenes this method caches some string encoder classes in a ThreadLocal.  When I try to benchmark, say something as simple as:

    @Benchmark

    public long bench(int reps) throws Exception {

        long dummy = 0;

        for (int i = 0; i < reps; i++) {

            dummy |= "foobar".getBytes().hashCode();

        }

        return dummy;

    }

the benchmark run fails with

java.lang.IllegalStateException: Your benchmark appears to have non-deterministic allocation behavior.

But even if I call String.getBytes() in a static field initializer in the benchmarking class (in order to pre-load the class caches), the benchmark still fails with the same exception.  I believe it is because the benchmarks are being run on a different thread.  Anyone know any way around this problem?  Thanks.

[Gregory Kick]

I think you have a few options:

1) I think that the simple way to address the problem would be to explicitly pass in the Charset.  (It's a best practice anyway: https://plus.google.com/u/0/118010414872916542489/posts/FGpFSai9dVo)

2) Call your benchmark method explicitly from a @BeforeExperiment method. Since they are guaranteed to run first, that should initialize the data.

HTH

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Greg Kick
Java Core Libraries Team