/* * Copyright (c) 2001, 2010, 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" #ifndef SERIALGC #include "gc_implementation/g1/concurrentMark.hpp" #include "gc_implementation/g1/g1SATBCardTableModRefBS.hpp" #endif CardTableRS::CardTableRS(MemRegion whole_heap, int max_covered_regions) : GenRemSet(), _cur_youngergen_card_val(youngergenP1_card), _regions_to_iterate(max_covered_regions - 1) { #ifndef SERIALGC 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 set_bs(_ct_bs); _last_cur_val_in_gen = new jbyte[GenCollectedHeap::max_gens + 1]; if (_last_cur_val_in_gen == NULL) { vm_exit_during_initialization("Could not 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); } 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); } class ClearNoncleanCardWrapper: public MemRegionClosure { MemRegionClosure* _dirty_card_closure; CardTableRS* _ct; bool _is_par; private: // Clears the given card, return true if the corresponding card should be // processed. bool clear_card(jbyte* entry) { if (_is_par) { 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; } else { 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; } } public: ClearNoncleanCardWrapper(MemRegionClosure* dirty_card_closure, CardTableRS* ct) : _dirty_card_closure(dirty_card_closure), _ct(ct) { _is_par = (SharedHeap::heap()->n_par_threads() > 0); } void do_MemRegion(MemRegion mr) { // We start at the high end of "mr", walking backwards // while accumulating a contiguous dirty range of cards in // [start_of_non_clean, end_of_non_clean) which we then // process en masse. HeapWord* end_of_non_clean = mr.end(); HeapWord* start_of_non_clean = end_of_non_clean; jbyte* entry = _ct->byte_for(mr.last()); const jbyte* first_entry = _ct->byte_for(mr.start()); while (entry >= first_entry) { HeapWord* cur = _ct->addr_for(entry); if (!clear_card(entry)) { // We hit a clean card; process any non-empty // dirty range accumulated so far. 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); } // Reset the dirty window while continuing to // look for the next dirty window to process. end_of_non_clean = cur; start_of_non_clean = end_of_non_clean; } // Open the left end of the window one card to the left. start_of_non_clean = cur; // Note that "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". 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) { MemRegion mr2(start_of_non_clean, end_of_non_clean); _dirty_card_closure->do_MemRegion(mr2); } } }; // 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) { DirtyCardToOopClosure* dcto_cl = sp->new_dcto_cl(cl, _ct_bs->precision(), cl->gen_boundary()); ClearNoncleanCardWrapper clear_cl(dcto_cl, this); _ct_bs->non_clean_card_iterate(sp, sp->used_region_at_save_marks(), dcto_cl, &clear_cl, false); } void CardTableRS::clear_into_younger(Generation* gen, bool clear_perm) { GenCollectedHeap* gch = GenCollectedHeap::heap(); // Generations younger than gen have been evacuated. We can clear // card table entries for gen (we know that it has no pointers // to younger gens) and for those below. The card tables for // the youngest gen need never be cleared, and those for perm gen // will be cleared based on the parameter clear_perm. // 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 Generation* g = gen; for(Generation* prev_gen = gch->prev_gen(g); prev_gen != NULL; g = prev_gen, prev_gen = gch->prev_gen(g)) { MemRegion to_be_cleared_mr = g->prev_used_region(); clear(to_be_cleared_mr); } // Clear perm gen cards if asked to do so. if (clear_perm) { MemRegion to_be_cleared_mr = gch->perm_gen()->prev_used_region(); clear(to_be_cleared_mr); } } void CardTableRS::invalidate_or_clear(Generation* gen, bool younger, bool perm) { GenCollectedHeap* gch = GenCollectedHeap::heap(); // For each generation gen (and younger and/or perm) // invalidate the cards for the currently occupied part // of that 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. Generation* g = gen; for(Generation* prev_gen = gch->prev_gen(g); prev_gen != NULL; g = prev_gen, prev_gen = gch->prev_gen(g)) { MemRegion used_mr = g->used_region(); MemRegion to_be_cleared_mr = g->prev_used_region().minus(used_mr); if (!to_be_cleared_mr.is_empty()) { clear(to_be_cleared_mr); } invalidate(used_mr); if (!younger) break; } // Clear perm gen cards if asked to do so. if (perm) { g = gch->perm_gen(); MemRegion used_mr = g->used_region(); MemRegion to_be_cleared_mr = g->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 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 ")", _begin, _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, (HeapWord*)obj, jp, _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, b, begin)); assert(begin <= end, err_msg("Error: begin " PTR_FORMAT " should be strictly below end " PTR_FORMAT, begin, 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(&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(); // We will do the perm-gen portion of the card table, too. Generation* pg = SharedHeap::heap()->perm_gen(); HeapWord* pg_boundary = pg->reserved().start(); if (ch->kind() == CollectedHeap::GenCollectedHeap) { GenCollectedHeap::heap()->generation_iterate(&blk, false); _ct_bs->verify(); // If the old gen collections also collect perm, then we are only // interested in perm-to-young pointers, not perm-to-old pointers. GenCollectedHeap* gch = GenCollectedHeap::heap(); CollectorPolicy* cp = gch->collector_policy(); if (cp->is_mark_sweep_policy() || cp->is_concurrent_mark_sweep_policy()) { pg_boundary = gch->get_gen(1)->reserved().start(); } } VerifyCTSpaceClosure perm_space_blk(this, pg_boundary); SharedHeap::heap()->perm_gen()->space_iterate(&perm_space_blk, true); } 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"); } } }