RFR: Avoid false sharing by padding the ShenandoahEvacOOMHandler field in ShenandoahHeap

[Aleksey Shipilev]

On 10/19/2018 11:56 PM, Aleksey Shipilev wrote:
> On 10/19/2018 09:28 PM, Roman Kennke wrote:
>> The changeset "Sort ShenandoahHeap methods/fields into logical groups"
>> from Sept 21 reordered fields in ShenandoahHeap. Oddly, this has caused
>> a *severe* regression in Traversal GC (and presumably less severe with
>> adaptive) that caused throughput to drop by about 1/3. The reason for
>> this seems to be false sharing. Padding the evac_oom_handler field fixes it:
>>
>> http://cr.openjdk.java.net/~rkennke/evac-oom-false-sharing/webrev.00/

DEFAULT_CACHE_LINE_SIZE/2 is wrong, by the way; it needs to be just DEFAULT_CACHE_LINE_SIZE. I think
you have copy-pasted it from globalCounter.hpp, but that code is too optimistic. Copy-paste from
shenandoahSharedVariables.hpp.

-Aleksey