// 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++;
}
}
}
