Pause Mixed 的逻辑也是封装在VM_G1IncCollectionPause 中，所以本文的重点是分析触发 Pause Mixed 的条件，以及 Pause Mixed 相对于 Young 的不同点。
1. 触发条件
// hotspot/share/gc/g1/g1CollectedHeap.cpp#L2960
G1HeapVerifier::G1VerifyType verify_type;
FormatBuffer<> gc_string("Pause ");
if (collector_state()->during_initial_mark_pause()) {
gc_string.append("Initial Mark");
verify_type = G1HeapVerifier::G1VerifyInitialMark;
} else if (collector_state()->gcs_are_young()) {
gc_string.append("Young");
verify_type = G1HeapVerifier::G1VerifyYoungOnly;
} else {
gc_string.append("Mixed");
verify_type = G1HeapVerifier::G1VerifyMixed;
}
对于 Initial Mark 这里就不重复了，所以区别 Young 和 Mixed 的参数就是 gcs_are_young，该参数赋值的逻辑在上次 Pause 结束前调用的 record_collection_pause_end 方法中。
首先我们通过注释了解一下几个关键参数：
// hotspot/share/gc/g1/g1CollectorState.hpp#L35
// Indicates whether we are in "full young" or "mixed" GC mode.
bool _gcs_are_young;
// Was the last GC "young"?
bool _last_gc_was_young;
// Is this the "last young GC" before we start doing mixed GCs?
// Set after a concurrent mark has completed.
bool _last_young_gc;
在record_collection_pause_end方法中：
// hotspot/share/gc/g1/g1DefaultPolicy.cpp#L622
if (collector_state()->last_young_gc()) {
// This is supposed to to be the "last young GC" before we start
// doing mixed GCs. Here we decide whether to start mixed GCs or not.
assert(!last_pause_included_initial_mark, "The last young GC is not allowed to be an initial mark GC");
if (next_gc_should_be_mixed("start mixed GCs",
"do not start mixed GCs")) {
collector_state()->set_gcs_are_young(false);
} else {
// We aborted the mixed GC phase early.
abort_time_to_mixed_tracking();
}
collector_state()->set_last_young_gc(false);
}
if (!collector_state()->last_gc_was_young()) {
// This is a mixed GC. Here we decide whether to continue doing
// mixed GCs or not.
if (!next_gc_should_be_mixed("continue mixed GCs",
"do not continue mixed GCs")) {
collector_state()->set_gcs_are_young(true);
maybe_start_marking();
}
}
- 如果这是执行 mixed 之前最后一次 yong，那么调用
next_gc_should_be_mixed判断下次是否应该开始 mixed； - 如果这次是 mixed ，那么调用
next_gc_should_be_mixed判断下次是否应该继续 mixed。
继续分析 next_gc_should_be_mixed 的判断逻辑：
// hotspot/share/gc/g1/g1DefaultPolicy.cpp#L1073
bool G1DefaultPolicy::next_gc_should_be_mixed(const char* true_action_str,
const char* false_action_str) const {
if (cset_chooser()->is_empty()) {
log_debug(gc, ergo)("%s (candidate old regions not available)", false_action_str);
return false;
}
// Is the amount of uncollected reclaimable space above G1HeapWastePercent?
size_t reclaimable_bytes = cset_chooser()->remaining_reclaimable_bytes();
double reclaimable_percent = reclaimable_bytes_percent(reclaimable_bytes);
double threshold = (double) G1HeapWastePercent;
if (reclaimable_percent <= threshold) {
log_debug(gc, ergo)("%s (reclaimable percentage not over threshold). candidate old regions: %u reclaimable: " SIZE_FORMAT " (%1.2f) threshold: " UINTX_FORMAT,
false_action_str, cset_chooser()->remaining_regions(), reclaimable_bytes, reclaimable_percent, G1HeapWastePercent);
return false;
}
log_debug(gc, ergo)("%s (candidate old regions available). candidate old regions: %u reclaimable: " SIZE_FORMAT " (%1.2f) threshold: " UINTX_FORMAT,
true_action_str, cset_chooser()->remaining_regions(), reclaimable_bytes, reclaimable_percent, G1HeapWastePercent);
return true;
}
- 首先候选的老年代 region 回收集合不为空；
- 未收集的可回收空间的比例（注：相对于 committed 的大小）大于 G1HeapWastePercent。
last_young_gc 这个参数的设置在 Concurrent Mark 完成之后。
// hotspot/share/gc/g1/g1DefaultPolicy.cpp#L503
void G1DefaultPolicy::record_concurrent_mark_cleanup_completed() {
bool should_continue_with_reclaim = next_gc_should_be_mixed("request last young-only gc",
"skip last young-only gc");
collector_state()->set_last_young_gc(should_continue_with_reclaim);
// We skip the marking phase.
if (!should_continue_with_reclaim) {
abort_time_to_mixed_tracking();
}
collector_state()->set_in_marking_window(false);
}
调用 next_gc_should_be_mixed 判断是否应该执行 mixed，如果满足条件那么设置 last_young_gc 为 true。
2. 操作
Pause Mixed 相对于 Young GC 的不同点主要是 cset 的选择。
// hotspot/share/gc/g1/g1CollectionSet.cpp#L410
void 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()->gcs_are_young()) {
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()
_g1->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);
}
从 cset_chooser() 中获取老年代的 region 加入 cset，并且限制了每次加入 cset 的 region 个数。
而 cset_chooser 的构建发生在 Cleanup 这个 STW 操作中。
// hotspot/share/gc/g1/g1DefaultPolicy.cpp#L996
void G1DefaultPolicy::record_concurrent_mark_cleanup_end() {
cset_chooser()->rebuild(_g1->workers(), _g1->num_regions());
double end_sec = os::elapsedTime();
double elapsed_time_ms = (end_sec - _mark_cleanup_start_sec) * 1000.0;
_analytics->report_concurrent_mark_cleanup_times_ms(elapsed_time_ms);
_analytics->append_prev_collection_pause_end_ms(elapsed_time_ms);
record_pause(Cleanup, _mark_cleanup_start_sec, end_sec);
}
加入 cset 的条件是 region 中存活空间小于 HeapRegion::GrainBytes * (size_t) G1MixedGCLiveThresholdPercent / 100