Using the -Xcomp flag to disable interpreted method invocations

A little experiment with the -Xcomp flag on Java VM.  By default, method invocation is executed in interpreted mode unless the number of invocation has reached certain threshold. The threshold can be set by runtime options (e.g. -client, -server, and -XX:CompileThreshold).  The -Xcomp flag forces compilation of the code on first invocation.

The following is a comparison of running the sample (basically an empty method) of jmh benchmark.  Hardware platform is armv7l with 1GB of RAM.  Software is Arch Linux with kernel 4.1.5-1-ARCH, running Oracle Java 1.8.0_51 with HotSpot Server VM.

Using default jmh settings, i.e. 10 forks of 20 iterations, each with 20 warmup iterations.

When run without -Xcomp:

Result "testMethod":
  46954362.329 ?(99.9%) 121531.693 ops/s [Average]
  (min, avg, max) = (44999194.538, 46954362.329, 47305769.412), stdev = 514572.799
  CI (99.9%): [46832830.635, 47075894.022] (assumes normal distribution)


# Run complete. Total time: 00:06:55

Benchmark                Mode  Cnt         Score        Error  Units
MyBenchmark.testMethod  thrpt  200  46954362.329 ? 121531.693  ops/s

When run with -Xcomp:

Result "testMethod":
  47114913.482 ?(99.9%) 89902.070 ops/s [Average]
  (min, avg, max) = (46113318.172, 47114913.482, 47304787.674), stdev = 380650.996
  CI (99.9%): [47025011.412, 47204815.553] (assumes normal distribution)


# Run complete. Total time: 00:07:41

Benchmark                Mode  Cnt         Score       Error  Units
MyBenchmark.testMethod  thrpt  200  47114913.482 ? 89902.070  ops/s

Note that:

1. When -Xcomp is specified, the overall runtime is 11% longer.  That is because the VM needs to wait the code to be compiled.  i.e. the efficiency is lower

2. The error (i.e. variation) is 26% lower when -Xcomp is specified.  That is probably because all code has been compiled and so no need to spend time on compilation during execution.

3. The average score (i.e. throughput) is almost the same.  That is because the jmh does warmup iteration before each round of fork.