RFR: Fix and enable parallel safepoint workers

[Aleksey Shipilev]

On 09/14/2018 03:17 PM, Roman Kennke wrote:
> We do have an implementation of SH::get_safepoint_workers() but it is
> broken:
> - the workers don't get their thread local data setup correctly, which
> is needed for evacs (e.g. via oopDesc::cas_set_mark(..) )
> - the workers must not identify themselves as GC workers, otherwise
> OOM-evac protocol assumes that WBs are not needed. I implemented this by
> narrowing the condition in write-barrier to is_GC_task_thread() which
> should identify only actual GC workers, but not other threads like the
> safepoint workers.
> 
> - I also propose to enable it by default. ZGC and G1 do it now, and not
> doing it basically guarantees this code to rot. I lowish number of
> threads seems to give the best bang for the buck.
> 
> http://cr.openjdk.java.net/~rkennke/par-sp-workers/webrev.00/
> 
> Testing: tier3_gc_shenandoah
> 
> OK?

Kinda OK. I wonder why doesn't it blow up the assert here?

  static void initialize_gclab(Thread* thread) {
    assert (thread->is_Java_thread() || thread->is_Worker_thread(), "Only Java
and GC worker threads are allowed to get GCLABs");
    data(thread)->_gclab = new PLAB(PLAB::min_size());
    data(thread)->_gclab_size = 0;
  }


-Aleksey