title: What does @Group and @GroupThreads refer to in jmh?
body: <p>I have been looking at the way jmh implements the "multithreaded" benchmarks. From my understanding, the annotations <code>@Group("Identifier")</code> and <code>@GroupThreads(thread_number)</code> enable running in parallel the benchmarks that are within the same Group. </p>
  
  <pre><code>@Benchmark
  @Group("parallel")
  @GroupThreads(1)
  public void benchmark_1() {
      while(true) {}
  }
  
  @Benchmark
  @Group("parallel")
  @GroupThreads(1)
  public void benchmark_2() {
      while(true) {}
  }
  </code></pre>
  
  <p>Using the CPU monitor, I get that two CPU's are fully used.
  I would like to know how does the runner interpret those annotations.</p>