// 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); }
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); }
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 set_bs(_ct_bs); _last_cur_val_in_gen = NEW_C_HEAP_ARRAY3(jbyte, GenCollectedHeap::max_gens + 1, mtGC, 0, 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); }
bool G1SATBCardTableModRefBS::mark_card_deferred(size_t card_index) { jbyte val = _byte_map[card_index]; // It's already processed if ((val & (clean_card_mask_val() | deferred_card_val())) == deferred_card_val()) { return false; } if (val == g1_young_gen) { // the card is for a young gen region. We don't need to keep track of all pointers into young return false; } // Cached bit can be installed either on a clean card or on a claimed card. jbyte new_val = val; if (val == clean_card_val()) { new_val = (jbyte)deferred_card_val(); } else { if (val & claimed_card_val()) { new_val = val | (jbyte)deferred_card_val(); } } if (new_val != val) { Atomic::cmpxchg(new_val, &_byte_map[card_index], val); } return true; }
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); }
CardTableRS::CardTableRS(MemRegion whole_heap) : _bs(NULL), _cur_youngergen_card_val(youngergenP1_card) { _ct_bs = new CardTableModRefBSForCTRS(whole_heap); _ct_bs->initialize(); set_bs(_ct_bs); // max_gens is really GenCollectedHeap::heap()->gen_policy()->number_of_generations() // (which is always 2, young & old), but GenCollectedHeap has not been initialized yet. uint max_gens = 2; _last_cur_val_in_gen = NEW_C_HEAP_ARRAY3(jbyte, 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 (uint i = 0; i < max_gens + 1; i++) { _last_cur_val_in_gen[i] = clean_card_val(); } _ct_bs->set_CTRS(this); }
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 == clean_card_val()) { jbyte* first_dirty = cur_entry+1; while (first_dirty < limit && *first_dirty == clean_card_val()) { 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) != clean_card_val()) { 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 "young 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 nonetheless 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++; } } }