RFR (XS): 8155721: Sparse remset wastes half of entry memory

[Mikael Gerdin]

Hi Thomas,

On 2016-05-02 12:17, Thomas Schatzl wrote:
> Hi all,
>
>    can I have reviews for this pretty small change that (basically)
> halves memory usage of the sparse prt?
>
> Looking through the code showed that G1 allocates
> capacity SparsePRTEntrys (the container for the cards for a particular
> region), but already expands the hash table at capacity/2+1. So almost
> half of these SparsePRTEntrys are never used and need not be
> allocated/cleared/etc.
>
> This also uncovered a off-by-one bug in RSHashTable::alloc_entry() that
> was benign before but caused crashes when allocating just what is
> required.
>
> CR:
> https://bugs.openjdk.java.net/browse/JDK-8155721
> Webrev:
> http://cr.openjdk.java.net/~tschatzl/8155721/webrev/

Good catch,
however

I'm not sure I'm fond of keeping the inverse of the occupancy factor.
I think the code would be a bit clearer if we made it a floating point 
number.

Also, I think it should be CapitalizedCamelCase since it's a constant.

/Mikael


jprt, gcbasher with -XX:+VerifyRememberedSet
>
> Thanks,
>    Thomas
>