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

[Thomas Schatzl]

Hi,

On 20.11.19 11:42, Stefan Johansson wrote:
> 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.

Fixed in
http://cr.openjdk.java.net/~tschatzl/8233919/webrev.0_to_1/
http://cr.openjdk.java.net/~tschatzl/8233919/webrev.1/

Thanks for your review,
   Thomas