duke@435: /* drchase@6680: * Copyright (c) 2001, 2014, Oracle and/or its affiliates. All rights reserved. duke@435: * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. duke@435: * duke@435: * This code is free software; you can redistribute it and/or modify it duke@435: * under the terms of the GNU General Public License version 2 only, as duke@435: * published by the Free Software Foundation. duke@435: * duke@435: * This code is distributed in the hope that it will be useful, but WITHOUT duke@435: * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or duke@435: * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License duke@435: * version 2 for more details (a copy is included in the LICENSE file that duke@435: * accompanied this code). duke@435: * duke@435: * You should have received a copy of the GNU General Public License version duke@435: * 2 along with this work; if not, write to the Free Software Foundation, duke@435: * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. duke@435: * trims@1907: * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA trims@1907: * or visit www.oracle.com if you need additional information or have any trims@1907: * questions. duke@435: * duke@435: */ duke@435: stefank@2314: #include "precompiled.hpp" stefank@2314: #include "memory/allocation.inline.hpp" stefank@2314: #include "memory/cardTableRS.hpp" stefank@2314: #include "memory/genCollectedHeap.hpp" stefank@2314: #include "memory/generation.hpp" stefank@2314: #include "memory/space.hpp" stefank@2314: #include "oops/oop.inline.hpp" stefank@2314: #include "runtime/java.hpp" stefank@2314: #include "runtime/os.hpp" jprovino@4542: #include "utilities/macros.hpp" jprovino@4542: #if INCLUDE_ALL_GCS stefank@2314: #include "gc_implementation/g1/concurrentMark.hpp" stefank@2314: #include "gc_implementation/g1/g1SATBCardTableModRefBS.hpp" jprovino@4542: #endif // INCLUDE_ALL_GCS duke@435: duke@435: CardTableRS::CardTableRS(MemRegion whole_heap, duke@435: int max_covered_regions) : ysr@777: GenRemSet(), ysr@777: _cur_youngergen_card_val(youngergenP1_card), ysr@777: _regions_to_iterate(max_covered_regions - 1) duke@435: { jprovino@4542: #if INCLUDE_ALL_GCS ysr@777: if (UseG1GC) { ysr@777: _ct_bs = new G1SATBCardTableLoggingModRefBS(whole_heap, ysr@777: max_covered_regions); ysr@777: } else { ysr@777: _ct_bs = new CardTableModRefBSForCTRS(whole_heap, max_covered_regions); ysr@777: } ysr@777: #else ysr@777: _ct_bs = new CardTableModRefBSForCTRS(whole_heap, max_covered_regions); ysr@777: #endif tschatzl@7051: _ct_bs->initialize(); ysr@777: set_bs(_ct_bs); minqi@5103: _last_cur_val_in_gen = NEW_C_HEAP_ARRAY3(jbyte, GenCollectedHeap::max_gens + 1, zgu@7074: mtGC, CURRENT_PC, AllocFailStrategy::RETURN_NULL); duke@435: if (_last_cur_val_in_gen == NULL) { minqi@5103: vm_exit_during_initialization("Could not create last_cur_val_in_gen array."); duke@435: } duke@435: for (int i = 0; i < GenCollectedHeap::max_gens + 1; i++) { duke@435: _last_cur_val_in_gen[i] = clean_card_val(); duke@435: } ysr@777: _ct_bs->set_CTRS(this); duke@435: } duke@435: minqi@5103: CardTableRS::~CardTableRS() { minqi@5103: if (_ct_bs) { minqi@5103: delete _ct_bs; minqi@5103: _ct_bs = NULL; minqi@5103: } minqi@5103: if (_last_cur_val_in_gen) { minqi@5103: FREE_C_HEAP_ARRAY(jbyte, _last_cur_val_in_gen, mtInternal); minqi@5103: } minqi@5103: } minqi@5103: duke@435: void CardTableRS::resize_covered_region(MemRegion new_region) { ysr@777: _ct_bs->resize_covered_region(new_region); duke@435: } duke@435: duke@435: jbyte CardTableRS::find_unused_youngergenP_card_value() { duke@435: for (jbyte v = youngergenP1_card; duke@435: v < cur_youngergen_and_prev_nonclean_card; duke@435: v++) { duke@435: bool seen = false; ysr@777: for (int g = 0; g < _regions_to_iterate; g++) { duke@435: if (_last_cur_val_in_gen[g] == v) { duke@435: seen = true; duke@435: break; duke@435: } duke@435: } duke@435: if (!seen) return v; duke@435: } duke@435: ShouldNotReachHere(); duke@435: return 0; duke@435: } duke@435: duke@435: void CardTableRS::prepare_for_younger_refs_iterate(bool parallel) { duke@435: // Parallel or sequential, we must always set the prev to equal the duke@435: // last one written. duke@435: if (parallel) { duke@435: // Find a parallel value to be used next. duke@435: jbyte next_val = find_unused_youngergenP_card_value(); duke@435: set_cur_youngergen_card_val(next_val); duke@435: duke@435: } else { duke@435: // In an sequential traversal we will always write youngergen, so that duke@435: // the inline barrier is correct. duke@435: set_cur_youngergen_card_val(youngergen_card); duke@435: } duke@435: } duke@435: duke@435: void CardTableRS::younger_refs_iterate(Generation* g, duke@435: OopsInGenClosure* blk) { duke@435: _last_cur_val_in_gen[g->level()+1] = cur_youngergen_card_val(); duke@435: g->younger_refs_iterate(blk); duke@435: } duke@435: ysr@2819: inline bool ClearNoncleanCardWrapper::clear_card(jbyte* entry) { ysr@2819: if (_is_par) { ysr@2819: return clear_card_parallel(entry); ysr@2819: } else { ysr@2819: return clear_card_serial(entry); ysr@2819: } ysr@2819: } ysr@2819: ysr@2819: inline bool ClearNoncleanCardWrapper::clear_card_parallel(jbyte* entry) { ysr@2819: while (true) { ysr@2819: // In the parallel case, we may have to do this several times. ysr@2819: jbyte entry_val = *entry; ysr@2819: assert(entry_val != CardTableRS::clean_card_val(), ysr@2819: "We shouldn't be looking at clean cards, and this should " ysr@2819: "be the only place they get cleaned."); ysr@2819: if (CardTableRS::card_is_dirty_wrt_gen_iter(entry_val) ysr@2819: || _ct->is_prev_youngergen_card_val(entry_val)) { ysr@2819: jbyte res = ysr@2819: Atomic::cmpxchg(CardTableRS::clean_card_val(), entry, entry_val); ysr@2819: if (res == entry_val) { ysr@2819: break; ysr@2819: } else { ysr@2819: assert(res == CardTableRS::cur_youngergen_and_prev_nonclean_card, ysr@2819: "The CAS above should only fail if another thread did " ysr@2819: "a GC write barrier."); duke@435: } ysr@2819: } else if (entry_val == ysr@2819: CardTableRS::cur_youngergen_and_prev_nonclean_card) { ysr@2819: // Parallelism shouldn't matter in this case. Only the thread ysr@2819: // assigned to scan the card should change this value. ysr@2819: *entry = _ct->cur_youngergen_card_val(); ysr@2819: break; duke@435: } else { ysr@2819: assert(entry_val == _ct->cur_youngergen_card_val(), ysr@2819: "Should be the only possibility."); ysr@2819: // In this case, the card was clean before, and become ysr@2819: // cur_youngergen only because of processing of a promoted object. ysr@2819: // We don't have to look at the card. ysr@2819: return false; duke@435: } duke@435: } ysr@2819: return true; ysr@2819: } duke@435: ysr@2819: ysr@2819: inline bool ClearNoncleanCardWrapper::clear_card_serial(jbyte* entry) { ysr@2819: jbyte entry_val = *entry; ysr@2819: assert(entry_val != CardTableRS::clean_card_val(), ysr@2819: "We shouldn't be looking at clean cards, and this should " ysr@2819: "be the only place they get cleaned."); ysr@2819: assert(entry_val != CardTableRS::cur_youngergen_and_prev_nonclean_card, ysr@2819: "This should be possible in the sequential case."); ysr@2819: *entry = CardTableRS::clean_card_val(); ysr@2819: return true; ysr@2819: } ysr@2819: ysr@2819: ClearNoncleanCardWrapper::ClearNoncleanCardWrapper( ysr@2889: DirtyCardToOopClosure* dirty_card_closure, CardTableRS* ct) : duke@435: _dirty_card_closure(dirty_card_closure), _ct(ct) { jmasa@3294: // Cannot yet substitute active_workers for n_par_threads jmasa@3294: // in the case where parallelism is being turned off by jmasa@3294: // setting n_par_threads to 0. duke@435: _is_par = (SharedHeap::heap()->n_par_threads() > 0); jmasa@3294: assert(!_is_par || jmasa@3294: (SharedHeap::heap()->n_par_threads() == jmasa@3294: SharedHeap::heap()->workers()->active_workers()), "Mismatch"); ysr@2819: } ysr@2819: brutisso@3642: bool ClearNoncleanCardWrapper::is_word_aligned(jbyte* entry) { brutisso@3642: return (((intptr_t)entry) & (BytesPerWord-1)) == 0; brutisso@3642: } brutisso@3642: ysr@2819: void ClearNoncleanCardWrapper::do_MemRegion(MemRegion mr) { ysr@2819: assert(mr.word_size() > 0, "Error"); ysr@2819: assert(_ct->is_aligned(mr.start()), "mr.start() should be card aligned"); ysr@2819: // mr.end() may not necessarily be card aligned. ysr@2819: jbyte* cur_entry = _ct->byte_for(mr.last()); ysr@2819: const jbyte* limit = _ct->byte_for(mr.start()); ysr@2819: HeapWord* end_of_non_clean = mr.end(); ysr@2819: HeapWord* start_of_non_clean = end_of_non_clean; ysr@2819: while (cur_entry >= limit) { ysr@2819: HeapWord* cur_hw = _ct->addr_for(cur_entry); ysr@2819: if ((*cur_entry != CardTableRS::clean_card_val()) && clear_card(cur_entry)) { ysr@2819: // Continue the dirty range by opening the ysr@2819: // dirty window one card to the left. ysr@2819: start_of_non_clean = cur_hw; ysr@2819: } else { ysr@2819: // We hit a "clean" card; process any non-empty ysr@2819: // "dirty" range accumulated so far. ysr@2819: if (start_of_non_clean < end_of_non_clean) { ysr@2819: const MemRegion mrd(start_of_non_clean, end_of_non_clean); ysr@2819: _dirty_card_closure->do_MemRegion(mrd); ysr@2819: } brutisso@3642: brutisso@3642: // fast forward through potential continuous whole-word range of clean cards beginning at a word-boundary brutisso@3642: if (is_word_aligned(cur_entry)) { brutisso@3642: jbyte* cur_row = cur_entry - BytesPerWord; brutisso@3642: while (cur_row >= limit && *((intptr_t*)cur_row) == CardTableRS::clean_card_row()) { brutisso@3642: cur_row -= BytesPerWord; brutisso@3642: } brutisso@3642: cur_entry = cur_row + BytesPerWord; brutisso@3642: cur_hw = _ct->addr_for(cur_entry); brutisso@3642: } brutisso@3642: ysr@2819: // Reset the dirty window, while continuing to look ysr@2819: // for the next dirty card that will start a ysr@2819: // new dirty window. ysr@2819: end_of_non_clean = cur_hw; ysr@2819: start_of_non_clean = cur_hw; ysr@2819: } ysr@2819: // Note that "cur_entry" leads "start_of_non_clean" in ysr@2819: // its leftward excursion after this point ysr@2819: // in the loop and, when we hit the left end of "mr", ysr@2819: // will point off of the left end of the card-table ysr@2819: // for "mr". ysr@2819: cur_entry--; duke@435: } ysr@2819: // If the first card of "mr" was dirty, we will have ysr@2819: // been left with a dirty window, co-initial with "mr", ysr@2819: // which we now process. ysr@2819: if (start_of_non_clean < end_of_non_clean) { ysr@2819: const MemRegion mrd(start_of_non_clean, end_of_non_clean); ysr@2819: _dirty_card_closure->do_MemRegion(mrd); duke@435: } ysr@2819: } ysr@2819: duke@435: // clean (by dirty->clean before) ==> cur_younger_gen duke@435: // dirty ==> cur_youngergen_and_prev_nonclean_card duke@435: // precleaned ==> cur_youngergen_and_prev_nonclean_card duke@435: // prev-younger-gen ==> cur_youngergen_and_prev_nonclean_card duke@435: // cur-younger-gen ==> cur_younger_gen duke@435: // cur_youngergen_and_prev_nonclean_card ==> no change. coleenp@548: void CardTableRS::write_ref_field_gc_par(void* field, oop new_val) { duke@435: jbyte* entry = ct_bs()->byte_for(field); duke@435: do { duke@435: jbyte entry_val = *entry; duke@435: // We put this first because it's probably the most common case. duke@435: if (entry_val == clean_card_val()) { duke@435: // No threat of contention with cleaning threads. duke@435: *entry = cur_youngergen_card_val(); duke@435: return; duke@435: } else if (card_is_dirty_wrt_gen_iter(entry_val) duke@435: || is_prev_youngergen_card_val(entry_val)) { duke@435: // Mark it as both cur and prev youngergen; card cleaning thread will duke@435: // eventually remove the previous stuff. duke@435: jbyte new_val = cur_youngergen_and_prev_nonclean_card; duke@435: jbyte res = Atomic::cmpxchg(new_val, entry, entry_val); duke@435: // Did the CAS succeed? duke@435: if (res == entry_val) return; duke@435: // Otherwise, retry, to see the new value. duke@435: continue; duke@435: } else { duke@435: assert(entry_val == cur_youngergen_and_prev_nonclean_card duke@435: || entry_val == cur_youngergen_card_val(), duke@435: "should be only possibilities."); duke@435: return; duke@435: } duke@435: } while (true); duke@435: } duke@435: duke@435: void CardTableRS::younger_refs_in_space_iterate(Space* sp, duke@435: OopsInGenClosure* cl) { ysr@2825: const MemRegion urasm = sp->used_region_at_save_marks(); ysr@2825: #ifdef ASSERT ysr@2825: // Convert the assertion check to a warning if we are running ysr@2825: // CMS+ParNew until related bug is fixed. ysr@2825: MemRegion ur = sp->used_region(); ysr@2825: assert(ur.contains(urasm) || (UseConcMarkSweepGC && UseParNewGC), ysr@2825: err_msg("Did you forget to call save_marks()? " ysr@2825: "[" PTR_FORMAT ", " PTR_FORMAT ") is not contained in " ysr@2825: "[" PTR_FORMAT ", " PTR_FORMAT ")", drchase@6680: p2i(urasm.start()), p2i(urasm.end()), p2i(ur.start()), p2i(ur.end()))); ysr@2825: // In the case of CMS+ParNew, issue a warning ysr@2825: if (!ur.contains(urasm)) { ysr@2825: assert(UseConcMarkSweepGC && UseParNewGC, "Tautology: see assert above"); ysr@2825: warning("CMS+ParNew: Did you forget to call save_marks()? " ysr@2825: "[" PTR_FORMAT ", " PTR_FORMAT ") is not contained in " ysr@2825: "[" PTR_FORMAT ", " PTR_FORMAT ")", drchase@6680: p2i(urasm.start()), p2i(urasm.end()), p2i(ur.start()), p2i(ur.end())); ysr@2825: MemRegion ur2 = sp->used_region(); ysr@2825: MemRegion urasm2 = sp->used_region_at_save_marks(); ysr@2825: if (!ur.equals(ur2)) { ysr@2825: warning("CMS+ParNew: Flickering used_region()!!"); ysr@2825: } ysr@2825: if (!urasm.equals(urasm2)) { ysr@2825: warning("CMS+ParNew: Flickering used_region_at_save_marks()!!"); ysr@2825: } ysr@2889: ShouldNotReachHere(); ysr@2825: } ysr@2825: #endif ysr@2889: _ct_bs->non_clean_card_iterate_possibly_parallel(sp, urasm, cl, this); duke@435: } duke@435: brutisso@5519: void CardTableRS::clear_into_younger(Generation* old_gen) { brutisso@5519: assert(old_gen->level() == 1, "Should only be called for the old generation"); brutisso@5519: // The card tables for the youngest gen need never be cleared. duke@435: // There's a bit of subtlety in the clear() and invalidate() duke@435: // methods that we exploit here and in invalidate_or_clear() duke@435: // below to avoid missing cards at the fringes. If clear() or duke@435: // invalidate() are changed in the future, this code should duke@435: // be revisited. 20040107.ysr brutisso@5516: clear(old_gen->prev_used_region()); duke@435: } duke@435: brutisso@5519: void CardTableRS::invalidate_or_clear(Generation* old_gen) { brutisso@5519: assert(old_gen->level() == 1, "Should only be called for the old generation"); brutisso@5519: // Invalidate the cards for the currently occupied part of brutisso@5519: // the old generation and clear the cards for the duke@435: // unoccupied part of the generation (if any, making use duke@435: // of that generation's prev_used_region to determine that duke@435: // region). No need to do anything for the youngest duke@435: // generation. Also see note#20040107.ysr above. brutisso@5519: MemRegion used_mr = old_gen->used_region(); brutisso@5519: MemRegion to_be_cleared_mr = old_gen->prev_used_region().minus(used_mr); brutisso@5516: if (!to_be_cleared_mr.is_empty()) { brutisso@5516: clear(to_be_cleared_mr); duke@435: } brutisso@5516: invalidate(used_mr); duke@435: } duke@435: duke@435: duke@435: class VerifyCleanCardClosure: public OopClosure { coleenp@548: private: coleenp@548: HeapWord* _boundary; coleenp@548: HeapWord* _begin; coleenp@548: HeapWord* _end; coleenp@548: protected: coleenp@548: template void do_oop_work(T* p) { duke@435: HeapWord* jp = (HeapWord*)p; ysr@2710: assert(jp >= _begin && jp < _end, ysr@2710: err_msg("Error: jp " PTR_FORMAT " should be within " ysr@2710: "[_begin, _end) = [" PTR_FORMAT "," PTR_FORMAT ")", drchase@6680: p2i(jp), p2i(_begin), p2i(_end))); ysr@2710: oop obj = oopDesc::load_decode_heap_oop(p); ysr@2710: guarantee(obj == NULL || (HeapWord*)obj >= _boundary, ysr@2710: err_msg("pointer " PTR_FORMAT " at " PTR_FORMAT " on " ysr@2710: "clean card crosses boundary" PTR_FORMAT, drchase@6680: p2i((HeapWord*)obj), p2i(jp), p2i(_boundary))); duke@435: } ysr@2710: coleenp@548: public: coleenp@548: VerifyCleanCardClosure(HeapWord* b, HeapWord* begin, HeapWord* end) : ysr@2710: _boundary(b), _begin(begin), _end(end) { ysr@2710: assert(b <= begin, ysr@2710: err_msg("Error: boundary " PTR_FORMAT " should be at or below begin " PTR_FORMAT, drchase@6680: p2i(b), p2i(begin))); ysr@2710: assert(begin <= end, ysr@2710: err_msg("Error: begin " PTR_FORMAT " should be strictly below end " PTR_FORMAT, drchase@6680: p2i(begin), p2i(end))); ysr@2710: } ysr@2710: coleenp@548: virtual void do_oop(oop* p) { VerifyCleanCardClosure::do_oop_work(p); } coleenp@548: virtual void do_oop(narrowOop* p) { VerifyCleanCardClosure::do_oop_work(p); } duke@435: }; duke@435: duke@435: class VerifyCTSpaceClosure: public SpaceClosure { coleenp@548: private: duke@435: CardTableRS* _ct; duke@435: HeapWord* _boundary; duke@435: public: duke@435: VerifyCTSpaceClosure(CardTableRS* ct, HeapWord* boundary) : duke@435: _ct(ct), _boundary(boundary) {} coleenp@548: virtual void do_space(Space* s) { _ct->verify_space(s, _boundary); } duke@435: }; duke@435: duke@435: class VerifyCTGenClosure: public GenCollectedHeap::GenClosure { duke@435: CardTableRS* _ct; duke@435: public: duke@435: VerifyCTGenClosure(CardTableRS* ct) : _ct(ct) {} duke@435: void do_generation(Generation* gen) { duke@435: // Skip the youngest generation. duke@435: if (gen->level() == 0) return; duke@435: // Normally, we're interested in pointers to younger generations. duke@435: VerifyCTSpaceClosure blk(_ct, gen->reserved().start()); duke@435: gen->space_iterate(&blk, true); duke@435: } duke@435: }; duke@435: duke@435: void CardTableRS::verify_space(Space* s, HeapWord* gen_boundary) { duke@435: // We don't need to do young-gen spaces. duke@435: if (s->end() <= gen_boundary) return; duke@435: MemRegion used = s->used_region(); duke@435: duke@435: jbyte* cur_entry = byte_for(used.start()); duke@435: jbyte* limit = byte_after(used.last()); duke@435: while (cur_entry < limit) { duke@435: if (*cur_entry == CardTableModRefBS::clean_card) { duke@435: jbyte* first_dirty = cur_entry+1; duke@435: while (first_dirty < limit && duke@435: *first_dirty == CardTableModRefBS::clean_card) { duke@435: first_dirty++; duke@435: } duke@435: // If the first object is a regular object, and it has a duke@435: // young-to-old field, that would mark the previous card. duke@435: HeapWord* boundary = addr_for(cur_entry); duke@435: HeapWord* end = (first_dirty >= limit) ? used.end() : addr_for(first_dirty); duke@435: HeapWord* boundary_block = s->block_start(boundary); duke@435: HeapWord* begin = boundary; // Until proven otherwise. duke@435: HeapWord* start_block = boundary_block; // Until proven otherwise. duke@435: if (boundary_block < boundary) { duke@435: if (s->block_is_obj(boundary_block) && s->obj_is_alive(boundary_block)) { duke@435: oop boundary_obj = oop(boundary_block); duke@435: if (!boundary_obj->is_objArray() && duke@435: !boundary_obj->is_typeArray()) { duke@435: guarantee(cur_entry > byte_for(used.start()), duke@435: "else boundary would be boundary_block"); duke@435: if (*byte_for(boundary_block) != CardTableModRefBS::clean_card) { duke@435: begin = boundary_block + s->block_size(boundary_block); duke@435: start_block = begin; duke@435: } duke@435: } duke@435: } duke@435: } duke@435: // Now traverse objects until end. ysr@2710: if (begin < end) { ysr@2710: MemRegion mr(begin, end); ysr@2710: VerifyCleanCardClosure verify_blk(gen_boundary, begin, end); ysr@2710: for (HeapWord* cur = start_block; cur < end; cur += s->block_size(cur)) { ysr@2710: if (s->block_is_obj(cur) && s->obj_is_alive(cur)) { coleenp@4037: oop(cur)->oop_iterate_no_header(&verify_blk, mr); ysr@2710: } duke@435: } duke@435: } duke@435: cur_entry = first_dirty; duke@435: } else { duke@435: // We'd normally expect that cur_youngergen_and_prev_nonclean_card duke@435: // is a transient value, that cannot be in the card table duke@435: // except during GC, and thus assert that: duke@435: // guarantee(*cur_entry != cur_youngergen_and_prev_nonclean_card, duke@435: // "Illegal CT value"); duke@435: // That however, need not hold, as will become clear in the duke@435: // following... duke@435: duke@435: // We'd normally expect that if we are in the parallel case, duke@435: // we can't have left a prev value (which would be different duke@435: // from the current value) in the card table, and so we'd like to duke@435: // assert that: duke@435: // guarantee(cur_youngergen_card_val() == youngergen_card duke@435: // || !is_prev_youngergen_card_val(*cur_entry), duke@435: // "Illegal CT value"); duke@435: // That, however, may not hold occasionally, because of duke@435: // CMS or MSC in the old gen. To wit, consider the duke@435: // following two simple illustrative scenarios: duke@435: // (a) CMS: Consider the case where a large object L duke@435: // spanning several cards is allocated in the old duke@435: // gen, and has a young gen reference stored in it, dirtying duke@435: // some interior cards. A young collection scans the card, duke@435: // finds a young ref and installs a youngergenP_n value. duke@435: // L then goes dead. Now a CMS collection starts, duke@435: // finds L dead and sweeps it up. Assume that L is duke@435: // abutting _unallocated_blk, so _unallocated_blk is duke@435: // adjusted down to (below) L. Assume further that duke@435: // no young collection intervenes during this CMS cycle. duke@435: // The next young gen cycle will not get to look at this duke@435: // youngergenP_n card since it lies in the unoccupied duke@435: // part of the space. duke@435: // Some young collections later the blocks on this duke@435: // card can be re-allocated either due to direct allocation duke@435: // or due to absorbing promotions. At this time, the duke@435: // before-gc verification will fail the above assert. duke@435: // (b) MSC: In this case, an object L with a young reference duke@435: // is on a card that (therefore) holds a youngergen_n value. duke@435: // Suppose also that L lies towards the end of the used duke@435: // the used space before GC. An MSC collection duke@435: // occurs that compacts to such an extent that this duke@435: // card is no longer in the occupied part of the space. duke@435: // Since current code in MSC does not always clear cards duke@435: // in the unused part of old gen, this stale youngergen_n duke@435: // value is left behind and can later be covered by duke@435: // an object when promotion or direct allocation duke@435: // re-allocates that part of the heap. duke@435: // duke@435: // Fortunately, the presence of such stale card values is duke@435: // "only" a minor annoyance in that subsequent young collections duke@435: // might needlessly scan such cards, but would still never corrupt duke@435: // the heap as a result. However, it's likely not to be a significant duke@435: // performance inhibitor in practice. For instance, duke@435: // some recent measurements with unoccupied cards eagerly cleared duke@435: // out to maintain this invariant, showed next to no duke@435: // change in young collection times; of course one can construct duke@435: // degenerate examples where the cost can be significant.) duke@435: // Note, in particular, that if the "stale" card is modified duke@435: // after re-allocation, it would be dirty, not "stale". Thus, duke@435: // we can never have a younger ref in such a card and it is duke@435: // safe not to scan that card in any collection. [As we see duke@435: // below, we do some unnecessary scanning duke@435: // in some cases in the current parallel scanning algorithm.] duke@435: // duke@435: // The main point below is that the parallel card scanning code duke@435: // deals correctly with these stale card values. There are two main duke@435: // cases to consider where we have a stale "younger gen" value and a duke@435: // "derivative" case to consider, where we have a stale duke@435: // "cur_younger_gen_and_prev_non_clean" value, as will become duke@435: // apparent in the case analysis below. duke@435: // o Case 1. If the stale value corresponds to a younger_gen_n duke@435: // value other than the cur_younger_gen value then the code duke@435: // treats this as being tantamount to a prev_younger_gen duke@435: // card. This means that the card may be unnecessarily scanned. duke@435: // There are two sub-cases to consider: duke@435: // o Case 1a. Let us say that the card is in the occupied part duke@435: // of the generation at the time the collection begins. In duke@435: // that case the card will be either cleared when it is scanned duke@435: // for young pointers, or will be set to cur_younger_gen as a duke@435: // result of promotion. (We have elided the normal case where duke@435: // the scanning thread and the promoting thread interleave duke@435: // possibly resulting in a transient duke@435: // cur_younger_gen_and_prev_non_clean value before settling duke@435: // to cur_younger_gen. [End Case 1a.] duke@435: // o Case 1b. Consider now the case when the card is in the unoccupied duke@435: // part of the space which becomes occupied because of promotions duke@435: // into it during the current young GC. In this case the card duke@435: // will never be scanned for young references. The current duke@435: // code will set the card value to either duke@435: // cur_younger_gen_and_prev_non_clean or leave duke@435: // it with its stale value -- because the promotions didn't duke@435: // result in any younger refs on that card. Of these two duke@435: // cases, the latter will be covered in Case 1a during duke@435: // a subsequent scan. To deal with the former case, we need duke@435: // to further consider how we deal with a stale value of duke@435: // cur_younger_gen_and_prev_non_clean in our case analysis duke@435: // below. This we do in Case 3 below. [End Case 1b] duke@435: // [End Case 1] duke@435: // o Case 2. If the stale value corresponds to cur_younger_gen being duke@435: // a value not necessarily written by a current promotion, the duke@435: // card will not be scanned by the younger refs scanning code. duke@435: // (This is OK since as we argued above such cards cannot contain duke@435: // any younger refs.) The result is that this value will be duke@435: // treated as a prev_younger_gen value in a subsequent collection, duke@435: // which is addressed in Case 1 above. [End Case 2] duke@435: // o Case 3. We here consider the "derivative" case from Case 1b. above duke@435: // because of which we may find a stale duke@435: // cur_younger_gen_and_prev_non_clean card value in the table. duke@435: // Once again, as in Case 1, we consider two subcases, depending duke@435: // on whether the card lies in the occupied or unoccupied part duke@435: // of the space at the start of the young collection. duke@435: // o Case 3a. Let us say the card is in the occupied part of duke@435: // the old gen at the start of the young collection. In that duke@435: // case, the card will be scanned by the younger refs scanning duke@435: // code which will set it to cur_younger_gen. In a subsequent duke@435: // scan, the card will be considered again and get its final duke@435: // correct value. [End Case 3a] duke@435: // o Case 3b. Now consider the case where the card is in the duke@435: // unoccupied part of the old gen, and is occupied as a result duke@435: // of promotions during thus young gc. In that case, duke@435: // the card will not be scanned for younger refs. The presence duke@435: // of newly promoted objects on the card will then result in duke@435: // its keeping the value cur_younger_gen_and_prev_non_clean duke@435: // value, which we have dealt with in Case 3 here. [End Case 3b] duke@435: // [End Case 3] duke@435: // duke@435: // (Please refer to the code in the helper class duke@435: // ClearNonCleanCardWrapper and in CardTableModRefBS for details.) duke@435: // duke@435: // The informal arguments above can be tightened into a formal duke@435: // correctness proof and it behooves us to write up such a proof, duke@435: // or to use model checking to prove that there are no lingering duke@435: // concerns. duke@435: // duke@435: // Clearly because of Case 3b one cannot bound the time for duke@435: // which a card will retain what we have called a "stale" value. duke@435: // However, one can obtain a Loose upper bound on the redundant duke@435: // work as a result of such stale values. Note first that any duke@435: // time a stale card lies in the occupied part of the space at duke@435: // the start of the collection, it is scanned by younger refs duke@435: // code and we can define a rank function on card values that duke@435: // declines when this is so. Note also that when a card does not duke@435: // lie in the occupied part of the space at the beginning of a duke@435: // young collection, its rank can either decline or stay unchanged. duke@435: // In this case, no extra work is done in terms of redundant duke@435: // younger refs scanning of that card. duke@435: // Then, the case analysis above reveals that, in the worst case, duke@435: // any such stale card will be scanned unnecessarily at most twice. duke@435: // duke@435: // It is nonethelss advisable to try and get rid of some of this duke@435: // redundant work in a subsequent (low priority) re-design of duke@435: // the card-scanning code, if only to simplify the underlying duke@435: // state machine analysis/proof. ysr 1/28/2002. XXX duke@435: cur_entry++; duke@435: } duke@435: } duke@435: } duke@435: duke@435: void CardTableRS::verify() { duke@435: // At present, we only know how to verify the card table RS for duke@435: // generational heaps. duke@435: VerifyCTGenClosure blk(this); duke@435: CollectedHeap* ch = Universe::heap(); duke@435: duke@435: if (ch->kind() == CollectedHeap::GenCollectedHeap) { duke@435: GenCollectedHeap::heap()->generation_iterate(&blk, false); ysr@777: _ct_bs->verify(); duke@435: } duke@435: } duke@435: duke@435: jmasa@441: void CardTableRS::verify_aligned_region_empty(MemRegion mr) { duke@435: if (!mr.is_empty()) { duke@435: jbyte* cur_entry = byte_for(mr.start()); duke@435: jbyte* limit = byte_after(mr.last()); jmasa@441: // The region mr may not start on a card boundary so jmasa@441: // the first card may reflect a write to the space jmasa@441: // just prior to mr. jmasa@441: if (!is_aligned(mr.start())) { jmasa@441: cur_entry++; jmasa@441: } duke@435: for (;cur_entry < limit; cur_entry++) { duke@435: guarantee(*cur_entry == CardTableModRefBS::clean_card, duke@435: "Unexpected dirty card found"); duke@435: } duke@435: } duke@435: }