RFR (S): 8037344: Use the "next" field to iterate over fine remembered instead of using the hash table

[Thomas Schatzl]

Hi,

On Wed, 2014-04-09 at 13:46 +0200, Mikael Gerdin wrote:
> Hi Thomas,
> 
> On Wednesday 09 April 2014 12.02.45 Thomas Schatzl wrote:
> > Hi all,
> > 
> >   I have been notified that the webrev does not apply to the jdk9/hs-gc
> > repository any more.
> > 
> > Here is an updated version:
> > http://cr.openjdk.java.net/~tschatzl/8037344/webrev.1/
> 
> Just to make sure that I follow the subtleties in the code here, you set 
> _cur_region_cur_card to -1 here. Then when fine_has_next is called it computes 
> _cur_region_cur_card + 1 to get 0 and calls _bm.get_next_one_offset?

Exactly, this is to get the start condition right after switching from
coarse to fine entries or to the next fine PRT.
Get_next_one_offset() starts searching including the passed offset.

I will add a comment.

> 1115 void HeapRegionRemSetIterator::switch_to_next_prt(PerRegionTable* prt) {
> 1116   assert(prt != NULL, "Cannot switch to NULL prt");
> 1117   _fine_cur_prt = prt;
> 1118 
> 1119   HeapWord* r_bot = _fine_cur_prt->hr()->bottom();
> 1120   _cur_region_card_offset = _bosa->index_for(r_bot);
> 1121 
> 1122   _cur_region_cur_card = (size_t)-1;
> 1123 }
> 
> The rest of the code change looks ok, but this code is really complex and I 
> hope that you've tested this with verification enabled to catch any tricky 
> bugs.

It took me a while to get it completely right to pass various benchmarks
with verification. I do not exactly remember though what programs the
change has been tested with - however this change is part of my stack of
pending changes that is continuously used in developing new changes, so
in the last month or so it got a lot of implicit testing.

This testing includes the usual benchmarks, and bigapps containing
kitchensink 24h runs and such.

Overall the change simplifies the old code significantly imo, however I
agree that especially this iterator's interface complicates matters a
lot.

Thanks,
  Thomas