RFR: Fix and enable parallel safepoint workers

[Roman Kennke]

Am 14.09.2018 um 17:03 schrieb 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;
>   }
> 

Because those safepoint workers are worker threads.

Roman