jdk8-mips64-public/hotspot: src/share/vm/memory/cardTableRS.cpp@8c95980d0b66 (annotated)

src/share/vm/memory/cardTableRS.cpp@8c95980d0b66 (annotated)

src/share/vm/memory/cardTableRS.cpp

Sat, 07 Nov 2020 10:30:02 +0800

author: aoqi
date: Sat, 07 Nov 2020 10:30:02 +0800
changeset 10026: 8c95980d0b66
parent 9465: 1556c6d89036
permissions: -rw-r--r--

Added tag mips-jdk8u275-b01 for changeset d3b4d62f391f

 /*
  * Copyright (c) 2001, 2014, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
  * under the terms of the GNU General Public License version 2 only, as
  * published by the Free Software Foundation.
  *
  * This code is distributed in the hope that it will be useful, but WITHOUT
  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  * version 2 for more details (a copy is included in the LICENSE file that
  * accompanied this code).
  *
  * You should have received a copy of the GNU General Public License version
  * 2 along with this work; if not, write to the Free Software Foundation,
  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
  *
  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
  * or visit www.oracle.com if you need additional information or have any
  * questions.
  *
  */
 #include "precompiled.hpp"
 #include "memory/allocation.inline.hpp"
 #include "memory/cardTableRS.hpp"
 #include "memory/genCollectedHeap.hpp"
 #include "memory/generation.hpp"
 #include "memory/space.hpp"
 #include "oops/oop.inline.hpp"
 #include "runtime/java.hpp"
 #include "runtime/os.hpp"
 #include "utilities/macros.hpp"
 #if INCLUDE_ALL_GCS
 #include "gc_implementation/g1/concurrentMark.hpp"
 #include "gc_implementation/g1/g1SATBCardTableModRefBS.hpp"
 #endif // INCLUDE_ALL_GCS
 CardTableRS::CardTableRS(MemRegion whole_heap,
                          int max_covered_regions) :
   GenRemSet(),
   _cur_youngergen_card_val(youngergenP1_card),
   _regions_to_iterate(max_covered_regions - 1)
 {
 #if INCLUDE_ALL_GCS
   if (UseG1GC) {
       _ct_bs = new G1SATBCardTableLoggingModRefBS(whole_heap,
                                                   max_covered_regions);
   } else {
     _ct_bs = new CardTableModRefBSForCTRS(whole_heap, max_covered_regions);
   }
 #else
   _ct_bs = new CardTableModRefBSForCTRS(whole_heap, max_covered_regions);
 #endif
   _ct_bs->initialize();
   set_bs(_ct_bs);
   _last_cur_val_in_gen = NEW_C_HEAP_ARRAY3(jbyte, GenCollectedHeap::max_gens + 1,
                          mtGC, CURRENT_PC, AllocFailStrategy::RETURN_NULL);
   if (_last_cur_val_in_gen == NULL) {
     vm_exit_during_initialization("Could not create last_cur_val_in_gen array.");
   }
   for (int i = 0; i < GenCollectedHeap::max_gens + 1; i++) {
     _last_cur_val_in_gen[i] = clean_card_val();
   }
   _ct_bs->set_CTRS(this);
 }
 CardTableRS::~CardTableRS() {
   if (_ct_bs) {
     delete _ct_bs;
     _ct_bs = NULL;
   }
   if (_last_cur_val_in_gen) {
     FREE_C_HEAP_ARRAY(jbyte, _last_cur_val_in_gen, mtInternal);
   }
 }
 void CardTableRS::resize_covered_region(MemRegion new_region) {
   _ct_bs->resize_covered_region(new_region);
 }
 jbyte CardTableRS::find_unused_youngergenP_card_value() {
   for (jbyte v = youngergenP1_card;
        v < cur_youngergen_and_prev_nonclean_card;
        v++) {
     bool seen = false;
     for (int g = 0; g < _regions_to_iterate; g++) {
       if (_last_cur_val_in_gen[g] == v) {
         seen = true;
         break;
       }
     }
     if (!seen) return v;
   }
   ShouldNotReachHere();
   return 0;
 }
 void CardTableRS::prepare_for_younger_refs_iterate(bool parallel) {
   // Parallel or sequential, we must always set the prev to equal the
   // last one written.
   if (parallel) {
     // Find a parallel value to be used next.
     jbyte next_val = find_unused_youngergenP_card_value();
     set_cur_youngergen_card_val(next_val);
   } else {
     // In an sequential traversal we will always write youngergen, so that
     // the inline barrier is  correct.
     set_cur_youngergen_card_val(youngergen_card);
   }
 }
 void CardTableRS::younger_refs_iterate(Generation* g,
                                        OopsInGenClosure* blk) {
   _last_cur_val_in_gen[g->level()+1] = cur_youngergen_card_val();
   g->younger_refs_iterate(blk);
 }
 inline bool ClearNoncleanCardWrapper::clear_card(jbyte* entry) {
   if (_is_par) {
     return clear_card_parallel(entry);
   } else {
     return clear_card_serial(entry);
   }
 }
 inline bool ClearNoncleanCardWrapper::clear_card_parallel(jbyte* entry) {
   while (true) {
     // In the parallel case, we may have to do this several times.
     jbyte entry_val = *entry;
     assert(entry_val != CardTableRS::clean_card_val(),
            "We shouldn't be looking at clean cards, and this should "
            "be the only place they get cleaned.");
     if (CardTableRS::card_is_dirty_wrt_gen_iter(entry_val)
         || _ct->is_prev_youngergen_card_val(entry_val)) {
       jbyte res =
         Atomic::cmpxchg(CardTableRS::clean_card_val(), entry, entry_val);
       if (res == entry_val) {
         break;
       } else {
         assert(res == CardTableRS::cur_youngergen_and_prev_nonclean_card,
                "The CAS above should only fail if another thread did "
                "a GC write barrier.");
       }
     } else if (entry_val ==
                CardTableRS::cur_youngergen_and_prev_nonclean_card) {
       // Parallelism shouldn't matter in this case.  Only the thread
       // assigned to scan the card should change this value.
       *entry = _ct->cur_youngergen_card_val();
       break;
     } else {
       assert(entry_val == _ct->cur_youngergen_card_val(),
              "Should be the only possibility.");
       // In this case, the card was clean before, and become
       // cur_youngergen only because of processing of a promoted object.
       // We don't have to look at the card.
       return false;
     }
   }
   return true;
 }
 inline bool ClearNoncleanCardWrapper::clear_card_serial(jbyte* entry) {
   jbyte entry_val = *entry;
   assert(entry_val != CardTableRS::clean_card_val(),
          "We shouldn't be looking at clean cards, and this should "
          "be the only place they get cleaned.");
   assert(entry_val != CardTableRS::cur_youngergen_and_prev_nonclean_card,
          "This should be possible in the sequential case.");
   *entry = CardTableRS::clean_card_val();
   return true;
 }
 ClearNoncleanCardWrapper::ClearNoncleanCardWrapper(
   DirtyCardToOopClosure* dirty_card_closure, CardTableRS* ct) :
     _dirty_card_closure(dirty_card_closure), _ct(ct) {
     // Cannot yet substitute active_workers for n_par_threads
     // in the case where parallelism is being turned off by
     // setting n_par_threads to 0.
     _is_par = (SharedHeap::heap()->n_par_threads() > 0);
     assert(!_is_par ||
            (SharedHeap::heap()->n_par_threads() ==
             SharedHeap::heap()->workers()->active_workers()), "Mismatch");
 }
 bool ClearNoncleanCardWrapper::is_word_aligned(jbyte* entry) {
   return (((intptr_t)entry) & (BytesPerWord-1)) == 0;
 }
 void ClearNoncleanCardWrapper::do_MemRegion(MemRegion mr) {
   assert(mr.word_size() > 0, "Error");
   assert(_ct->is_aligned(mr.start()), "mr.start() should be card aligned");
   // mr.end() may not necessarily be card aligned.
   jbyte* cur_entry = _ct->byte_for(mr.last());
   const jbyte* limit = _ct->byte_for(mr.start());
   HeapWord* end_of_non_clean = mr.end();
   HeapWord* start_of_non_clean = end_of_non_clean;
   while (cur_entry >= limit) {
     HeapWord* cur_hw = _ct->addr_for(cur_entry);
     if ((*cur_entry != CardTableRS::clean_card_val()) && clear_card(cur_entry)) {
       // Continue the dirty range by opening the
       // dirty window one card to the left.
       start_of_non_clean = cur_hw;
     } else {
       // We hit a "clean" card; process any non-empty
       // "dirty" range accumulated so far.
       if (start_of_non_clean < end_of_non_clean) {
         const MemRegion mrd(start_of_non_clean, end_of_non_clean);
         _dirty_card_closure->do_MemRegion(mrd);
       }
       // fast forward through potential continuous whole-word range of clean cards beginning at a word-boundary
       if (is_word_aligned(cur_entry)) {
         jbyte* cur_row = cur_entry - BytesPerWord;
         while (cur_row >= limit && *((intptr_t*)cur_row) ==  CardTableRS::clean_card_row()) {
           cur_row -= BytesPerWord;
         }
         cur_entry = cur_row + BytesPerWord;
         cur_hw = _ct->addr_for(cur_entry);
       }
       // Reset the dirty window, while continuing to look
       // for the next dirty card that will start a
       // new dirty window.
       end_of_non_clean = cur_hw;
       start_of_non_clean = cur_hw;
     }
     // Note that "cur_entry" leads "start_of_non_clean" in
     // its leftward excursion after this point
     // in the loop and, when we hit the left end of "mr",
     // will point off of the left end of the card-table
     // for "mr".
     cur_entry--;
   }
   // If the first card of "mr" was dirty, we will have
   // been left with a dirty window, co-initial with "mr",
   // which we now process.
   if (start_of_non_clean < end_of_non_clean) {
     const MemRegion mrd(start_of_non_clean, end_of_non_clean);
     _dirty_card_closure->do_MemRegion(mrd);
   }
 }
 // clean (by dirty->clean before) ==> cur_younger_gen
 // dirty                          ==> cur_youngergen_and_prev_nonclean_card
 // precleaned                     ==> cur_youngergen_and_prev_nonclean_card
 // prev-younger-gen               ==> cur_youngergen_and_prev_nonclean_card
 // cur-younger-gen                ==> cur_younger_gen
 // cur_youngergen_and_prev_nonclean_card ==> no change.
 void CardTableRS::write_ref_field_gc_par(void* field, oop new_val) {
   jbyte* entry = ct_bs()->byte_for(field);
 #if defined MIPS && !defined ZERO
   if (UseSyncLevel >= 2000) OrderAccess::fence();
 #endif
   do {
     jbyte entry_val = *entry;
     // We put this first because it's probably the most common case.
     if (entry_val == clean_card_val()) {
       // No threat of contention with cleaning threads.
       *entry = cur_youngergen_card_val();
       return;
     } else if (card_is_dirty_wrt_gen_iter(entry_val)
                || is_prev_youngergen_card_val(entry_val)) {
       // Mark it as both cur and prev youngergen; card cleaning thread will
       // eventually remove the previous stuff.
       jbyte new_val = cur_youngergen_and_prev_nonclean_card;
       jbyte res = Atomic::cmpxchg(new_val, entry, entry_val);
       // Did the CAS succeed?
       if (res == entry_val) {
 #if defined MIPS && !defined ZERO
          if (UseSyncLevel >= 2000) OrderAccess::fence();
 #endif
          return;
       }
       // Otherwise, retry, to see the new value.
       continue;
     } else {
       assert(entry_val == cur_youngergen_and_prev_nonclean_card
              || entry_val == cur_youngergen_card_val(),
              "should be only possibilities.");
       return;
     }
   } while (true);
 }
 void CardTableRS::younger_refs_in_space_iterate(Space* sp,
                                                 OopsInGenClosure* cl) {
   const MemRegion urasm = sp->used_region_at_save_marks();
 #ifdef ASSERT
   // Convert the assertion check to a warning if we are running
   // CMS+ParNew until related bug is fixed.
   MemRegion ur    = sp->used_region();
   assert(ur.contains(urasm) || (UseConcMarkSweepGC && UseParNewGC),
          err_msg("Did you forget to call save_marks()? "
                  "[" PTR_FORMAT ", " PTR_FORMAT ") is not contained in "
                  "[" PTR_FORMAT ", " PTR_FORMAT ")",
                  p2i(urasm.start()), p2i(urasm.end()), p2i(ur.start()), p2i(ur.end())));
   // In the case of CMS+ParNew, issue a warning
   if (!ur.contains(urasm)) {
     assert(UseConcMarkSweepGC && UseParNewGC, "Tautology: see assert above");
     warning("CMS+ParNew: Did you forget to call save_marks()? "
             "[" PTR_FORMAT ", " PTR_FORMAT ") is not contained in "
             "[" PTR_FORMAT ", " PTR_FORMAT ")",
              p2i(urasm.start()), p2i(urasm.end()), p2i(ur.start()), p2i(ur.end()));
     MemRegion ur2 = sp->used_region();
     MemRegion urasm2 = sp->used_region_at_save_marks();
     if (!ur.equals(ur2)) {
       warning("CMS+ParNew: Flickering used_region()!!");
     }
     if (!urasm.equals(urasm2)) {
       warning("CMS+ParNew: Flickering used_region_at_save_marks()!!");
     }
     ShouldNotReachHere();
   }
 #endif
   _ct_bs->non_clean_card_iterate_possibly_parallel(sp, urasm, cl, this);
 }
 void CardTableRS::clear_into_younger(Generation* old_gen) {
   assert(old_gen->level() == 1, "Should only be called for the old generation");
   // The card tables for the youngest gen need never be cleared.
   // There's a bit of subtlety in the clear() and invalidate()
   // methods that we exploit here and in invalidate_or_clear()
   // below to avoid missing cards at the fringes. If clear() or
   // invalidate() are changed in the future, this code should
   // be revisited. 20040107.ysr
   clear(old_gen->prev_used_region());
 }
 void CardTableRS::invalidate_or_clear(Generation* old_gen) {
   assert(old_gen->level() == 1, "Should only be called for the old generation");
   // Invalidate the cards for the currently occupied part of
   // the old generation and clear the cards for the
   // unoccupied part of the generation (if any, making use
   // of that generation's prev_used_region to determine that
   // region). No need to do anything for the youngest
   // generation. Also see note#20040107.ysr above.
   MemRegion used_mr = old_gen->used_region();
   MemRegion to_be_cleared_mr = old_gen->prev_used_region().minus(used_mr);
   if (!to_be_cleared_mr.is_empty()) {
     clear(to_be_cleared_mr);
   }
   invalidate(used_mr);
 }
 class VerifyCleanCardClosure: public OopClosure {
 private:
   HeapWord* _boundary;
   HeapWord* _begin;
   HeapWord* _end;
 protected:
   template <class T> void do_oop_work(T* p) {
     HeapWord* jp = (HeapWord*)p;
     assert(jp >= _begin && jp < _end,
            err_msg("Error: jp " PTR_FORMAT " should be within "
                    "[_begin, _end) = [" PTR_FORMAT "," PTR_FORMAT ")",
                    p2i(jp), p2i(_begin), p2i(_end)));
     oop obj = oopDesc::load_decode_heap_oop(p);
     guarantee(obj == NULL || (HeapWord*)obj >= _boundary,
               err_msg("pointer " PTR_FORMAT " at " PTR_FORMAT " on "
                       "clean card crosses boundary" PTR_FORMAT,
                       p2i((HeapWord*)obj), p2i(jp), p2i(_boundary)));
   }
 public:
   VerifyCleanCardClosure(HeapWord* b, HeapWord* begin, HeapWord* end) :
     _boundary(b), _begin(begin), _end(end) {
     assert(b <= begin,
            err_msg("Error: boundary " PTR_FORMAT " should be at or below begin " PTR_FORMAT,
                    p2i(b), p2i(begin)));
     assert(begin <= end,
            err_msg("Error: begin " PTR_FORMAT " should be strictly below end " PTR_FORMAT,
                    p2i(begin), p2i(end)));
   }
   virtual void do_oop(oop* p)       { VerifyCleanCardClosure::do_oop_work(p); }
   virtual void do_oop(narrowOop* p) { VerifyCleanCardClosure::do_oop_work(p); }
 };
 class VerifyCTSpaceClosure: public SpaceClosure {
 private:
   CardTableRS* _ct;
   HeapWord* _boundary;
 public:
   VerifyCTSpaceClosure(CardTableRS* ct, HeapWord* boundary) :
     _ct(ct), _boundary(boundary) {}
   virtual void do_space(Space* s) { _ct->verify_space(s, _boundary); }
 };
 class VerifyCTGenClosure: public GenCollectedHeap::GenClosure {
   CardTableRS* _ct;
 public:
   VerifyCTGenClosure(CardTableRS* ct) : _ct(ct) {}
   void do_generation(Generation* gen) {
     // Skip the youngest generation.
     if (gen->level() == 0) return;
     // Normally, we're interested in pointers to younger generations.
     VerifyCTSpaceClosure blk(_ct, gen->reserved().start());
     gen->space_iterate(&blk, true);
   }
 };
 void CardTableRS::verify_space(Space* s, HeapWord* gen_boundary) {
   // We don't need to do young-gen spaces.
   if (s->end() <= gen_boundary) return;
   MemRegion used = s->used_region();
   jbyte* cur_entry = byte_for(used.start());
   jbyte* limit = byte_after(used.last());
   while (cur_entry < limit) {
     if (*cur_entry == CardTableModRefBS::clean_card) {
       jbyte* first_dirty = cur_entry+1;
       while (first_dirty < limit &&
              *first_dirty == CardTableModRefBS::clean_card) {
         first_dirty++;
       }
       // If the first object is a regular object, and it has a
       // young-to-old field, that would mark the previous card.
       HeapWord* boundary = addr_for(cur_entry);
       HeapWord* end = (first_dirty >= limit) ? used.end() : addr_for(first_dirty);
       HeapWord* boundary_block = s->block_start(boundary);
       HeapWord* begin = boundary;             // Until proven otherwise.
       HeapWord* start_block = boundary_block; // Until proven otherwise.
       if (boundary_block < boundary) {
         if (s->block_is_obj(boundary_block) && s->obj_is_alive(boundary_block)) {
           oop boundary_obj = oop(boundary_block);
           if (!boundary_obj->is_objArray() &&
               !boundary_obj->is_typeArray()) {
             guarantee(cur_entry > byte_for(used.start()),
                       "else boundary would be boundary_block");
             if (*byte_for(boundary_block) != CardTableModRefBS::clean_card) {
               begin = boundary_block + s->block_size(boundary_block);
               start_block = begin;
             }
           }
         }
       }
       // Now traverse objects until end.
       if (begin < end) {
         MemRegion mr(begin, end);
         VerifyCleanCardClosure verify_blk(gen_boundary, begin, end);
         for (HeapWord* cur = start_block; cur < end; cur += s->block_size(cur)) {
           if (s->block_is_obj(cur) && s->obj_is_alive(cur)) {
             oop(cur)->oop_iterate_no_header(&verify_blk, mr);
           }
         }
       }
       cur_entry = first_dirty;
     } else {
       // We'd normally expect that cur_youngergen_and_prev_nonclean_card
       // is a transient value, that cannot be in the card table
       // except during GC, and thus assert that:
       // guarantee(*cur_entry != cur_youngergen_and_prev_nonclean_card,
       //        "Illegal CT value");
       // That however, need not hold, as will become clear in the
       // following...
       // We'd normally expect that if we are in the parallel case,
       // we can't have left a prev value (which would be different
       // from the current value) in the card table, and so we'd like to
       // assert that:
       // guarantee(cur_youngergen_card_val() == youngergen_card
       //           || !is_prev_youngergen_card_val(*cur_entry),
       //           "Illegal CT value");
       // That, however, may not hold occasionally, because of
       // CMS or MSC in the old gen. To wit, consider the
       // following two simple illustrative scenarios:
       // (a) CMS: Consider the case where a large object L
       //     spanning several cards is allocated in the old
       //     gen, and has a young gen reference stored in it, dirtying
       //     some interior cards. A young collection scans the card,
       //     finds a young ref and installs a youngergenP_n value.
       //     L then goes dead. Now a CMS collection starts,
       //     finds L dead and sweeps it up. Assume that L is
       //     abutting _unallocated_blk, so _unallocated_blk is
       //     adjusted down to (below) L. Assume further that
       //     no young collection intervenes during this CMS cycle.
       //     The next young gen cycle will not get to look at this
       //     youngergenP_n card since it lies in the unoccupied
       //     part of the space.
       //     Some young collections later the blocks on this
       //     card can be re-allocated either due to direct allocation
       //     or due to absorbing promotions. At this time, the
       //     before-gc verification will fail the above assert.
       // (b) MSC: In this case, an object L with a young reference
       //     is on a card that (therefore) holds a youngergen_n value.
       //     Suppose also that L lies towards the end of the used
       //     the used space before GC. An MSC collection
       //     occurs that compacts to such an extent that this
       //     card is no longer in the occupied part of the space.
       //     Since current code in MSC does not always clear cards
       //     in the unused part of old gen, this stale youngergen_n
       //     value is left behind and can later be covered by
       //     an object when promotion or direct allocation
       //     re-allocates that part of the heap.
       //
       // Fortunately, the presence of such stale card values is
       // "only" a minor annoyance in that subsequent young collections
       // might needlessly scan such cards, but would still never corrupt
       // the heap as a result. However, it's likely not to be a significant
       // performance inhibitor in practice. For instance,
       // some recent measurements with unoccupied cards eagerly cleared
       // out to maintain this invariant, showed next to no
       // change in young collection times; of course one can construct
       // degenerate examples where the cost can be significant.)
       // Note, in particular, that if the "stale" card is modified
       // after re-allocation, it would be dirty, not "stale". Thus,
       // we can never have a younger ref in such a card and it is
       // safe not to scan that card in any collection. [As we see
       // below, we do some unnecessary scanning
       // in some cases in the current parallel scanning algorithm.]
       //
       // The main point below is that the parallel card scanning code
       // deals correctly with these stale card values. There are two main
       // cases to consider where we have a stale "younger gen" value and a
       // "derivative" case to consider, where we have a stale
       // "cur_younger_gen_and_prev_non_clean" value, as will become
       // apparent in the case analysis below.
       // o Case 1. If the stale value corresponds to a younger_gen_n
       //   value other than the cur_younger_gen value then the code
       //   treats this as being tantamount to a prev_younger_gen
       //   card. This means that the card may be unnecessarily scanned.
       //   There are two sub-cases to consider:
       //   o Case 1a. Let us say that the card is in the occupied part
       //     of the generation at the time the collection begins. In
       //     that case the card will be either cleared when it is scanned
       //     for young pointers, or will be set to cur_younger_gen as a
       //     result of promotion. (We have elided the normal case where
       //     the scanning thread and the promoting thread interleave
       //     possibly resulting in a transient
       //     cur_younger_gen_and_prev_non_clean value before settling
       //     to cur_younger_gen. [End Case 1a.]
       //   o Case 1b. Consider now the case when the card is in the unoccupied
       //     part of the space which becomes occupied because of promotions
       //     into it during the current young GC. In this case the card
       //     will never be scanned for young references. The current
       //     code will set the card value to either
       //     cur_younger_gen_and_prev_non_clean or leave
       //     it with its stale value -- because the promotions didn't
       //     result in any younger refs on that card. Of these two
       //     cases, the latter will be covered in Case 1a during
       //     a subsequent scan. To deal with the former case, we need
       //     to further consider how we deal with a stale value of
       //     cur_younger_gen_and_prev_non_clean in our case analysis
       //     below. This we do in Case 3 below. [End Case 1b]
       //   [End Case 1]
       // o Case 2. If the stale value corresponds to cur_younger_gen being
       //   a value not necessarily written by a current promotion, the
       //   card will not be scanned by the younger refs scanning code.
       //   (This is OK since as we argued above such cards cannot contain
       //   any younger refs.) The result is that this value will be
       //   treated as a prev_younger_gen value in a subsequent collection,
       //   which is addressed in Case 1 above. [End Case 2]
       // o Case 3. We here consider the "derivative" case from Case 1b. above
       //   because of which we may find a stale
       //   cur_younger_gen_and_prev_non_clean card value in the table.
       //   Once again, as in Case 1, we consider two subcases, depending
       //   on whether the card lies in the occupied or unoccupied part
       //   of the space at the start of the young collection.
       //   o Case 3a. Let us say the card is in the occupied part of
       //     the old gen at the start of the young collection. In that
       //     case, the card will be scanned by the younger refs scanning
       //     code which will set it to cur_younger_gen. In a subsequent
       //     scan, the card will be considered again and get its final
       //     correct value. [End Case 3a]
       //   o Case 3b. Now consider the case where the card is in the
       //     unoccupied part of the old gen, and is occupied as a result
       //     of promotions during thus young gc. In that case,
       //     the card will not be scanned for younger refs. The presence
       //     of newly promoted objects on the card will then result in
       //     its keeping the value cur_younger_gen_and_prev_non_clean
       //     value, which we have dealt with in Case 3 here. [End Case 3b]
       //   [End Case 3]
       //
       // (Please refer to the code in the helper class
       // ClearNonCleanCardWrapper and in CardTableModRefBS for details.)
       //
       // The informal arguments above can be tightened into a formal
       // correctness proof and it behooves us to write up such a proof,
       // or to use model checking to prove that there are no lingering
       // concerns.
       //
       // Clearly because of Case 3b one cannot bound the time for
       // which a card will retain what we have called a "stale" value.
       // However, one can obtain a Loose upper bound on the redundant
       // work as a result of such stale values. Note first that any
       // time a stale card lies in the occupied part of the space at
       // the start of the collection, it is scanned by younger refs
       // code and we can define a rank function on card values that
       // declines when this is so. Note also that when a card does not
       // lie in the occupied part of the space at the beginning of a
       // young collection, its rank can either decline or stay unchanged.
       // In this case, no extra work is done in terms of redundant
       // younger refs scanning of that card.
       // Then, the case analysis above reveals that, in the worst case,
       // any such stale card will be scanned unnecessarily at most twice.
       //
       // It is nonethelss advisable to try and get rid of some of this
       // redundant work in a subsequent (low priority) re-design of
       // the card-scanning code, if only to simplify the underlying
       // state machine analysis/proof. ysr 1/28/2002. XXX
       cur_entry++;
     }
   }
 }
 void CardTableRS::verify() {
   // At present, we only know how to verify the card table RS for
   // generational heaps.
   VerifyCTGenClosure blk(this);
   CollectedHeap* ch = Universe::heap();
   if (ch->kind() == CollectedHeap::GenCollectedHeap) {
     GenCollectedHeap::heap()->generation_iterate(&blk, false);
     _ct_bs->verify();
     }
   }
 void CardTableRS::verify_aligned_region_empty(MemRegion mr) {
   if (!mr.is_empty()) {
     jbyte* cur_entry = byte_for(mr.start());
     jbyte* limit = byte_after(mr.last());
     // The region mr may not start on a card boundary so
     // the first card may reflect a write to the space
     // just prior to mr.
     if (!is_aligned(mr.start())) {
       cur_entry++;
     }
     for (;cur_entry < limit; cur_entry++) {
       guarantee(*cur_entry == CardTableModRefBS::clean_card,
                 "Unexpected dirty card found");
     }
   }
 }

Mercurial > jdk8-mips64-public > hotspot / annotate

src/share/vm/memory/cardTableRS.cpp@8c95980d0b66 (annotated)

src/share/vm/memory/cardTableRS.cpp