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src/share/vm/gc_implementation/g1/g1CollectedHeap.cpp
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rev 7323 : 8069367: Eagerly reclaimed humongous objects left on mark stack
Summary: Prevent eager reclaim of objects that might be on mark stack.
Reviewed-by: brutisso, tschatzl
@@ -1851,11 +1851,11 @@
_refine_cte_cl(NULL),
_full_collection(false),
_secondary_free_list("Secondary Free List", new SecondaryFreeRegionListMtSafeChecker()),
_old_set("Old Set", false /* humongous */, new OldRegionSetMtSafeChecker()),
_humongous_set("Master Humongous Set", true /* humongous */, new HumongousRegionSetMtSafeChecker()),
- _humongous_is_live(),
+ _humongous_reclaim_candidates(),
_has_humongous_reclaim_candidates(false),
_free_regions_coming(false),
_young_list(new YoungList(this)),
_gc_time_stamp(0),
_survivor_plab_stats(YoungPLABSize, PLABWeight),
@@ -2046,12 +2046,18 @@
_bot_shared = new G1BlockOffsetSharedArray(_reserved, bot_storage);
_g1h = this;
- _in_cset_fast_test.initialize(_hrm.reserved().start(), _hrm.reserved().end(), HeapRegion::GrainBytes);
- _humongous_is_live.initialize(_hrm.reserved().start(), _hrm.reserved().end(), HeapRegion::GrainBytes);
+ {
+ HeapWord* start = _hrm.reserved().start();
+ HeapWord* end = _hrm.reserved().end();
+ size_t granularity = HeapRegion::GrainBytes;
+
+ _in_cset_fast_test.initialize(start, end, granularity);
+ _humongous_reclaim_candidates.initialize(start, end, granularity);
+ }
// Create the ConcurrentMark data structure and thread.
// (Must do this late, so that "max_regions" is defined.)
_cm = new ConcurrentMark(this, prev_bitmap_storage, next_bitmap_storage);
if (_cm == NULL || !_cm->completed_initialization()) {
@@ -2139,15 +2145,10 @@
if (G1StringDedup::is_enabled()) {
G1StringDedup::stop();
}
}
-void G1CollectedHeap::clear_humongous_is_live_table() {
- guarantee(G1EagerReclaimHumongousObjects, "Should only be called if true");
- _humongous_is_live.clear();
-}
-
size_t G1CollectedHeap::conservative_max_heap_alignment() {
return HeapRegion::max_region_size();
}
void G1CollectedHeap::ref_processing_init() {
@@ -3664,31 +3665,70 @@
size_t G1CollectedHeap::cards_scanned() {
return g1_rem_set()->cardsScanned();
}
-bool G1CollectedHeap::humongous_region_is_always_live(uint index) {
- HeapRegion* region = region_at(index);
- assert(region->startsHumongous(), "Must start a humongous object");
- return oop(region->bottom())->is_objArray() || !region->rem_set()->is_empty();
-}
-
class RegisterHumongousWithInCSetFastTestClosure : public HeapRegionClosure {
private:
size_t _total_humongous;
size_t _candidate_humongous;
DirtyCardQueue _dcq;
- bool humongous_region_is_candidate(uint index) {
- HeapRegion* region = G1CollectedHeap::heap()->region_at(index);
- assert(region->startsHumongous(), "Must start a humongous object");
+ // We don't nominate objects with many remembered set entries, on
+ // the assumption that such objects are likely still live.
+ bool is_remset_small(HeapRegion* region) const {
HeapRegionRemSet* const rset = region->rem_set();
- bool const allow_stale_refs = G1EagerReclaimHumongousObjectsWithStaleRefs;
- return !oop(region->bottom())->is_objArray() &&
- ((allow_stale_refs && rset->occupancy_less_or_equal_than(G1RSetSparseRegionEntries)) ||
- (!allow_stale_refs && rset->is_empty()));
+ return G1EagerReclaimHumongousObjectsWithStaleRefs
+ ? rset->occupancy_less_or_equal_than(G1RSetSparseRegionEntries)
+ : rset->is_empty();
+ }
+
+ bool is_typeArray_region(HeapRegion* region) const {
+ return oop(region->bottom())->is_typeArray();
+ }
+
+ bool humongous_region_is_candidate(G1CollectedHeap* heap, HeapRegion* region) const {
+ assert(region->startsHumongous(), "Must start a humongous object");
+
+ // Candidate selection must satisfy the following constraints
+ // while concurrent marking is in progress:
+ //
+ // * In order to maintain SATB invariants, an object must not be
+ // reclaimed if it was allocated before the start of marking and
+ // has not had its references scanned. Such an object must have
+ // its references (including type metadata) scanned to ensure no
+ // live objects are missed by the marking process. Objects
+ // allocated after the start of concurrent marking don't need to
+ // be scanned.
+ //
+ // * An object must not be reclaimed if it is on the concurrent
+ // mark stack. Objects allocated after the start of concurrent
+ // marking are never pushed on the mark stack.
+ //
+ // Nominating only objects allocated after the start of concurrent
+ // marking is sufficient to meet both constraints. This may miss
+ // some objects that satisfy the constraints, but the marking data
+ // structures don't support efficiently performing the needed
+ // additional tests or scrubbing of the mark stack.
+ //
+ // However, we presently only nominate is_typeArray() objects.
+ // A humongous object containing references induces remembered
+ // set entries on other regions. In order to reclaim such an
+ // object, those remembered sets would need to be cleaned up.
+ //
+ // We also treat is_typeArray() objects specially, allowing them
+ // to be reclaimed even if allocated before the start of
+ // concurrent mark. For this we rely on mark stack insertion to
+ // exclude is_typeArray() objects, preventing reclaiming an object
+ // that is in the mark stack. We also rely on the metadata for
+ // such objects to be built-in and so ensured to be kept live.
+ // Frequent allocation and drop of large binary blobs is an
+ // important use case for eager reclaim, and this special handling
+ // may reduce needed headroom.
+
+ return is_typeArray_region(region) && is_remset_small(region);
}
public:
RegisterHumongousWithInCSetFastTestClosure()
: _total_humongous(0),
@@ -3700,18 +3740,21 @@
if (!r->startsHumongous()) {
return false;
}
G1CollectedHeap* g1h = G1CollectedHeap::heap();
- uint region_idx = r->hrm_index();
- bool is_candidate = humongous_region_is_candidate(region_idx);
+ bool is_candidate = humongous_region_is_candidate(g1h, r);
+ uint rindex = r->hrm_index();
+ g1h->set_humongous_reclaim_candidate(rindex, is_candidate);
+ if (is_candidate) {
+ _candidate_humongous++;
+ g1h->register_humongous_region_with_in_cset_fast_test(rindex);
// Is_candidate already filters out humongous object with large remembered sets.
// If we have a humongous object with a few remembered sets, we simply flush these
// remembered set entries into the DCQS. That will result in automatic
// re-evaluation of their remembered set entries during the following evacuation
// phase.
- if (is_candidate) {
if (!r->rem_set()->is_empty()) {
guarantee(r->rem_set()->occupancy_less_or_equal_than(G1RSetSparseRegionEntries),
"Found a not-small remembered set here. This is inconsistent with previous assumptions.");
G1SATBCardTableLoggingModRefBS* bs = g1h->g1_barrier_set();
HeapRegionRemSetIterator hrrs(r->rem_set());
@@ -3724,12 +3767,10 @@
}
}
r->rem_set()->clear_locked();
}
assert(r->rem_set()->is_empty(), "At this point any humongous candidate remembered set must be empty.");
- g1h->register_humongous_region_with_in_cset_fast_test(region_idx);
- _candidate_humongous++;
}
_total_humongous++;
return false;
}
@@ -3745,23 +3786,20 @@
g1_policy()->phase_times()->record_fast_reclaim_humongous_stats(0.0, 0, 0);
return;
}
double time = os::elapsed_counter();
+ // Collect reclaim candidate information and register candidates with cset.
RegisterHumongousWithInCSetFastTestClosure cl;
heap_region_iterate(&cl);
time = ((double)(os::elapsed_counter() - time) / os::elapsed_frequency()) * 1000.0;
g1_policy()->phase_times()->record_fast_reclaim_humongous_stats(time,
cl.total_humongous(),
cl.candidate_humongous());
_has_humongous_reclaim_candidates = cl.candidate_humongous() > 0;
- if (_has_humongous_reclaim_candidates || G1TraceEagerReclaimHumongousObjects) {
- clear_humongous_is_live_table();
- }
-
// Finally flush all remembered set entries to re-check into the global DCQS.
cl.flush_rem_set_entries();
}
void
@@ -6319,45 +6357,46 @@
// - never consider object arrays at this time because they would pose
// considerable effort for cleaning up the the remembered sets. This is
// required because stale remembered sets might reference locations that
// are currently allocated into.
uint region_idx = r->hrm_index();
- if (g1h->humongous_is_live(region_idx) ||
- g1h->humongous_region_is_always_live(region_idx)) {
+ if (!g1h->is_humongous_reclaim_candidate(region_idx) ||
+ !r->rem_set()->is_empty()) {
if (G1TraceEagerReclaimHumongousObjects) {
- gclog_or_tty->print_cr("Live humongous region %u size "SIZE_FORMAT" start "PTR_FORMAT" length "UINT32_FORMAT" with remset "SIZE_FORMAT" code roots "SIZE_FORMAT" is marked %d live-other %d obj array %d",
+ gclog_or_tty->print_cr("Live humongous region %u size "SIZE_FORMAT" start "PTR_FORMAT" length "UINT32_FORMAT" with remset "SIZE_FORMAT" code roots "SIZE_FORMAT" is marked %d reclaim candidate %d type array %d",
region_idx,
obj->size()*HeapWordSize,
r->bottom(),
r->region_num(),
r->rem_set()->occupied(),
r->rem_set()->strong_code_roots_list_length(),
next_bitmap->isMarked(r->bottom()),
- g1h->humongous_is_live(region_idx),
- obj->is_objArray()
+ g1h->is_humongous_reclaim_candidate(region_idx),
+ obj->is_typeArray()
);
}
return false;
}
- guarantee(!obj->is_objArray(),
- err_msg("Eagerly reclaiming object arrays is not supported, but the object "PTR_FORMAT" is.",
+ guarantee(obj->is_typeArray(),
+ err_msg("Only eagerly reclaiming type arrays is supported, but the object "
+ PTR_FORMAT " is not.",
r->bottom()));
if (G1TraceEagerReclaimHumongousObjects) {
- gclog_or_tty->print_cr("Dead humongous region %u size "SIZE_FORMAT" start "PTR_FORMAT" length "UINT32_FORMAT" with remset "SIZE_FORMAT" code roots "SIZE_FORMAT" is marked %d live-other %d obj array %d",
+ gclog_or_tty->print_cr("Dead humongous region %u size "SIZE_FORMAT" start "PTR_FORMAT" length "UINT32_FORMAT" with remset "SIZE_FORMAT" code roots "SIZE_FORMAT" is marked %d reclaim candidate %d type array %d",
region_idx,
obj->size()*HeapWordSize,
r->bottom(),
r->region_num(),
r->rem_set()->occupied(),
r->rem_set()->strong_code_roots_list_length(),
next_bitmap->isMarked(r->bottom()),
- g1h->humongous_is_live(region_idx),
- obj->is_objArray()
+ g1h->is_humongous_reclaim_candidate(region_idx),
+ obj->is_typeArray()
);
}
// Need to clear mark bit of the humongous object if already set.
if (next_bitmap->isMarked(r->bottom())) {
next_bitmap->clear(r->bottom());
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