diff -r fd16c54261b3 -r 489c9b5090e2 hotspot/src/share/vm/memory/cardTableRS.cpp --- /dev/null Thu Jan 01 00:00:00 1970 +0000 +++ b/hotspot/src/share/vm/memory/cardTableRS.cpp Sat Dec 01 00:00:00 2007 +0000 @@ -0,0 +1,568 @@ +/* + * Copyright 2001-2006 Sun Microsystems, Inc. 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 Sun Microsystems, Inc., 4150 Network Circle, Santa Clara, + * CA 95054 USA or visit www.sun.com if you need additional information or + * have any questions. + * + */ + +# include "incls/_precompiled.incl" +# include "incls/_cardTableRS.cpp.incl" + +CardTableRS::CardTableRS(MemRegion whole_heap, + int max_covered_regions) : + GenRemSet(&_ct_bs), + _ct_bs(whole_heap, max_covered_regions), + _cur_youngergen_card_val(youngergenP1_card) +{ + _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() { + GenCollectedHeap* gch = GenCollectedHeap::heap(); + for (jbyte v = youngergenP1_card; + v < cur_youngergen_and_prev_nonclean_card; + v++) { + bool seen = false; + for (int g = 0; g < gch->n_gens()+1; 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(oop* 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 { + HeapWord* boundary; + HeapWord* begin; HeapWord* end; +public: + void do_oop(oop* p) { + HeapWord* jp = (HeapWord*)p; + if (jp >= begin && jp < end) { + guarantee(*p == NULL || (HeapWord*)p < boundary + || (HeapWord*)(*p) >= boundary, + "pointer on clean card crosses boundary"); + } + } + VerifyCleanCardClosure(HeapWord* b, HeapWord* _begin, HeapWord* _end) : + boundary(b), begin(_begin), end(_end) {} +}; + +class VerifyCTSpaceClosure: public SpaceClosure { + CardTableRS* _ct; + HeapWord* _boundary; +public: + VerifyCTSpaceClosure(CardTableRS* ct, HeapWord* boundary) : + _ct(ct), _boundary(boundary) {} + 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. + HeapWord* cur = start_block; + VerifyCleanCardClosure verify_blk(gen_boundary, begin, end); + while (cur < end) { + if (s->block_is_obj(cur) && s->obj_is_alive(cur)) { + oop(cur)->oop_iterate(&verify_blk); + } + cur += s->block_size(cur); + } + 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_empty(MemRegion mr) { + if (!mr.is_empty()) { + jbyte* cur_entry = byte_for(mr.start()); + jbyte* limit = byte_after(mr.last()); + for (;cur_entry < limit; cur_entry++) { + guarantee(*cur_entry == CardTableModRefBS::clean_card, + "Unexpected dirty card found"); + } + } +}