| /* |
| * 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); |
| 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) 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"); |
| } |
| } |
| } |
