RFR: Use PLAB for evacuations instead of TLAB

[Roman Kennke]

Am 14.05.2018 um 10:52 schrieb Aleksey Shipilev:
> On 05/11/2018 11:25 PM, Roman Kennke wrote:
>> Full:
>> http://cr.openjdk.java.net/~rkennke/plab-gclab/webrev.05/
> 
> Looks OK. I have a few final questions:
> 
>  *) On this path we can fire assert with -UseTLAB?
> 
>  256 inline HeapWord* ShenandoahHeap::allocate_from_gclab(Thread* thread, size_t size) {
>  257   PLAB* gclab = ShenandoahThreadLocalData::gclab(thread);
>  258   if (gclab == NULL) {
>  259     assert(!thread->is_Java_thread() && !thread->is_Worker_thread(),
>  260            "Performance: thread should have GCLAB: %s", thread->name());
>  261     // No GCLABs in this thread, fallback to shared allocation
>  262     return NULL;
>  263   }
> 
> Otherwise OK.

GCLAB usage is no longer tied to +UseTLAB. I.e. even if we -UseTLAB,
we'd still be using GCLABs. We can change this if you prefer, or even
introduce a new flag (e.g. +ShenandoahUseGCLAB) ?

Roman