[PATCH] Adding limitation while checking for cardTable

[Tim Bell]

Forwarded message

Please update your address books.  Stop using hotspot-gc-dev at openjdk.dev.java.net
                                                                      ^^^^
and use hotspot-gc-dev at openjdk.java.net instead (note the missing .dev).


-------- Original Message --------
From: Neo Jia <neojia at gmail.com>
Subject: Re: [PATCH] Adding limitation while checking for cardTable

On 5/10/07, Y Srinivas Ramakrishna <Y.S.Ramakrishna at sun.com> wrote:
 > Hi Neo --
 >
 > Thanks for the heads-up. This seems worth fixing, at least
 > for hygiene, How about the following instead, where we avoid the
 > addr_for() translation until we are sure it will be safe,
 > provided that the mr argument to the method is backed by
 > card table entries. This avoids the need for the
 > extra compare-and-branch inside the loop in your patch.
 >

Ramki,

Thanks. Your fix makes more sense than mine.

Neo

 >   void do_MemRegion(MemRegion mr) {
 >     HeapWord* end_of_non_clean = mr.end();
 >     HeapWord* start_of_non_clean = end_of_non_clean;
 >     jbyte*       entry = _ct->byte_for(mr.last());
 >     jbyte* first_entry = _ct->byte_for(mr.start());
 >     while (entry >= first_entry) {
 >       jbyte entry_val = *entry;
 >       HeapWord* cur = _ct->addr_for(entry);
 >       if (!clear_card(entry)) {
 >         if (start_of_non_clean < end_of_non_clean) {
 >           MemRegion mr2(start_of_non_clean, end_of_non_clean);
 >           _dirty_card_closure->do_MemRegion(mr2);
 >         }
 >         end_of_non_clean = cur;
 >         start_of_non_clean = end_of_non_clean;
 >       }
 >       start_of_non_clean = cur;
 >       entry--;
 >     }
 >     if (start_of_non_clean < end_of_non_clean) {
 >       MemRegion mr2(start_of_non_clean, end_of_non_clean);
 >       _dirty_card_closure->do_MemRegion(mr2);
 >     }
 >   }
 >
 > -- Ramki Ramakrishna
 >
 > Neo Jia wrote On 05/09/07 03:31 PM,:
 >
 > > hi,
 > >
 > > Acturally, the existing code works well although it misses the
 > > limitation checking, because the current heap layout will make sure
 > > the memory region checking failed before it goes beyond the limitation
 > > of byte map.
 > >
 > > The reason is that the nursery space is on the lower address, which
 > > will make its committed card table also on the lower address of inside
 > > the byte maps. While walking through the card table of the mature
 > > space and keeping reduce the entry, we will finally hit the boundary
 > > of its byte map before doing the check of memory region, which will
 > > fire the assert of the addr_for, if it is on the lower address.
 > >
 > > So, just adding more safety for this function.
 > >
 > > Thanks,
 > > Neo
 > > Index: memory/cardTableRS.cpp
 > > ===================================================================
 > > --- memory/cardTableRS.cpp  (revision 86)
 > > +++ memory/cardTableRS.cpp  (working copy)
 > > @@ -163,6 +163,9 @@
 > >     start_of_non_clean = end_of_non_clean;
 > >        }
 > >        entry--;
 > > +      // Adding a limit checking for safety.
 > > +      if (entry < _ct->base_map())
 > > +        break;
 > >        start_of_non_clean = cur;
 > >        cur = _ct->addr_for(entry);
 > >      }
 > > Index: memory/cardTableRS.hpp
 > > ===================================================================
 > > --- memory/cardTableRS.hpp  (revision 86)
 > > +++ memory/cardTableRS.hpp  (working copy)
 > > @@ -101,6 +101,8 @@
 > >
 > >    CardTableModRefBS* ct_bs() { return &_ct_bs; }
 > >
 > > +  jbyte * base_map() { return &(_ct_bs._byte_map[0]); }
 > > +
 > >    // Override.
 > >    void prepare_for_younger_refs_iterate(bool parallel);
 > >
 >
 > --
 >
 > ----------------------------------------------------------------------------
 > Y. Srinivas Ramakrishna               HotSpot JVM
 > Sun Microsystems, Inc., USCA 14-102   JWS / Software Group
 > 4140 Network Circle                   408 276 7250 (x17250)
 > Santa Clara, CA  95054, U.S.A.        Y dot S dot Ramakrishna at Sun dot COM
 > ----------------------------------------------------------------------------
 >
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 >


-- 
I would remember that if researchers were not ambitious
probably today we haven't the technology we are using!

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