RFR(M): 8218049: Survivor MemoryMXBean used() size granularity is region based

[Kim Barrett]

> On Apr 4, 2019, at 5:02 AM, Thomas Schatzl <thomas.schatzl at oracle.com> wrote:
> 
> On Wed, 2019-04-03 at 18:43 -0400, Kim Barrett wrote:
>> src/hotspot/share/gc/g1/g1CollectedHeap.cpp
>> 4589   OrderAccess::storestore();
>> 
>> src/hotspot/share/gc/g1/g1MonitoringSupport.cpp
>> 233   OrderAccess::loadload();
>> 
>> […]
>> I think these new uses of OrderAccess are an attempt to ensure the
>> monitoring code gets a consistent pair of values.  But I don't think
>> it accomplishes that goal.  We have
>> […]
> 
>> guess there is some reason not to use a lock here? Double buffering
>> or GlobalCounter could be used instead.
> 
> Double buffering (if I think it is what you think it is) would have
> been what I would have suggested, but it seems at least some effort.

Agreed.

> For this change I suggest going back to use the subtract_until_zero()
> method (whether we use the racy region counts or the actual "used
> bytes" makes not difference with respect to the problem) and
> investigate improvements in a separate CR.

That seems like a reasonable plan.