JVM源码分析之G1垃圾收集器分析一
对 Java 的 gc 实现比较感兴趣，原先一般都是看周志明的书，但其实并没有讲具体的 gc 源码，而是把整个思路和流程讲解了一下
特别是 G1 的具体实现
一般对 G1 的理解其实就是把原先整块的新生代老年代分成了以 region 为单位的小块内存，简而言之，就是原先对新生代老年代的收集会涉及到整个代的堆内存空间，而G1 把它变成了更细致的小块内存
这带来了一个很明显的好处和一个很明显的坏处，好处是内存收集可以更灵活，耗时会变短，但整个收集的处理复杂度就变高了
目前看了一点点关于 G1 收集的预期时间相关的代码1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21HeapWord* G1CollectedHeap::do_collection_pause(size_t word_size,
uint gc_count_before,
bool* succeeded,
GCCause::Cause gc_cause) {
assert_heap_not_locked_and_not_at_safepoint();
VM_G1CollectForAllocation op(word_size,
gc_count_before,
gc_cause,
false, /* should_initiate_conc_mark */
g1_policy()->max_pause_time_ms());
VMThread::execute(&op);
HeapWord* result = op.result();
bool ret_succeeded = op.prologue_succeeded() && op.pause_succeeded();
assert(result == NULL || ret_succeeded,
"the result should be NULL if the VM did not succeed");
*succeeded = ret_succeeded;
assert_heap_not_locked();
return result;
}
这里就是收集时需要停顿的，其中VMThread::execute(&op);是具体执行的，真正执行的是VM_G1CollectForAllocation::doit方法1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85void VM_G1CollectForAllocation::doit() {
G1CollectedHeap* g1h = G1CollectedHeap::heap();
assert(!_should_initiate_conc_mark || g1h->should_do_concurrent_full_gc(_gc_cause),
"only a GC locker, a System.gc(), stats update, whitebox, or a hum allocation induced GC should start a cycle");
if (_word_size > 0) {
// An allocation has been requested. So, try to do that first.
_result = g1h->attempt_allocation_at_safepoint(_word_size,
false /* expect_null_cur_alloc_region */);
if (_result != NULL) {
// If we can successfully allocate before we actually do the
// pause then we will consider this pause successful.
_pause_succeeded = true;
return;
}
}
GCCauseSetter x(g1h, _gc_cause);
if (_should_initiate_conc_mark) {
// It's safer to read old_marking_cycles_completed() here, given
// that noone else will be updating it concurrently. Since we'll
// only need it if we're initiating a marking cycle, no point in
// setting it earlier.
_old_marking_cycles_completed_before = g1h->old_marking_cycles_completed();
// At this point we are supposed to start a concurrent cycle. We
// will do so if one is not already in progress.
bool res = g1h->g1_policy()->force_initial_mark_if_outside_cycle(_gc_cause);
// The above routine returns true if we were able to force the
// next GC pause to be an initial mark; it returns false if a
// marking cycle is already in progress.
//
// If a marking cycle is already in progress just return and skip the
// pause below - if the reason for requesting this initial mark pause
// was due to a System.gc() then the requesting thread should block in
// doit_epilogue() until the marking cycle is complete.
//
// If this initial mark pause was requested as part of a humongous
// allocation then we know that the marking cycle must just have
// been started by another thread (possibly also allocating a humongous
// object) as there was no active marking cycle when the requesting
// thread checked before calling collect() in
// attempt_allocation_humongous(). Retrying the GC, in this case,
// will cause the requesting thread to spin inside collect() until the
// just started marking cycle is complete - which may be a while. So
// we do NOT retry the GC.
if (!res) {
assert(_word_size == 0, "Concurrent Full GC/Humongous Object IM shouldn't be allocating");
if (_gc_cause != GCCause::_g1_humongous_allocation) {
_should_retry_gc = true;
}
return;
}
}
// Try a partial collection of some kind.
_pause_succeeded = g1h->do_collection_pause_at_safepoint(_target_pause_time_ms);
if (_pause_succeeded) {
if (_word_size > 0) {
// An allocation had been requested. Do it, eventually trying a stronger
// kind of GC.
_result = g1h->satisfy_failed_allocation(_word_size, &_pause_succeeded);
} else {
bool should_upgrade_to_full = !g1h->should_do_concurrent_full_gc(_gc_cause) &&
!g1h->has_regions_left_for_allocation();
if (should_upgrade_to_full) {
// There has been a request to perform a GC to free some space. We have no
// information on how much memory has been asked for. In case there are
// absolutely no regions left to allocate into, do a maximally compacting full GC.
log_info(gc, ergo)("Attempting maximally compacting collection");
_pause_succeeded = g1h->do_full_collection(false, /* explicit gc */
true /* clear_all_soft_refs */);
}
}
guarantee(_pause_succeeded, "Elevated collections during the safepoint must always succeed.");
} else {
assert(_result == NULL, "invariant");
// The only reason for the pause to not be successful is that, the GC locker is
// active (or has become active since the prologue was executed). In this case
// we should retry the pause after waiting for the GC locker to become inactive.
_should_retry_gc = true;
}
}
这里可以看到核心的是G1CollectedHeap::do_collection_pause_at_safepoint这个方法，它带上了目标暂停时间的值1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330G1CollectedHeap::do_collection_pause_at_safepoint(double target_pause_time_ms) {
assert_at_safepoint_on_vm_thread();
guarantee(!is_gc_active(), "collection is not reentrant");
if (GCLocker::check_active_before_gc()) {
return false;
}
_gc_timer_stw->register_gc_start();
GCIdMark gc_id_mark;
_gc_tracer_stw->report_gc_start(gc_cause(), _gc_timer_stw->gc_start());
SvcGCMarker sgcm(SvcGCMarker::MINOR);
ResourceMark rm;
g1_policy()->note_gc_start();
wait_for_root_region_scanning();
print_heap_before_gc();
print_heap_regions();
trace_heap_before_gc(_gc_tracer_stw);
_verifier->verify_region_sets_optional();
_verifier->verify_dirty_young_regions();
// We should not be doing initial mark unless the conc mark thread is running
if (!_cm_thread->should_terminate()) {
// This call will decide whether this pause is an initial-mark
// pause. If it is, in_initial_mark_gc() will return true
// for the duration of this pause.
g1_policy()->decide_on_conc_mark_initiation();
}
// We do not allow initial-mark to be piggy-backed on a mixed GC.
assert(!collector_state()->in_initial_mark_gc() ||
collector_state()->in_young_only_phase(), "sanity");
// We also do not allow mixed GCs during marking.
assert(!collector_state()->mark_or_rebuild_in_progress() || collector_state()->in_young_only_phase(), "sanity");
// Record whether this pause is an initial mark. When the current
// thread has completed its logging output and it's safe to signal
// the CM thread, the flag's value in the policy has been reset.
bool should_start_conc_mark = collector_state()->in_initial_mark_gc();
// Inner scope for scope based logging, timers, and stats collection
{
EvacuationInfo evacuation_info;
if (collector_state()->in_initial_mark_gc()) {
// We are about to start a marking cycle, so we increment the
// full collection counter.
increment_old_marking_cycles_started();
_cm->gc_tracer_cm()->set_gc_cause(gc_cause());
}
_gc_tracer_stw->report_yc_type(collector_state()->yc_type());
GCTraceCPUTime tcpu;
G1HeapVerifier::G1VerifyType verify_type;
FormatBuffer<> gc_string("Pause Young ");
if (collector_state()->in_initial_mark_gc()) {
gc_string.append("(Concurrent Start)");
verify_type = G1HeapVerifier::G1VerifyConcurrentStart;
} else if (collector_state()->in_young_only_phase()) {
if (collector_state()->in_young_gc_before_mixed()) {
gc_string.append("(Prepare Mixed)");
} else {
gc_string.append("(Normal)");
}
verify_type = G1HeapVerifier::G1VerifyYoungNormal;
} else {
gc_string.append("(Mixed)");
verify_type = G1HeapVerifier::G1VerifyMixed;
}
GCTraceTime(Info, gc) tm(gc_string, NULL, gc_cause(), true);
uint active_workers = AdaptiveSizePolicy::calc_active_workers(workers()->total_workers(),
workers()->active_workers(),
Threads::number_of_non_daemon_threads());
active_workers = workers()->update_active_workers(active_workers);
log_info(gc,task)("Using %u workers of %u for evacuation", active_workers, workers()->total_workers());
TraceCollectorStats tcs(g1mm()->incremental_collection_counters());
TraceMemoryManagerStats tms(&_memory_manager, gc_cause(),
collector_state()->yc_type() == Mixed /* allMemoryPoolsAffected */);
G1HeapTransition heap_transition(this);
size_t heap_used_bytes_before_gc = used();
// Don't dynamically change the number of GC threads this early. A value of
// 0 is used to indicate serial work. When parallel work is done,
// it will be set.
{ // Call to jvmpi::post_class_unload_events must occur outside of active GC
IsGCActiveMark x;
gc_prologue(false);
if (VerifyRememberedSets) {
log_info(gc, verify)("[Verifying RemSets before GC]");
VerifyRegionRemSetClosure v_cl;
heap_region_iterate(&v_cl);
}
_verifier->verify_before_gc(verify_type);
_verifier->check_bitmaps("GC Start");
DerivedPointerTable::clear();
// Please see comment in g1CollectedHeap.hpp and
// G1CollectedHeap::ref_processing_init() to see how
// reference processing currently works in G1.
// Enable discovery in the STW reference processor
_ref_processor_stw->enable_discovery();
{
// We want to temporarily turn off discovery by the
// CM ref processor, if necessary, and turn it back on
// on again later if we do. Using a scoped
// NoRefDiscovery object will do this.
NoRefDiscovery no_cm_discovery(_ref_processor_cm);
// Forget the current alloc region (we might even choose it to be part
// of the collection set!).
_allocator->release_mutator_alloc_region();
// This timing is only used by the ergonomics to handle our pause target.
// It is unclear why this should not include the full pause. We will
// investigate this in CR 7178365.
//
// Preserving the old comment here if that helps the investigation:
//
// The elapsed time induced by the start time below deliberately elides
// the possible verification above.
double sample_start_time_sec = os::elapsedTime();
g1_policy()->record_collection_pause_start(sample_start_time_sec);
if (collector_state()->in_initial_mark_gc()) {
concurrent_mark()->pre_initial_mark();
}
g1_policy()->finalize_collection_set(target_pause_time_ms, &_survivor);
evacuation_info.set_collectionset_regions(collection_set()->region_length());
// Make sure the remembered sets are up to date. This needs to be
// done before register_humongous_regions_with_cset(), because the
// remembered sets are used there to choose eager reclaim candidates.
// If the remembered sets are not up to date we might miss some
// entries that need to be handled.
g1_rem_set()->cleanupHRRS();
register_humongous_regions_with_cset();
assert(_verifier->check_cset_fast_test(), "Inconsistency in the InCSetState table.");
// We call this after finalize_cset() to
// ensure that the CSet has been finalized.
_cm->verify_no_cset_oops();
if (_hr_printer.is_active()) {
G1PrintCollectionSetClosure cl(&_hr_printer);
_collection_set.iterate(&cl);
}
// Initialize the GC alloc regions.
_allocator->init_gc_alloc_regions(evacuation_info);
G1ParScanThreadStateSet per_thread_states(this, workers()->active_workers(), collection_set()->young_region_length());
pre_evacuate_collection_set();
// Actually do the work...
evacuate_collection_set(&per_thread_states);
post_evacuate_collection_set(evacuation_info, &per_thread_states);
const size_t* surviving_young_words = per_thread_states.surviving_young_words();
free_collection_set(&_collection_set, evacuation_info, surviving_young_words);
eagerly_reclaim_humongous_regions();
record_obj_copy_mem_stats();
_survivor_evac_stats.adjust_desired_plab_sz();
_old_evac_stats.adjust_desired_plab_sz();
double start = os::elapsedTime();
start_new_collection_set();
g1_policy()->phase_times()->record_start_new_cset_time_ms((os::elapsedTime() - start) * 1000.0);
if (evacuation_failed()) {
set_used(recalculate_used());
if (_archive_allocator != NULL) {
_archive_allocator->clear_used();
}
for (uint i = 0; i < ParallelGCThreads; i++) {
if (_evacuation_failed_info_array[i].has_failed()) {
_gc_tracer_stw->report_evacuation_failed(_evacuation_failed_info_array[i]);
}
}
} else {
// The "used" of the the collection set have already been subtracted
// when they were freed. Add in the bytes evacuated.
increase_used(g1_policy()->bytes_copied_during_gc());
}
if (collector_state()->in_initial_mark_gc()) {
// We have to do this before we notify the CM threads that
// they can start working to make sure that all the
// appropriate initialization is done on the CM object.
concurrent_mark()->post_initial_mark();
// Note that we don't actually trigger the CM thread at
// this point. We do that later when we're sure that
// the current thread has completed its logging output.
}
allocate_dummy_regions();
_allocator->init_mutator_alloc_region();
{
size_t expand_bytes = _heap_sizing_policy->expansion_amount();
if (expand_bytes > 0) {
size_t bytes_before = capacity();
// No need for an ergo logging here,
// expansion_amount() does this when it returns a value > 0.
double expand_ms;
if (!expand(expand_bytes, _workers, &expand_ms)) {
// We failed to expand the heap. Cannot do anything about it.
}
g1_policy()->phase_times()->record_expand_heap_time(expand_ms);
}
}
// We redo the verification but now wrt to the new CSet which
// has just got initialized after the previous CSet was freed.
_cm->verify_no_cset_oops();
// This timing is only used by the ergonomics to handle our pause target.
// It is unclear why this should not include the full pause. We will
// investigate this in CR 7178365.
double sample_end_time_sec = os::elapsedTime();
double pause_time_ms = (sample_end_time_sec - sample_start_time_sec) * MILLIUNITS;
size_t total_cards_scanned = g1_policy()->phase_times()->sum_thread_work_items(G1GCPhaseTimes::ScanRS, G1GCPhaseTimes::ScanRSScannedCards);
g1_policy()->record_collection_pause_end(pause_time_ms, total_cards_scanned, heap_used_bytes_before_gc);
evacuation_info.set_collectionset_used_before(collection_set()->bytes_used_before());
evacuation_info.set_bytes_copied(g1_policy()->bytes_copied_during_gc());
if (VerifyRememberedSets) {
log_info(gc, verify)("[Verifying RemSets after GC]");
VerifyRegionRemSetClosure v_cl;
heap_region_iterate(&v_cl);
}
_verifier->verify_after_gc(verify_type);
_verifier->check_bitmaps("GC End");
assert(!_ref_processor_stw->discovery_enabled(), "Postcondition");
_ref_processor_stw->verify_no_references_recorded();
// CM reference discovery will be re-enabled if necessary.
}
ParallelTaskTerminator::print_termination_counts();
gc_epilogue(false);
}
// Print the remainder of the GC log output.
if (evacuation_failed()) {
log_info(gc)("To-space exhausted");
}
g1_policy()->print_phases();
heap_transition.print();
// It is not yet to safe to tell the concurrent mark to
// start as we have some optional output below. We don't want the
// output from the concurrent mark thread interfering with this
// logging output either.
_hrm.verify_optional();
_verifier->verify_region_sets_optional();
TASKQUEUE_STATS_ONLY(print_taskqueue_stats());
TASKQUEUE_STATS_ONLY(reset_taskqueue_stats());
print_heap_after_gc();
print_heap_regions();
trace_heap_after_gc(_gc_tracer_stw);
// We must call G1MonitoringSupport::update_sizes() in the same scoping level
// as an active TraceMemoryManagerStats object (i.e. before the destructor for the
// TraceMemoryManagerStats is called) so that the G1 memory pools are updated
// before any GC notifications are raised.
g1mm()->update_sizes();
_gc_tracer_stw->report_evacuation_info(&evacuation_info);
_gc_tracer_stw->report_tenuring_threshold(_g1_policy->tenuring_threshold());
_gc_timer_stw->register_gc_end();
_gc_tracer_stw->report_gc_end(_gc_timer_stw->gc_end(), _gc_timer_stw->time_partitions());
}
// It should now be safe to tell the concurrent mark thread to start
// without its logging output interfering with the logging output
// that came from the pause.
if (should_start_conc_mark) {
// CAUTION: after the doConcurrentMark() call below,
// the concurrent marking thread(s) could be running
// concurrently with us. Make sure that anything after
// this point does not assume that we are the only GC thread
// running. Note: of course, the actual marking work will
// not start until the safepoint itself is released in
// SuspendibleThreadSet::desynchronize().
do_concurrent_mark();
}
return true;
}
往下走就是这一步G1Policy::finalize_collection_set,去处理新生代和老年代1
2
3
4void G1Policy::finalize_collection_set(double target_pause_time_ms, G1SurvivorRegions* survivor) {
double time_remaining_ms = _collection_set->finalize_young_part(target_pause_time_ms, survivor);
_collection_set->finalize_old_part(time_remaining_ms);
}
这里分别调用了两个方法，可以看到剩余时间是往下传的，来看一下具体的方法1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43double G1CollectionSet::finalize_young_part(double target_pause_time_ms, G1SurvivorRegions* survivors) {
double young_start_time_sec = os::elapsedTime();
finalize_incremental_building();
guarantee(target_pause_time_ms > 0.0,
"target_pause_time_ms = %1.6lf should be positive", target_pause_time_ms);
size_t pending_cards = _policy->pending_cards();
double base_time_ms = _policy->predict_base_elapsed_time_ms(pending_cards);
double time_remaining_ms = MAX2(target_pause_time_ms - base_time_ms, 0.0);
log_trace(gc, ergo, cset)("Start choosing CSet. pending cards: " SIZE_FORMAT " predicted base time: %1.2fms remaining time: %1.2fms target pause time: %1.2fms",
pending_cards, base_time_ms, time_remaining_ms, target_pause_time_ms);
// The young list is laid with the survivor regions from the previous
// pause are appended to the RHS of the young list, i.e.
// [Newly Young Regions ++ Survivors from last pause].
uint survivor_region_length = survivors->length();
uint eden_region_length = _g1h->eden_regions_count();
init_region_lengths(eden_region_length, survivor_region_length);
verify_young_cset_indices();
// Clear the fields that point to the survivor list - they are all young now.
survivors->convert_to_eden();
_bytes_used_before = _inc_bytes_used_before;
time_remaining_ms = MAX2(time_remaining_ms - _inc_predicted_elapsed_time_ms, 0.0);
log_trace(gc, ergo, cset)("Add young regions to CSet. eden: %u regions, survivors: %u regions, predicted young region time: %1.2fms, target pause time: %1.2fms",
eden_region_length, survivor_region_length, _inc_predicted_elapsed_time_ms, target_pause_time_ms);
// The number of recorded young regions is the incremental
// collection set's current size
set_recorded_rs_lengths(_inc_recorded_rs_lengths);
double young_end_time_sec = os::elapsedTime();
phase_times()->record_young_cset_choice_time_ms((young_end_time_sec - young_start_time_sec) * 1000.0);
return time_remaining_ms;
}
下面是老年代的部分1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100void G1CollectionSet::finalize_old_part(double time_remaining_ms) {
double non_young_start_time_sec = os::elapsedTime();
double predicted_old_time_ms = 0.0;
if (collector_state()->in_mixed_phase()) {
cset_chooser()->verify();
const uint min_old_cset_length = _policy->calc_min_old_cset_length();
const uint max_old_cset_length = _policy->calc_max_old_cset_length();
uint expensive_region_num = 0;
bool check_time_remaining = _policy->adaptive_young_list_length();
HeapRegion* hr = cset_chooser()->peek();
while (hr != NULL) {
if (old_region_length() >= max_old_cset_length) {
// Added maximum number of old regions to the CSet.
log_debug(gc, ergo, cset)("Finish adding old regions to CSet (old CSet region num reached max). old %u regions, max %u regions",
old_region_length(), max_old_cset_length);
break;
}
// Stop adding regions if the remaining reclaimable space is
// not above G1HeapWastePercent.
size_t reclaimable_bytes = cset_chooser()->remaining_reclaimable_bytes();
double reclaimable_percent = _policy->reclaimable_bytes_percent(reclaimable_bytes);
double threshold = (double) G1HeapWastePercent;
if (reclaimable_percent <= threshold) {
// We've added enough old regions that the amount of uncollected
// reclaimable space is at or below the waste threshold. Stop
// adding old regions to the CSet.
log_debug(gc, ergo, cset)("Finish adding old regions to CSet (reclaimable percentage not over threshold). "
"old %u regions, max %u regions, reclaimable: " SIZE_FORMAT "B (%1.2f%%) threshold: " UINTX_FORMAT "%%",
old_region_length(), max_old_cset_length, reclaimable_bytes, reclaimable_percent, G1HeapWastePercent);
break;
}
double predicted_time_ms = predict_region_elapsed_time_ms(hr);
if (check_time_remaining) {
if (predicted_time_ms > time_remaining_ms) {
// Too expensive for the current CSet.
if (old_region_length() >= min_old_cset_length) {
// We have added the minimum number of old regions to the CSet,
// we are done with this CSet.
log_debug(gc, ergo, cset)("Finish adding old regions to CSet (predicted time is too high). "
"predicted time: %1.2fms, remaining time: %1.2fms old %u regions, min %u regions",
predicted_time_ms, time_remaining_ms, old_region_length(), min_old_cset_length);
break;
}
// We'll add it anyway given that we haven't reached the
// minimum number of old regions.
expensive_region_num += 1;
}
} else {
if (old_region_length() >= min_old_cset_length) {
// In the non-auto-tuning case, we'll finish adding regions
// to the CSet if we reach the minimum.
log_debug(gc, ergo, cset)("Finish adding old regions to CSet (old CSet region num reached min). old %u regions, min %u regions",
old_region_length(), min_old_cset_length);
break;
}
}
// We will add this region to the CSet.
time_remaining_ms = MAX2(time_remaining_ms - predicted_time_ms, 0.0);
predicted_old_time_ms += predicted_time_ms;
cset_chooser()->pop(); // already have region via peek()
_g1h->old_set_remove(hr);
add_old_region(hr);
hr = cset_chooser()->peek();
}
if (hr == NULL) {
log_debug(gc, ergo, cset)("Finish adding old regions to CSet (candidate old regions not available)");
}
if (expensive_region_num > 0) {
// We print the information once here at the end, predicated on
// whether we added any apparently expensive regions or not, to
// avoid generating output per region.
log_debug(gc, ergo, cset)("Added expensive regions to CSet (old CSet region num not reached min)."
"old: %u regions, expensive: %u regions, min: %u regions, remaining time: %1.2fms",
old_region_length(), expensive_region_num, min_old_cset_length, time_remaining_ms);
}
cset_chooser()->verify();
}
stop_incremental_building();
log_debug(gc, ergo, cset)("Finish choosing CSet. old: %u regions, predicted old region time: %1.2fms, time remaining: %1.2f",
old_region_length(), predicted_old_time_ms, time_remaining_ms);
double non_young_end_time_sec = os::elapsedTime();
phase_times()->record_non_young_cset_choice_time_ms((non_young_end_time_sec - non_young_start_time_sec) * 1000.0);
QuickSort::sort(_collection_set_regions, _collection_set_cur_length, compare_region_idx, true);
}
上面第三行是个判断，当前是否是 mixed 回收阶段，如果不是的话其实是没有老年代什么事的，所以可以看到代码基本是从这个 if 判断if (collector_state()->in_mixed_phase()) {开始往下走的
先写到这，偏向于做笔记用，有错轻拍
再看这个图，我们发现在这的时候还没有进行替换
好像是这里了
，图中一共有四个槽的存储空间，依次访问顺序是 A B C D E D F，
，这样就不会被覆盖
+]]>
,假如我的 U 盘的盘符是
就可以搜索到占用这个盘符下文件的进程，这里千万小心‼️‼️，不可轻易杀掉这些进程，有些系统进程如果轻易杀掉会导致蓝屏等问题，不可轻易尝试，除非能确认这些进程的作用。
，这个可以看到当前最近 Windows 发生的事件，打开这个后就点击U盘推出，因为推不出来也是一种错误事件，点击下刷新就能在这看到具体是因为什么推出不了，具体的进程信息
-
