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8252752: Clear card table for old regions during scan in G1

Reviewed-by: kbarrett, iwalulya, ayang
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Thomas Schatzl
Thomas Schatzl committed Sep 28, 2020
1 parent 276fcee commit e9c178276ffdb7d8152e75bb54e478ac1228f71d
@@ -84,6 +84,7 @@ class G1CardTable : public CardTable {

static CardValue g1_young_card_val() { return g1_young_gen; }
static CardValue g1_scanned_card_val() { return g1_card_already_scanned; }

void verify_g1_young_region(MemRegion mr) PRODUCT_RETURN;
void g1_mark_as_young(const MemRegion& mr);
@@ -103,8 +104,8 @@ class G1CardTable : public CardTable {
// be inaccurate as it does not perform the dirtying atomically.
inline size_t mark_region_dirty(size_t start_card_index, size_t num_cards);

// Mark the given range of cards as Scanned. All of these cards must be Dirty.
inline void mark_as_scanned(size_t start_card_index, size_t num_cards);
// Change the given range of dirty cards to "which". All of these cards must be Dirty.
inline void change_dirty_cards_to(size_t start_card_index, size_t num_cards, CardValue which);

inline uint region_idx_for(CardValue* p);

@@ -77,14 +77,14 @@ inline size_t G1CardTable::mark_region_dirty(size_t start_card_index, size_t num
return result;

inline void G1CardTable::mark_as_scanned(size_t start_card_index, size_t num_cards) {
inline void G1CardTable::change_dirty_cards_to(size_t start_card_index, size_t num_cards, CardValue which) {
CardValue* start = &_byte_map[start_card_index];
CardValue* const end = start + num_cards;
while (start < end) {
CardValue value = *start;
assert(value == dirty_card_val(),
"Must have been dirty %d start " PTR_FORMAT " " PTR_FORMAT, value, p2i(start), p2i(end));
*start++ = g1_card_already_scanned;
*start++ = which;

@@ -3045,10 +3045,11 @@ void G1CollectedHeap::do_collection_pause_at_safepoint_helper(double target_paus
pre_evacuate_collection_set(evacuation_info, &per_thread_states);

bool may_do_optional_evacuation = _collection_set.optional_region_length() != 0;
// Actually do the work...
evacuate_initial_collection_set(&per_thread_states, may_do_optional_evacuation);

if (_collection_set.optional_region_length() != 0) {
if (may_do_optional_evacuation) {
post_evacuate_collection_set(evacuation_info, &rdcqs, &per_thread_states);
@@ -3814,10 +3815,11 @@ class G1EvacuateRegionsBaseTask : public AbstractGangTask {

class G1EvacuateRegionsTask : public G1EvacuateRegionsBaseTask {
G1RootProcessor* _root_processor;
bool _has_optional_evacuation_work;

void scan_roots(G1ParScanThreadState* pss, uint worker_id) {
_root_processor->evacuate_roots(pss, worker_id);
_g1h->rem_set()->scan_heap_roots(pss, worker_id, G1GCPhaseTimes::ScanHR, G1GCPhaseTimes::ObjCopy);
_g1h->rem_set()->scan_heap_roots(pss, worker_id, G1GCPhaseTimes::ScanHR, G1GCPhaseTimes::ObjCopy, _has_optional_evacuation_work);
_g1h->rem_set()->scan_collection_set_regions(pss, worker_id, G1GCPhaseTimes::ScanHR, G1GCPhaseTimes::CodeRoots, G1GCPhaseTimes::ObjCopy);

@@ -3838,13 +3840,16 @@ class G1EvacuateRegionsTask : public G1EvacuateRegionsBaseTask {
G1ParScanThreadStateSet* per_thread_states,
G1ScannerTasksQueueSet* task_queues,
G1RootProcessor* root_processor,
uint num_workers) :
uint num_workers,
bool has_optional_evacuation_work) :
G1EvacuateRegionsBaseTask("G1 Evacuate Regions", per_thread_states, task_queues, num_workers),
{ }

void G1CollectedHeap::evacuate_initial_collection_set(G1ParScanThreadStateSet* per_thread_states) {
void G1CollectedHeap::evacuate_initial_collection_set(G1ParScanThreadStateSet* per_thread_states,
bool has_optional_evacuation_work) {
G1GCPhaseTimes* p = phase_times();

@@ -3859,7 +3864,12 @@ void G1CollectedHeap::evacuate_initial_collection_set(G1ParScanThreadStateSet* p
Ticks start_processing = Ticks::now();
G1RootProcessor root_processor(this, num_workers);
G1EvacuateRegionsTask g1_par_task(this, per_thread_states, _task_queues, &root_processor, num_workers);
G1EvacuateRegionsTask g1_par_task(this,
task_time = run_task_timed(&g1_par_task);
// Closing the inner scope will execute the destructor for the G1RootProcessor object.
// To extract its code root fixup time we measure total time of this scope and
@@ -3869,12 +3879,14 @@ void G1CollectedHeap::evacuate_initial_collection_set(G1ParScanThreadStateSet* p

p->record_initial_evac_time(task_time.seconds() * 1000.0);
p->record_or_add_code_root_fixup_time((total_processing - task_time).seconds() * 1000.0);


class G1EvacuateOptionalRegionsTask : public G1EvacuateRegionsBaseTask {

void scan_roots(G1ParScanThreadState* pss, uint worker_id) {
_g1h->rem_set()->scan_heap_roots(pss, worker_id, G1GCPhaseTimes::OptScanHR, G1GCPhaseTimes::OptObjCopy);
_g1h->rem_set()->scan_heap_roots(pss, worker_id, G1GCPhaseTimes::OptScanHR, G1GCPhaseTimes::OptObjCopy, true /* remember_already_scanned_cards */);
_g1h->rem_set()->scan_collection_set_regions(pss, worker_id, G1GCPhaseTimes::OptScanHR, G1GCPhaseTimes::OptCodeRoots, G1GCPhaseTimes::OptObjCopy);

@@ -3934,6 +3946,8 @@ void G1CollectedHeap::evacuate_optional_collection_set(G1ParScanThreadStateSet*
phase_times()->record_or_add_optional_evac_time((Ticks::now() - start).seconds() * 1000.0);

rem_set()->complete_evac_phase(true /* has_more_than_one_evacuation_phase */);

@@ -785,7 +785,23 @@ class G1CollectedHeap : public CollectedHeap {
void calculate_collection_set(G1EvacuationInfo& evacuation_info, double target_pause_time_ms);

// Actually do the work of evacuating the parts of the collection set.
void evacuate_initial_collection_set(G1ParScanThreadStateSet* per_thread_states);
// The has_optional_evacuation_work flag for the initial collection set
// evacuation indicates whether one or more optional evacuation steps may
// follow.
// If not set, G1 can avoid clearing the card tables of regions that we scan
// for roots from the heap: when scanning the card table for dirty cards after
// all remembered sets have been dumped onto it, for optional evacuation we
// mark these cards as "Scanned" to know that we do not need to re-scan them
// in the additional optional evacuation passes. This means that in the "Clear
// Card Table" phase we need to clear those marks. However, if there is no
// optional evacuation, g1 can immediately clean the dirty cards it encounters
// as nobody else will be looking at them again, saving the clear card table
// work later.
// This case is very common (young only collections and most mixed gcs), so
// depending on the ratio between scanned and evacuated regions (which g1 always
// needs to clear), this is a big win.
void evacuate_initial_collection_set(G1ParScanThreadStateSet* per_thread_states,
bool has_optional_evacuation_work);
void evacuate_optional_collection_set(G1ParScanThreadStateSet* per_thread_states);
// Evacuate the next set of optional regions.
@@ -131,8 +131,17 @@ class G1RemSetScanState : public CHeapObj<mtGC> {

// The complete set of regions which card table needs to be cleared at the end of GC because
// we scribbled all over them.
// The complete set of regions which card table needs to be cleared at the end
// of GC because we scribbled over these card tables.
// Regions may be added for two reasons:
// - they were part of the collection set: they may contain g1_young_card_val
// or regular card marks that we never scan so we must always clear their card
// table
// - or in case g1 does an optional evacuation pass, g1 marks the cards in there
// as g1_scanned_card_val. If G1 only did an initial evacuation pass, the
// scanning already cleared these cards. In that case they are not in this set
// at the end of the collection.
G1DirtyRegions* _all_dirty_regions;
// The set of regions which card table needs to be scanned for new dirty cards
// in the current evacuation pass.
@@ -321,16 +330,22 @@ class G1RemSetScanState : public CHeapObj<mtGC> {

void prepare_for_merge_heap_roots() {
assert(_next_dirty_regions->size() == 0, "next dirty regions must be empty");

for (size_t i = 0; i < _max_reserved_regions; i++) {
_card_table_scan_state[i] = 0;

::memset(_region_scan_chunks, false, _num_total_scan_chunks * sizeof(*_region_scan_chunks));

void complete_evac_phase(bool merge_dirty_regions) {
if (merge_dirty_regions) {

// Returns whether the given region contains cards we need to scan. The remembered
// set and other sources may contain cards that
// - are in uncommitted regions
@@ -374,8 +389,6 @@ class G1RemSetScanState : public CHeapObj<mtGC> {

void cleanup(WorkGang* workers) {


delete _all_dirty_regions;
@@ -448,7 +461,7 @@ class G1RemSetScanState : public CHeapObj<mtGC> {
#ifdef ASSERT
HeapRegion* hr = G1CollectedHeap::heap()->region_at(region);
"Only add young regions to all dirty regions directly but %u is %s",
"Only add collection set regions to all dirty regions directly but %u is %s",
hr->hrm_index(), hr->get_short_type_str());
@@ -641,6 +654,7 @@ class G1ScanHRForRegionClosure : public HeapRegionClosure {
// The address to which this thread already scanned (walked the heap) up to during
// card scanning (exclusive).
HeapWord* _scanned_to;
G1CardTable::CardValue _scanned_card_value;

HeapWord* scan_memregion(uint region_idx_for_card, MemRegion mr) {
HeapRegion* const card_region = _g1h->region_at(region_idx_for_card);
@@ -677,7 +691,7 @@ class G1ScanHRForRegionClosure : public HeapRegionClosure {

ALWAYSINLINE void do_card_block(uint const region_idx, size_t const first_card, size_t const num_cards) {
_ct->mark_as_scanned(first_card, num_cards);
_ct->change_dirty_cards_to(first_card, num_cards, _scanned_card_value);
do_claimed_block(region_idx, first_card, num_cards);
@@ -727,7 +741,8 @@ class G1ScanHRForRegionClosure : public HeapRegionClosure {
G1ScanHRForRegionClosure(G1RemSetScanState* scan_state,
G1ParScanThreadState* pss,
uint worker_id,
G1GCPhaseTimes::GCParPhases phase) :
G1GCPhaseTimes::GCParPhases phase,
bool remember_already_scanned_cards) :
@@ -740,7 +755,9 @@ class G1ScanHRForRegionClosure : public HeapRegionClosure {
_scanned_to(NULL) {
_scanned_card_value(remember_already_scanned_cards ? G1CardTable::g1_scanned_card_val()
: G1CardTable::clean_card_val()) {

bool do_heap_region(HeapRegion* r) {
@@ -765,10 +782,11 @@ class G1ScanHRForRegionClosure : public HeapRegionClosure {

void G1RemSet::scan_heap_roots(G1ParScanThreadState* pss,
uint worker_id,
G1GCPhaseTimes::GCParPhases scan_phase,
G1GCPhaseTimes::GCParPhases objcopy_phase) {
G1ScanHRForRegionClosure cl(_scan_state, pss, worker_id, scan_phase);
uint worker_id,
G1GCPhaseTimes::GCParPhases scan_phase,
G1GCPhaseTimes::GCParPhases objcopy_phase,
bool remember_already_scanned_cards) {
G1ScanHRForRegionClosure cl(_scan_state, pss, worker_id, scan_phase, remember_already_scanned_cards);
_scan_state->iterate_dirty_regions_from(&cl, worker_id);

G1GCPhaseTimes* p = _g1p->phase_times();
@@ -891,18 +909,11 @@ void G1RemSet::scan_collection_set_regions(G1ParScanThreadState* pss,
void G1RemSet::prepare_region_for_scan(HeapRegion* region) {
uint hrm_index = region->hrm_index();

if (region->in_collection_set()) {
// Young regions had their card table marked as young at their allocation;
// we need to make sure that these marks are cleared at the end of GC, *but*
// they should not be scanned for cards.
// So directly add them to the "all_dirty_regions".
// Same for regions in the (initial) collection set: they may contain cards from
// the log buffers, make sure they are cleaned.
} else if (region->is_old_or_humongous_or_archive()) {
if (region->is_old_or_humongous_or_archive()) {
_scan_state->set_scan_top(hrm_index, region->top());
} else {
assert(region->is_free(), "Should only be free region at this point %s", region->get_type_str());
assert(region->in_collection_set() || region->is_free(),
"Should only be free or in the collection set at this point %s", region->get_type_str());

@@ -984,13 +995,34 @@ class G1MergeHeapRootsTask : public AbstractGangTask {

virtual bool do_heap_region(HeapRegion* r) {
// Helper to put the remembered set cards for these regions onto the card
// table.
// Called directly for humongous starts regions because we should not add
// humongous eager reclaim candidates to the "all" list of regions to
// clear the card table by default as we do not know yet whether this region
// will be reclaimed (and reused).
// If the humongous region contains dirty cards, g1 will scan them
// because dumping the remembered set entries onto the card table will add
// the humongous region to the "dirty" region list to scan. Then scanning
// either clears the card during scan (if there is only an initial evacuation
// pass) or the "dirty" list will be merged with the "all" list later otherwise.
// (And there is no problem either way if the region does not contain dirty
// cards).
void dump_rem_set_for_region(HeapRegion* r) {
assert(r->in_collection_set() || r->is_starts_humongous(), "must be");

HeapRegionRemSet* rem_set = r->rem_set();
if (!rem_set->is_empty()) {

virtual bool do_heap_region(HeapRegion* r) {
assert(r->in_collection_set(), "must be");


return false;
@@ -1022,7 +1054,7 @@ class G1MergeHeapRootsTask : public AbstractGangTask {
"Found a not-small remembered set here. This is inconsistent with previous assumptions.");


// We should only clear the card based remembered set here as we will not
// implicitly rebuild anything else during eager reclaim. Note that at the moment
@@ -1239,6 +1271,10 @@ void G1RemSet::merge_heap_roots(bool initial_evacuation) {

void G1RemSet::complete_evac_phase(bool has_more_than_one_evacuation_phase) {

void G1RemSet::exclude_region_from_scan(uint region_idx) {
@@ -84,13 +84,15 @@ class G1RemSet: public CHeapObj<mtGC> {
void scan_heap_roots(G1ParScanThreadState* pss,
uint worker_id,
G1GCPhaseTimes::GCParPhases scan_phase,
G1GCPhaseTimes::GCParPhases objcopy_phase);
G1GCPhaseTimes::GCParPhases objcopy_phase,
bool remember_already_scanned_cards);

// Merge cards from various sources (remembered sets, hot card cache, log buffers)
// and calculate the cards that need to be scanned later (via scan_heap_roots()).
// If initial_evacuation is set, this is called during the initial evacuation.
void merge_heap_roots(bool initial_evacuation);

void complete_evac_phase(bool has_more_than_one_evacuation_phase);
// Prepare for and cleanup after scanning the heap roots. Must be called
// once before and after in sequential code.
void prepare_for_scan_heap_roots();

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