1. 简介

当晋升失败、疏散失败、大对象分配失败、Evac失败时，有可能触发Full GC，在JDK10之前，Full GC是串行的，JEP 307: Parallel Full GC for G1之后引入了并行Full GC。本文主要介绍并行Full GC机制。

2. 源码分析

Full GC的入口在g1CollectedHeap.cpp的G1CollectedHeap::do_full_collection

bool G1CollectedHeap::do_full_collection(bool explicit_gc,bool clear_all_soft_refs) {assert_at_safepoint_on_vm_thread();if (GCLocker::check_active_before_gc()) {// Full GC was not completed.return false;}const bool do_clear_all_soft_refs = clear_all_soft_refs ||soft_ref_policy()->should_clear_all_soft_refs();G1FullCollector collector(this, explicit_gc, do_clear_all_soft_refs);GCTraceTime(Info, gc) tm("Pause Full", NULL, gc_cause(), true);collector.prepare_collection();collector.collect();collector.complete_collection();// Full collection was successfully completed.return true;
}

准备回收，prepare_collection
回收，collect
回收后处理，complete_collection

2.1 准备阶段

void G1FullCollector::prepare_collection() {_heap->g1_policy()->record_full_collection_start();_heap->print_heap_before_gc();_heap->print_heap_regions();_heap->abort_concurrent_cycle();_heap->verify_before_full_collection(scope()->is_explicit_gc());_heap->gc_prologue(true);_heap->prepare_heap_for_full_collection();reference_processor()->enable_discovery();reference_processor()->setup_policy(scope()->should_clear_soft_refs());// When collecting the permanent generation Method*s may be moving,// so we either have to flush all bcp data or convert it into bci.CodeCache::gc_prologue();// We should save the marks of the currently locked biased monitors.// The marking doesn't preserve the marks of biased objects.BiasedLocking::preserve_marks();// Clear and activate derived pointer collection.clear_and_activate_derived_pointers();
}

Full GC应当清理软引用
由于Full GC过程中，永久代（元空间）中的方法可能被移动，需要保存bcp字节码指针数据或者转化为bci字节码索引
保存轻量级锁和重量级锁的对象头
清理和处理对象的派生关系

2.2 回收阶段

void G1FullCollector::collect() {phase1_mark_live_objects();verify_after_marking();// Don't add any more derived pointers during later phasesdeactivate_derived_pointers();phase2_prepare_compaction();phase3_adjust_pointers();phase4_do_compaction();
}

phase1 并行标记对象
phase2 并行准备压缩
phase3 并行调整指针
phase4 并行压缩

2.2.1 并行标记

从GC roots出发，递归标记所有的活跃对象。

void G1FullCollector::phase1_mark_live_objects() {// Recursively traverse all live objects and mark them.GCTraceTime(Info, gc, phases) info("Phase 1: Mark live objects", scope()->timer());// Do the actual marking.G1FullGCMarkTask marking_task(this);run_task(&marking_task);// Process references discovered during marking.G1FullGCReferenceProcessingExecutor reference_processing(this);reference_processing.execute(scope()->timer(), scope()->tracer());// Weak oops cleanup.{GCTraceTime(Debug, gc, phases) debug("Phase 1: Weak Processing", scope()->timer());WeakProcessor::weak_oops_do(_heap->workers(), &_is_alive, &do_nothing_cl, 1);}// Class unloading and cleanup.if (ClassUnloading) {GCTraceTime(Debug, gc, phases) debug("Phase 1: Class Unloading and Cleanup", scope()->timer());// Unload classes and purge the SystemDictionary.bool purged_class = SystemDictionary::do_unloading(scope()->timer());_heap->complete_cleaning(&_is_alive, purged_class);} else {GCTraceTime(Debug, gc, phases) debug("Phase 1: String and Symbol Tables Cleanup", scope()->timer());// If no class unloading just clean out strings._heap->partial_cleaning(&_is_alive, true, G1StringDedup::is_enabled());}scope()->tracer()->report_object_count_after_gc(&_is_alive);
}

标记对象，具体逻辑在G1FullGCMarkTask中
清理弱引用
卸载类的元数据（complete_cleaning）或仅清理字符串（partial_cleaning）
清理字符串会清理StringTable和字符串去重（JEP 192: String Deduplication in G1）

G1FullGCMarkTask

void G1FullGCMarkTask::work(uint worker_id) {Ticks start = Ticks::now();ResourceMark rm;G1FullGCMarker* marker = collector()->marker(worker_id);MarkingCodeBlobClosure code_closure(marker->mark_closure(), !CodeBlobToOopClosure::FixRelocations);if (ClassUnloading) {_root_processor.process_strong_roots(marker->mark_closure(),marker->cld_closure(),&code_closure);} else {_root_processor.process_all_roots_no_string_table(marker->mark_closure(),marker->cld_closure(),&code_closure);}// Mark stack is populated, now process and drain it.marker->complete_marking(collector()->oop_queue_set(), collector()->array_queue_set(), _terminator.terminator());// This is the point where the entire marking should have completed.assert(marker->oop_stack()->is_empty(), "Marking should have completed");assert(marker->objarray_stack()->is_empty(), "Array marking should have completed");log_task("Marking task", worker_id, start);
}void G1RootProcessor::process_strong_roots(OopClosure* oops,CLDClosure* clds,CodeBlobClosure* blobs) {StrongRootsClosures closures(oops, clds, blobs);process_java_roots(&closures, NULL, 0);process_vm_roots(&closures, NULL, 0);_process_strong_tasks.all_tasks_completed(n_workers());
}void G1RootProcessor::process_all_roots(OopClosure* oops,CLDClosure* clds,CodeBlobClosure* blobs,bool process_string_table) {AllRootsClosures closures(oops, clds);process_java_roots(&closures, NULL, 0);process_vm_roots(&closures, NULL, 0);if (process_string_table) {process_string_table_roots(&closures, NULL, 0);}process_code_cache_roots(blobs, NULL, 0);_process_strong_tasks.all_tasks_completed(n_workers());
}void G1RootProcessor::process_all_roots(OopClosure* oops,CLDClosure* clds,CodeBlobClosure* blobs) {process_all_roots(oops, clds, blobs, true);
}void G1RootProcessor::process_all_roots_no_string_table(OopClosure* oops,CLDClosure* clds,CodeBlobClosure* blobs) {assert(!ClassUnloading, "Should only be used when class unloading is disabled");process_all_roots(oops, clds, blobs, false);
}

如果允许卸载类的元数据，则调用process_strong_roots；否则调用process_all_roots_no_string_table
process_strong_roots的GC roots仅强根
process_all_roots_no_string_table的GC roots包括弱根、强根，但是不含StringTable
遍历标记栈中的所有对象

2.2.2 准备压缩

计算每个活跃对象应该在什么位置，即计算对象压缩后的新位置指针并写入对象头。

void G1FullCollector::phase2_prepare_compaction() {GCTraceTime(Info, gc, phases) info("Phase 2: Prepare for compaction", scope()->timer());G1FullGCPrepareTask task(this);run_task(&task);// To avoid OOM when there is memory left.if (!task.has_freed_regions()) {task.prepare_serial_compaction();}
}

调用G1FullGCPrepareTask准备压缩
如果任务没有空闲Region，则调用prepare_serial_compaction串行合并所有线程的最后一个分区，以避免OOM

G1FullGCPrepareTask

void G1FullGCPrepareTask::work(uint worker_id) {Ticks start = Ticks::now();G1FullGCCompactionPoint* compaction_point = collector()->compaction_point(worker_id);G1CalculatePointersClosure closure(collector()->mark_bitmap(), compaction_point);G1CollectedHeap::heap()->heap_region_par_iterate_from_start(&closure, &_hrclaimer);// Update humongous region setsclosure.update_sets();compaction_point->update();// Check if any regions was freed by this worker and store in task.if (closure.freed_regions()) {set_freed_regions();}log_task("Prepare compaction task", worker_id, start);
}

压缩对象具体逻辑在G1FullGCCompactionPoint中实现，执行完成后，对象头存储了对象的新地址
如果是大对象分区，且对象已经都死亡，则直接释放分区

2.2.3 调整指针

在上一步计算出所有活跃对象的新位置后，需要修改引用到新地址。

void G1FullCollector::phase3_adjust_pointers() {// Adjust the pointers to reflect the new locationsGCTraceTime(Info, gc, phases) info("Phase 3: Adjust pointers", scope()->timer());G1FullGCAdjustTask task(this);run_task(&task);
}

void G1FullGCAdjustTask::work(uint worker_id) {Ticks start = Ticks::now();ResourceMark rm;// Adjust preserved marks first since they are not balanced.G1FullGCMarker* marker = collector()->marker(worker_id);marker->preserved_stack()->adjust_during_full_gc();// Adjust the weak roots.if (Atomic::add(1u, &_references_done) == 1u) { // First incr claims task.G1CollectedHeap::heap()->ref_processor_stw()->weak_oops_do(&_adjust);}AlwaysTrueClosure always_alive;_weak_proc_task.work(worker_id, &always_alive, &_adjust);CLDToOopClosure adjust_cld(&_adjust, ClassLoaderData::_claim_strong);CodeBlobToOopClosure adjust_code(&_adjust, CodeBlobToOopClosure::FixRelocations);_root_processor.process_all_roots(&_adjust,&adjust_cld,&adjust_code);// Adjust string dedup if enabled.if (G1StringDedup::is_enabled()) {G1StringDedup::parallel_unlink(&_adjust_string_dedup, worker_id);}// Now adjust pointers region by regionG1AdjustRegionClosure blk(collector()->mark_bitmap(), worker_id);G1CollectedHeap::heap()->heap_region_par_iterate_from_worker_offset(&blk, &_hrclaimer, worker_id);log_task("Adjust task", worker_id, start);
}

调整之前保存的轻量级锁和重量级锁对象的引用地址
调整弱根
调整全部根对象
处理字符串去重逻辑
一个region一个region的调整引用地址

2.2.4 移动对象

对象的新地址和引用都已经更新，现在需要把对象移动到新位置

void G1FullCollector::phase4_do_compaction() {// Compact the heap using the compaction queues created in phase 2.GCTraceTime(Info, gc, phases) info("Phase 4: Compact heap", scope()->timer());G1FullGCCompactTask task(this);run_task(&task);// Serial compact to avoid OOM when very few free regions.if (serial_compaction_point()->has_regions()) {task.serial_compaction();}
}

具体压缩对象逻辑在G1FullGCCompactTask
如果phase2计算位置中使用了串行处理，则移动对象时也要使用串行处理移动每任务最后一个分区的对象

G1FullGCCompactTask

void G1FullGCCompactTask::work(uint worker_id) {Ticks start = Ticks::now();GrowableArray<HeapRegion*>* compaction_queue = collector()->compaction_point(worker_id)->regions();for (GrowableArrayIterator<HeapRegion*> it = compaction_queue->begin();it != compaction_queue->end();++it) {compact_region(*it);}G1ResetHumongousClosure hc(collector()->mark_bitmap());G1CollectedHeap::heap()->heap_region_par_iterate_from_worker_offset(&hc, &_claimer, worker_id);log_task("Compaction task", worker_id, start);
}size_t G1FullGCCompactTask::G1CompactRegionClosure::apply(oop obj) {size_t size = obj->size();HeapWord* destination = (HeapWord*)obj->forwardee();if (destination == NULL) {// Object not movingreturn size;}// copy object and reinit its markHeapWord* obj_addr = (HeapWord*) obj;assert(obj_addr != destination, "everything in this pass should be moving");Copy::aligned_conjoint_words(obj_addr, destination, size);oop(destination)->init_mark_raw();assert(oop(destination)->klass() != NULL, "should have a class");return size;
}

迭代处理每个Region
调用闭包G1CompactRegionClosure的apply函数移动对象到Region头部
如果Region中的全部对象都已清理，则回收该Region

3. 引用

jdk12源代码[https://hg.openjdk.java.net/jdk/jdk12]
JEP-307[http://openjdk.java.net/jeps/307]
JEP-192[http://openjdk.java.net/jeps/192]

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