RFR (M): 8233919: Incrementally calculate the occupied cards in a heap region remembered set

[Stefan Johansson]

Hi Thomas,

On 2019-11-12 16:24, Thomas Schatzl wrote:
> Hi all,
> 
>    may I have reviews for this change that ultimately makes sure that 
> the number of occupied cards in a remembered set is only growing by 
> providing a per-OtherRegionsTable count that is atomically updated when 
> adding a remembered set entry.
> 
> Note that this count may not be completely accurate due to races when 
> deleting a PerRegionTable (which is a known issue) from an 
> OtherRegionsTable; but that is no different than before.
> 
> This helps improving the predictions in the young gen remset sampling 
> thread, and increase the performance of getting the occupancy count.
> 
> Based on JDK-8233997, and JDK-8233998 also out for review.
> 
> CR:
> https://bugs.openjdk.java.net/browse/JDK-8233919
> Webrev:
> http://cr.openjdk.java.net/~tschatzl/8233919/webrev/
I like this change and it looks good in general, just one small comment:
src/hotspot/share/gc/g1/heapRegionRemSet.cpp
---
  247   bool added = prt->add_reference(from);
  248   Atomic::add(num_added_by_coarsening + (added ? 1 : 0), 
&_num_occupied, memory_order_relaxed);

I would prefer:
if (prt->add_reference(from)) {
   num_added_by_coarsening++;
}
Atomic::add...

I you disagree, leave it as is.
---

Thanks,
Stefan

> Testing:
> hs-tier1-5,
> 
> Thanks,
>    Thomas