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这一篇我们来看下JVM中的垃圾收集器，看下这些垃圾收集器是怎样选择以及初始化的一些信息。建议在看本篇文章的时候看下前面两篇

一、基本介绍

1、方法调用链

关于垃圾收集器选择初始化就是在initialized_heap()方法的create_heap()方法:

CollectedHeap*  Universe::_collectedHeap = NULL;

jint Universe::initialize_heap() {  jint status = JNI_ERR;  _collectedHeap = create_heap_ext();  if (_collectedHeap == NULL) {    _collectedHeap = create_heap();  }  status = _collectedHeap->initialize();  if (status != JNI_OK) {    return status;  }	.......}

2、垃圾的种类

CollectedHeap* Universe::create_heap() {  assert(_collectedHeap == NULL, "Heap already created");	..........  if (UseParallelGC) {    return Universe::create_heap_with_policy
   
    ();  } else if (UseG1GC) {    return Universe::create_heap_with_policy
    
     ();  } else if (UseConcMarkSweepGC) {    return Universe::create_heap_with_policy
     
      ();#endif  } else if (UseSerialGC) {    return Universe::create_heap_with_policy
      
       ();  }  ShouldNotReachHere();  return NULL;}
      
     
    
   

​ 可以看到这里就是根据选择的垃圾收集器然后决定对应的Heap&Policy。这里一共有四种：UseParallelGC,UseG1GC,UseConcMarkSweepGC,UseSerialGC：

及对应源码中的四种。

3、总括

template 
   
    CollectedHeap* Universe::create_heap_with_policy() {  Policy* policy = new Policy();  policy->initialize_all();  return new Heap(policy);}
   

​ 这个就是创建CollectedHeap的过程，上面的例如GenCollectedHeap、MarkSweepPolicy都是Heap、Policy的子类。

virtual void initialize_all() {  CollectorPolicy::initialize_all();  initialize_generations();}

​ 然后这里不同的Policy对initialize_generations()有不同的实现：

1）、MarkSweepPolicy：

void MarkSweepPolicy::initialize_generations() {  _young_gen_spec = new GenerationSpec(Generation::DefNew, _initial_young_size, _max_young_size, _gen_alignment);  _old_gen_spec   = new GenerationSpec(Generation::MarkSweepCompact, _initial_old_size, _max_old_size, _gen_alignment);}

2）、ConcurrentMarkSweepPolicy

void ConcurrentMarkSweepPolicy::initialize_generations() {  _young_gen_spec = new GenerationSpec(Generation::ParNew, _initial_young_size,                                       _max_young_size, _gen_alignment);  _old_gen_spec   = new GenerationSpec(Generation::ConcurrentMarkSweep,                                       _initial_old_size, _max_old_size, _gen_alignment);}

3）、继承关系

​ 不过MarkSweepPolicy、ConcurrentMarkSweepPolicy都是继承的GenCollectorPolicy：

class MarkSweepPolicy : public GenCollectorPolicy {

class ConcurrentMarkSweepPolicy : public GenCollectorPolicy {

​ 然后GenCollectorPolicy是继承的CollectorPolicy

class GenCollectorPolicy : public CollectorPolicy {

​ 其他的两种Policy：G1CollectorPolicy、GenerationSizer：

class GenerationSizer : public GenCollectorPolicy {

class G1CollectorPolicy: public CollectorPolicy {protected:  void initialize_alignments();public:  G1CollectorPolicy();};

二、serial垃圾收集器(UseSerialGC)空间管理

Universe::create_heap_with_policy
   
    ();
   

1、GenCollectedHeap

/ A "GenCollectedHeap" is a CollectedHeap that uses generational// collection.  It has two generations, young and old.class GenCollectedHeap : public CollectedHeap {  friend class GenCollectorPolicy;  friend class Generation;  friend class DefNewGeneration;  friend class TenuredGeneration;  friend class ConcurrentMarkSweepGeneration;  friend class CMSCollector;  friend class GenMarkSweep;  friend class VM_GenCollectForAllocation;  friend class VM_GenCollectFull;  friend class VM_GenCollectFullConcurrent;  	..............  enum GenerationType {    YoungGen,    OldGen  };private:  Generation* _young_gen;  Generation* _old_gen;	......  // The generational collector policy.  GenCollectorPolicy* _gen_policy;

这个是这个Heap的基本属性，下面我们来看下其创建后调用的_collectedHeap->initialize()方法。

1）、initialize()

jint GenCollectedHeap::initialize() {	......  char* heap_address;  ReservedSpace heap_rs;  size_t heap_alignment = collector_policy()->heap_alignment();  heap_address = allocate(heap_alignment, &heap_rs);	........  _rem_set = collector_policy()->create_rem_set(reserved_region());  set_barrier_set(rem_set()->bs());  ReservedSpace young_rs = heap_rs.first_part(gen_policy()->young_gen_spec()->max_size(), false, false);  _young_gen = gen_policy()->young_gen_spec()->init(young_rs, rem_set());  heap_rs = heap_rs.last_part(gen_policy()->young_gen_spec()->max_size());  ReservedSpace old_rs = heap_rs.first_part(gen_policy()->old_gen_spec()->max_size(), false, false);  _old_gen = gen_policy()->old_gen_spec()->init(old_rs, rem_set());  clear_incremental_collection_failed();	........  return JNI_OK;}

​ 这个方法其实在前面的文章也提到过，就是年轻代&老年代的初始化分配。然后这里主要是gen_policy()->young_gen_spec()->init(young_rs, rem_set())，也就是young_gen_spec()->init(young_rs, rem_set())，即GenerationSpec的init方法。

GenerationSpec* young_gen_spec() const {  assert(_young_gen_spec != NULL, "_young_gen_spec should have been initialized");  return _young_gen_spec;}

public: // The set of possible generation kinds. enum Name {   DefNew,   ParNew,   MarkSweepCompact,   ConcurrentMarkSweep,   Other };

Generation* GenerationSpec::init(ReservedSpace rs, CardTableRS* remset) {  switch (name()) {    case Generation::DefNew:      return new DefNewGeneration(rs, init_size());    case Generation::MarkSweepCompact:      return new TenuredGeneration(rs, init_size(), remset);#if INCLUDE_ALL_GCS    case Generation::ParNew:      return new ParNewGeneration(rs, init_size());    case Generation::ConcurrentMarkSweep: {   			.....      ConcurrentMarkSweepGeneration* g = NULL;      g = new ConcurrentMarkSweepGeneration(rs, init_size(), remset);		......    }#endif // INCLUDE_ALL_GCS    default:      guarantee(false, "unrecognized GenerationName");      return NULL;  }}

​ 我们当前是serial，所以这里的name()就会是Generation::DefNew、Generation::MarkSweepCompact，即年轻代使用Generation::DefNew、老年代使用Generation::MarkSweepCompact。但我们将这里的四种都看下，也就是我们使用UseSerialGC&UseConcMarkSweepGC垃圾收集器的时候，会将年轻代初始为DefNewGeneration或ParNewGeneration，老年代初始化为MarkSweepCompact或ConcurrentMarkSweepGeneration。这些Generation的实现都是用来空间管理的

2、DefNewGeneration(Generation::DefNew)

// DefNewGeneration is a young generation containing eden, from- and// to-space.class DefNewGeneration: public Generation {  friend class VMStructs;protected:  Generation* _old_gen;  uint        _tenuring_threshold;   // Tenuring threshold for next collection.  AgeTable    _age_table;  // Size of object to pretenure in words; command line provides bytes  size_t      _pretenure_size_threshold_words;  AgeTable*   age_table() { return &_age_table; }  ......      // Spaces  ContiguousSpace* _eden_space;  ContiguousSpace* _from_space;  ContiguousSpace* _to_space;    ......        // Printing  virtual const char* name() const;  virtual const char* short_name() const { return "DefNew"; }    ......

​ 这个是年轻代Generation，可以看到其有包含年龄表AgeTable，能找到老年代的指针_old_gen，对象晋升到老年代的阈值_tenuring_threshold。

3、TenuredGeneration

// TenuredGeneration models the heap containing old (promoted/tenured) objects// contained in a single contiguous space.//// Garbage collection is performed using mark-compact.class TenuredGeneration: public CardGeneration {  friend class VMStructs;  // Abstractly, this is a subtype that gets access to protected fields.  friend class VM_PopulateDumpSharedSpace; protected:  ContiguousSpace*    _the_space;       // Actual space holding objects  GenerationCounters* _gen_counters;  CSpaceCounters*     _space_counters;

​ 这个就是用来解析老年代的空间管理，通过注释可以知道其主要有两个信息：其是用来管理老年代的，同时其是一块连续的空间。

4、ConcurrentMarkSweepGeneration(Generation::ConcurrentMarkSweep)

class ConcurrentMarkSweepGeneration: public CardGeneration {  friend class VMStructs;  friend class ConcurrentMarkSweepThread;  friend class ConcurrentMarkSweep;  friend class CMSCollector; protected:  static CMSCollector*       _collector; // the collector that collects us  CompactibleFreeListSpace*  _cmsSpace;  // underlying space (only one for now)  // Performance Counters  GenerationCounters*      _gen_counters;  GSpaceCounters*          _space_counters;

三、CMS垃圾收集器(UseConcMarkSweepGC)空间管理

1、GenCollectedHeap

​ CMS与前面的serial用的是一样的GenCollectedHeap。

2、ParNewGeneration

// A Generation that does parallel young-gen collection.class ParNewGeneration: public DefNewGeneration {  friend class ParNewGenTask;  friend class ParNewRefProcTask;  friend class ParNewRefProcTaskExecutor;  friend class ParScanThreadStateSet;  friend class ParEvacuateFollowersClosure; private:  // The per-worker-thread work queues  ObjToScanQueueSet* _task_queues;  // Per-worker-thread local overflow stacks  Stack
   
    * _overflow_stacks;	......  // GC tracer that should be used during collection.  ParNewTracer _gc_tracer;    ......  virtual const char* name() const;  virtual const char* short_name() const { return "ParNew"; }
   

​ 其是多线程的用来处理年轻代的垃圾收集，同时其继承DefNewGeneration，也就是说ParNewGeneration是使用多线程去处理原来的DefNewGeneration单线程处理的内容。

3、ConcurrentMarkSweepPolicy

class ConcurrentMarkSweepPolicy : public GenCollectorPolicy { protected:  void initialize_alignments();  void initialize_generations(); public:  ConcurrentMarkSweepPolicy() {}  ConcurrentMarkSweepPolicy* as_concurrent_mark_sweep_policy() { return this; }  void initialize_gc_policy_counters();  virtual void initialize_size_policy(size_t init_eden_size,                                      size_t init_promo_size,                                      size_t init_survivor_size);};

同时ConcurrentMarkSweepPolicy也是直接继承原来的GenCollectorPolicy。

四、serial&CMS的垃圾收集

1、基本对象

class VM_Operation: public CHeapObj
   
     { public:  enum Mode {    _safepoint,       // blocking,        safepoint, vm_op C-heap allocated    _no_safepoint,    // blocking,     no safepoint, vm_op C-Heap allocated    _concurrent,      // non-blocking, no safepoint, vm_op C-Heap allocated    _async_safepoint  // non-blocking,    safepoint, vm_op C-Heap allocated  };  enum VMOp_Type {    VM_OPS_DO(VM_OP_ENUM)    VMOp_Terminating  };	....  // Called by VM thread - does in turn invoke doit(). Do not override this  void evaluate();  // evaluate() is called by the VMThread and in turn calls doit().  // If the thread invoking VMThread::execute((VM_Operation*) is a JavaThread,  // doit_prologue() is called in that thread before transferring control to  // the VMThread.  // If doit_prologue() returns true the VM operation will proceed, and  // doit_epilogue() will be called by the JavaThread once the VM operation  // completes. If doit_prologue() returns false the VM operation is cancelled.  virtual void doit()                            = 0;  virtual bool doit_prologue()                   { return true; };  virtual void doit_epilogue()                   {}; // Note: Not called if mode is: _concurrent
   

class VM_GC_Operation: public VM_Operation { private:  ReferencePendingListLocker _pending_list_locker; protected:  uint           _gc_count_before;         // gc count before acquiring PLL  uint           _full_gc_count_before;    // full gc count before acquiring PLL  bool           _full;                    // whether a "full" collection  bool           _prologue_succeeded;      // whether doit_prologue succeeded  GCCause::Cause _gc_cause;                // the putative cause for this gc op  bool           _gc_locked;               // will be set if gc was locked    ......

class VM_CollectForAllocation : public VM_GC_Operation { protected:  size_t    _word_size; // Size of object to be allocated (in number of words)  HeapWord* _result;    // Allocation result (NULL if allocation failed) public:  VM_CollectForAllocation(size_t word_size, uint gc_count_before, GCCause::Cause cause);  HeapWord* result() const {    return _result;  }};

以上三个类是父子关系，在VM_Operation中是有定义Mode4种状态。在垃圾收集的时候是另外的线程，线程的任务对象一般就是VM_CollectForAllocation的子类，然后过程就是通过evaluate()方法来调用doit*()方法来具体处理。

​ 然后不同的垃圾收集器即VM_CollectForAllocation的子类主要是对这两个方法的实现：

2、垃圾收集触发逻辑

HeapWord* GenCollectorPolicy::mem_allocate_work(size_t size,                                        bool is_tlab,                                        bool* gc_overhead_limit_was_exceeded) {  GenCollectedHeap *gch = GenCollectedHeap::heap();	.....  HeapWord* result = NULL;  // Loop until the allocation is satisfied, or unsatisfied after GC.  for (uint try_count = 1, gclocker_stalled_count = 0; /* return or throw */; try_count += 1) {    HandleMark hm; // Discard any handles allocated in each iteration.    // First allocation attempt is lock-free.    Generation *young = gch->young_gen();    assert(young->supports_inline_contig_alloc(),      "Otherwise, must do alloc within heap lock");    if (young->should_allocate(size, is_tlab)) {      result = young->par_allocate(size, is_tlab);      if (result != NULL) {        assert(gch->is_in_reserved(result), "result not in heap");        return result;      }    }    uint gc_count_before;  // Read inside the Heap_lock locked region.    {      MutexLocker ml(Heap_lock);      log_trace(gc, alloc)("GenCollectorPolicy::mem_allocate_work: attempting locked slow path allocation");      // Note that only large objects get a shot at being      // allocated in later generations.      bool first_only = ! should_try_older_generation_allocation(size);      result = gch->attempt_allocation(size, is_tlab, first_only);      if (result != NULL) {        assert(gch->is_in_reserved(result), "result not in heap");        return result;      }		..........      // Read the gc count while the heap lock is held.      gc_count_before = gch->total_collections();    }    VM_GenCollectForAllocation op(size, is_tlab, gc_count_before);    VMThread::execute(&op);    ..........}

​ 我们以VM_GenCollectForAllocation为例，在进行内存申请的时候gch->attempt_allocatio我们看到如果是申请成功了，result != NULL，就直接返回了，如果失败了，就会进行垃圾收集的线程VMThread::execute(&op)：

​ 然后在垃圾收集的线程中，就通过evaluate()方法调用doit()。下面我们就来具体分析下VM_GenCollectForAllocation，因为其是用来处理GenCollectedHeap的，同时CMS&serial都用的GenCollectedHeap。

3、VM_GenCollectForAllocation

class VM_GenCollectForAllocation : public VM_CollectForAllocation { private:  bool        _tlab;                       // alloc is of a tlab.

void VM_GenCollectForAllocation::doit() {  SvcGCMarker sgcm(SvcGCMarker::MINOR);  GenCollectedHeap* gch = GenCollectedHeap::heap();  GCCauseSetter gccs(gch, _gc_cause);  _result = gch->satisfy_failed_allocation(_word_size, _tlab);  assert(gch->is_in_reserved_or_null(_result), "result not in heap");  if (_result == NULL && GCLocker::is_active_and_needs_gc()) {    set_gc_locked();  }}

​ 这里实现是获取GenCollectedHeap，然后通过satisfy_failed_allocation处理：

HeapWord* GenCollectedHeap::satisfy_failed_allocation(size_t size, bool is_tlab) {  return gen_policy()->satisfy_failed_allocation(size, is_tlab);}

​ 可以看到这里是先获取policy，而policy 两个垃圾收集器是不同的，CMS是ConcurrentMarkSweepPolicy、serial是MarkSweepPolicy。

HeapWord* GenCollectorPolicy::satisfy_failed_allocation(size_t size,                                                        bool   is_tlab) {  GenCollectedHeap *gch = GenCollectedHeap::heap();  GCCauseSetter x(gch, GCCause::_allocation_failure);  HeapWord* result = NULL;		....    log_trace(gc)(" :: Trying full because partial may fail :: ");    // Try a full collection; see delta for bug id 6266275    // for the original code and why this has been simplified    // with from-space allocation criteria modified and    // such allocation moved out of the safepoint path.    gch->do_collection(true,                      // full                       false,                     // clear_all_soft_refs                       size,                      // size                       is_tlab,                   // is_tlab                       GenCollectedHeap::OldGen); // max_generation  }  result = gch->attempt_allocation(size, is_tlab, false /*first_only*/);	......}

void GenCollectedHeap::do_collection(bool           full,                                     bool           clear_all_soft_refs,                                     size_t         size,                                     bool           is_tlab,                                     GenerationType max_generation) { 		....  {    FlagSetting fl(_is_gc_active, true);    bool complete = full && (max_generation == OldGen);    bool old_collects_young = complete && !ScavengeBeforeFullGC;    bool do_young_collection = !old_collects_young && _young_gen->should_collect(full, size, is_tlab);    FormatBuffer<> gc_string("%s", "Pause ");    if (do_young_collection) {      gc_string.append("Young");    } else {      gc_string.append("Full");    }	.....    bool collected_old = false;		....      if (do_young_collection) {      		........        collect_generation(_old_gen, full, size, is_tlab, run_verification && VerifyGCLevel <= 1, do_clear_all_soft_refs, true);      } else {        // No young GC done. Use the same GC id as was set up earlier in this method.        collect_generation(_old_gen, full, size, is_tlab, run_verification && VerifyGCLevel <= 1, do_clear_all_soft_refs, true);      }      must_restore_marks_for_biased_locking = true;      collected_old = true;    }	......}

void GenCollectedHeap::collect_generation(Generation* gen, bool full, size_t size,                                          bool is_tlab, bool run_verification, bool clear_soft_refs,                                          bool restore_marks_for_biased_locking) {  		.........    gen->collect(full, clear_soft_refs, size, is_tlab);   	........}

​ 这里两种不同的垃圾处理器的划分关键就是这个，因为其使用的Generation是不同的，目前这个是老年代，所以serial收集器是用的TenuredGeneration、CMS用的是ConcurrentMarkSweepGeneration。下面我们就来看下这两个的collect()方法实现：

1）、TenuredGeneration

void TenuredGeneration::collect(bool   full,                                bool   clear_all_soft_refs,                                size_t size,                                bool   is_tlab) {  GenCollectedHeap* gch = GenCollectedHeap::heap();  // Temporarily expand the span of our ref processor, so  // refs discovery is over the entire heap, not just this generation  ReferenceProcessorSpanMutator    x(ref_processor(), gch->reserved_region());  STWGCTimer* gc_timer = GenMarkSweep::gc_timer();  gc_timer->register_gc_start();  SerialOldTracer* gc_tracer = GenMarkSweep::gc_tracer();  gc_tracer->report_gc_start(gch->gc_cause(), gc_timer->gc_start());  gch->pre_full_gc_dump(gc_timer);  GenMarkSweep::invoke_at_safepoint(ref_processor(), clear_all_soft_refs);  gch->post_full_gc_dump(gc_timer);  gc_timer->register_gc_end();  gc_tracer->report_gc_end(gc_timer->gc_end(), gc_timer->time_partitions());}

void GenMarkSweep::invoke_at_safepoint(ReferenceProcessor* rp, bool clear_all_softrefs) {  assert(SafepointSynchronize::is_at_safepoint(), "must be at a safepoint");  GenCollectedHeap* gch = GenCollectedHeap::heap();#ifdef ASSERT  if (gch->collector_policy()->should_clear_all_soft_refs()) {    assert(clear_all_softrefs, "Policy should have been checked earlier");  }#endif  // hook up weak ref data so it can be used during Mark-Sweep  assert(ref_processor() == NULL, "no stomping");  assert(rp != NULL, "should be non-NULL");  set_ref_processor(rp);  rp->setup_policy(clear_all_softrefs);  gch->trace_heap_before_gc(_gc_tracer);  // 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();  // Increment the invocation count  _total_invocations++;  // Capture used regions for each generation that will be  // subject to collection, so that card table adjustments can  // be made intelligently (see clear / invalidate further below).  gch->save_used_regions();  allocate_stacks();  mark_sweep_phase1(clear_all_softrefs);  mark_sweep_phase2();  // Don't add any more derived pointers during phase3#if defined(COMPILER2) || INCLUDE_JVMCI  assert(DerivedPointerTable::is_active(), "Sanity");  DerivedPointerTable::set_active(false);#endif  mark_sweep_phase3();  mark_sweep_phase4();  restore_marks();  // Set saved marks for allocation profiler (and other things? -- dld)  // (Should this be in general part?)  gch->save_marks();  deallocate_stacks();  // If compaction completely evacuated the young generation then we  // can clear the card table.  Otherwise, we must invalidate  // it (consider all cards dirty).  In the future, we might consider doing  // compaction within generations only, and doing card-table sliding.  CardTableRS* rs = gch->rem_set();  Generation* old_gen = gch->old_gen();  // Clear/invalidate below make use of the "prev_used_regions" saved earlier.  if (gch->young_gen()->used() == 0) {    // We've evacuated the young generation.    rs->clear_into_younger(old_gen);  } else {    // Invalidate the cards corresponding to the currently used    // region and clear those corresponding to the evacuated region.    rs->invalidate_or_clear(old_gen);  }  CodeCache::gc_epilogue();  JvmtiExport::gc_epilogue();  // refs processing: clean slate  set_ref_processor(NULL);  // Update heap occupancy information which is used as  // input to soft ref clearing policy at the next gc.  Universe::update_heap_info_at_gc();  // Update time of last gc for all generations we collected  // (which currently is all the generations in the heap).  // We need to use a monotonically non-decreasing time in ms  // or we will see time-warp warnings and os::javaTimeMillis()  // does not guarantee monotonicity.  jlong now = os::javaTimeNanos() / NANOSECS_PER_MILLISEC;  gch->update_time_of_last_gc(now);  gch->trace_heap_after_gc(_gc_tracer);}

​ 最终通过invoke_at_safepoint来进行垃圾清理。这个具体过程我们就先跳过。

2）、ConcurrentMarkSweepGeneration

bool GenCollectedHeap::create_cms_collector() {	........  CMSCollector* collector =    new CMSCollector((ConcurrentMarkSweepGeneration*)_old_gen,                     _rem_set,                     _gen_policy->as_concurrent_mark_sweep_policy()); ..........  return true;  // success}

void ConcurrentMarkSweepGeneration::collect(bool   full,                                            bool   clear_all_soft_refs,                                            size_t size,                                            bool   tlab){  collector()->collect(full, clear_all_soft_refs, size, tlab);}

​ 可以看到这两种Generation对于collect()方法订单实现是不同的，CMS是通过CMSCollector去处理的。

void CMSCollector::collect(bool   full,                           bool   clear_all_soft_refs,                           size_t size,                           bool   tlab){  .......  acquire_control_and_collect(full, clear_all_soft_refs);}

void CMSCollector::acquire_control_and_collect(bool full,        bool clear_all_soft_refs) { 		......  CollectorState first_state = _collectorState;		........  // Inform cms gen if this was due to partial collection failing.  // The CMS gen may use this fact to determine its expansion policy.  GenCollectedHeap* gch = GenCollectedHeap::heap();	..........  do_compaction_work(clear_all_soft_refs); 	......  return;}

// A work method used by the foreground collector to do// a mark-sweep-compact.void CMSCollector::do_compaction_work(bool clear_all_soft_refs) {  GenCollectedHeap* gch = GenCollectedHeap::heap();  	..........  GenMarkSweep::invoke_at_safepoint(ref_processor(), clear_all_soft_refs); 	......  // For a mark-sweep-compact, compute_new_size() will be called  // in the heap's do_collection() method.}

​ 可以看到其最终还是调用的GenMarkSweep::invoke_at_safepoint(ref_processor(), clear_all_soft_refs),与前面的TenuredGeneration是一样的。

五、ParallelGC

​ 下面我们按照上面的思路来梳理下ParallelGC的初始化创建及垃圾收集过程。

按照前面对Heap的创建过程，在使用UseParallelGC的时候：

if (UseParallelGC) {  return Universe::create_heap_with_policy
   
    ();}
   

1、ParallelScavengeHeap

class ParallelScavengeHeap : public CollectedHeap {  friend class VMStructs; private:  static PSYoungGen* _young_gen;  static PSOldGen*   _old_gen;  // Sizing policy for entire heap  static PSAdaptiveSizePolicy*       _size_policy;  static PSGCAdaptivePolicyCounters* _gc_policy_counters;  GenerationSizer* _collector_policy;  // Collection of generations that are adjacent in the  // space reserved for the heap.  AdjoiningGenerations* _gens;  unsigned int _death_march_count;	.......  // For use by VM operations  enum CollectionType {    Scavenge,    MarkSweep  };	.....  virtual const char* name() const {    return "Parallel";  }    ......

这里的年轻代是PSYoungGen、老年代时候PSOldGen：

1）、PSYoungGen

class PSYoungGen : public CHeapObj
   
     {  friend class VMStructs;  friend class ParallelScavengeHeap;  friend class AdjoiningGenerations; protected:  MemRegion       _reserved;  PSVirtualSpace* _virtual_space;  // Spaces  MutableSpace* _eden_space;  MutableSpace* _from_space;  MutableSpace* _to_space;  // MarkSweep Decorators  PSMarkSweepDecorator* _eden_mark_sweep;  PSMarkSweepDecorator* _from_mark_sweep;  PSMarkSweepDecorator* _to_mark_sweep;  // Sizing information, in bytes, set in constructor  const size_t _init_gen_size;  const size_t _min_gen_size;  const size_t _max_gen_size;  // Performance counters  PSGenerationCounters*     _gen_counters;  SpaceCounters*            _eden_counters;  SpaceCounters*            _from_counters;  SpaceCounters*            _to_counters;
   

​ 这些属性看名称就大概了解其是用来记录什么信息的了。

2）、PSOldGen

class PSOldGen : public CHeapObj
   
     {  friend class VMStructs;  friend class PSPromotionManager; // Uses the cas_allocate methods  friend class ParallelScavengeHeap;  friend class AdjoiningGenerations; protected:  MemRegion                _reserved;          // Used for simple containment tests  PSVirtualSpace*          _virtual_space;     // Controls mapping and unmapping of virtual mem  ObjectStartArray         _start_array;       // Keeps track of where objects start in a 512b block  MutableSpace*            _object_space;      // Where all the objects live  PSMarkSweepDecorator*    _object_mark_sweep; // The mark sweep view of _object_space  const char* const        _name;              // Name of this generation.  // Performance Counters  PSGenerationCounters*    _gen_counters;  SpaceCounters*           _space_counters;
   

3）、ASPSOldGen

class ASPSOldGen : public PSOldGen {    .......        // Debugging support  virtual const char* short_name() const { return "ASPSOldGen"; }    .........

4)、ASPSYoungGen

class ASPSYoungGen : public PSYoungGen {    ........ // Printing support  virtual const char* short_name() const { return "ASPSYoungGen"; }    ........

​ 可以看到ASPSYoungGen&ASPSOldGen都是直接继承的PSYoungGen&PSOldGen。这两个的区别我们后面会讲

5）、ParallelScavengeHeap的initialize方法

jint ParallelScavengeHeap::initialize() {  CollectedHeap::pre_initialize();  const size_t heap_size = _collector_policy->max_heap_byte_size();  ReservedSpace heap_rs = Universe::reserve_heap(heap_size, _collector_policy->heap_alignment());		.........  initialize_reserved_region((HeapWord*)heap_rs.base(), (HeapWord*)(heap_rs.base() + heap_rs.size()));  CardTableExtension* const barrier_set = new CardTableExtension(reserved_region());  barrier_set->initialize();  set_barrier_set(barrier_set);  // Make up the generations  // Calculate the maximum size that a generation can grow.  This  // includes growth into the other generation.  Note that the  // parameter _max_gen_size is kept as the maximum  // size of the generation as the boundaries currently stand.  		..........  _gens = new AdjoiningGenerations(heap_rs, _collector_policy, generation_alignment());  _old_gen = _gens->old_gen();  _young_gen = _gens->young_gen();	.......  _size_policy =    new PSAdaptiveSizePolicy(eden_capacity,                             initial_promo_size,                             young_gen()->to_space()->capacity_in_bytes(),                             _collector_policy->gen_alignment(),                             max_gc_pause_sec,                             max_gc_minor_pause_sec,                             GCTimeRatio                             );  _gc_policy_counters =    new PSGCAdaptivePolicyCounters("ParScav:MSC", 2, 3, _size_policy);  // Set up the GCTaskManager  _gc_task_manager = GCTaskManager::create(ParallelGCThreads);	.......  return JNI_OK;}

​ 这里主要是初始化了对空间处理的对象AdjoiningGenerations。

6）、AdjoiningGenerations

AdjoiningGenerations::AdjoiningGenerations(ReservedSpace old_young_rs,                                           GenerationSizer* policy,                                           size_t alignment) : 		.......  // Create the generations differently based on the option to  // move the boundary.  if (UseAdaptiveGCBoundary) {    // Initialize the adjoining virtual spaces.  Then pass the    // a virtual to each generation for initialization of the    // generation.    // Does the actual creation of the virtual spaces    _virtual_spaces.initialize(max_low_byte_size,                               init_low_byte_size,                               init_high_byte_size);    // Place the young gen at the high end.  Passes in the virtual space.    _young_gen = new ASPSYoungGen(_virtual_spaces.high(),                                  _virtual_spaces.high()->committed_size(),                                  min_high_byte_size,                                  _virtual_spaces.high_byte_size_limit());    // Place the old gen at the low end. Passes in the virtual space.    _old_gen = new ASPSOldGen(_virtual_spaces.low(),                              _virtual_spaces.low()->committed_size(),                              min_low_byte_size,                              _virtual_spaces.low_byte_size_limit(),                              "old", 1);    young_gen()->initialize_work();    old_gen()->initialize_work("old", 1);  } else {    // Layout the reserved space for the generations.    ReservedSpace old_rs   =      virtual_spaces()->reserved_space().first_part(max_low_byte_size);    ReservedSpace heap_rs  =      virtual_spaces()->reserved_space().last_part(max_low_byte_size);    ReservedSpace young_rs = heap_rs.first_part(max_high_byte_size);    assert(young_rs.size() == heap_rs.size(), "Didn't reserve all of the heap");    // Create the generations.  Virtual spaces are not passed in.    _young_gen = new PSYoungGen(init_high_byte_size,                                min_high_byte_size,                                max_high_byte_size);    _old_gen = new PSOldGen(init_low_byte_size,                            min_low_byte_size,                            max_low_byte_size,                            "old", 1);    // The virtual spaces are created by the initialization of the gens.    _young_gen->initialize(young_rs, alignment);  	......  }}

​ 这里主要是有一个参数UseAdaptiveGCBoundary，如果在JVM启动的时候有设置这个参数，就表示是由JVM自动决定年轻代、老年代的内存分配，可以看到如果有这个参数，其使用的是ASPSYoungGen&ASPSOldGen，没有的话就是使用PSYoungGen&PSOldGen。

PSOldGen::PSOldGen(ReservedSpace rs, size_t alignment,                   size_t initial_size, size_t min_size, size_t max_size,                   const char* perf_data_name, int level):  _name(select_name()), _init_gen_size(initial_size), _min_gen_size(min_size),  _max_gen_size(max_size){  initialize(rs, alignment, perf_data_name, level);}

​ 我们可以看到在初始化的时候，这里是会有设置名称的：

inline const char* PSOldGen::select_name() {  return UseParallelOldGC ? "ParOldGen" : "PSOldGen";}

也就是如果打印GC日志的话，如果使用到的UseParallelOldGC，老年代名称就会是ParOldGen，不然就会是PSOldGen，这里我最开始搞错了，因为使用的是short_name()的字符名称。

7）、Gen的初始化

void PSYoungGen::initialize(ReservedSpace rs, size_t alignment) {  initialize_virtual_space(rs, alignment);  initialize_work();}

void PSYoungGen::initialize_work() {  _reserved = MemRegion((HeapWord*)virtual_space()->low_boundary(),                        (HeapWord*)virtual_space()->high_boundary());		.......  if (UseNUMA) {    _eden_space = new MutableNUMASpace(virtual_space()->alignment());  } else {    _eden_space = new MutableSpace(virtual_space()->alignment());  }  _from_space = new MutableSpace(virtual_space()->alignment());  _to_space   = new MutableSpace(virtual_space()->alignment());  if (_eden_space == NULL || _from_space == NULL || _to_space == NULL) {    vm_exit_during_initialization("Could not allocate a young gen space");  }  // Allocate the mark sweep views of spaces  _eden_mark_sweep =      new PSMarkSweepDecorator(_eden_space, NULL, MarkSweepDeadRatio);  _from_mark_sweep =      new PSMarkSweepDecorator(_from_space, NULL, MarkSweepDeadRatio);  _to_mark_sweep =      new PSMarkSweepDecorator(_to_space, NULL, MarkSweepDeadRatio);  // Generation Counters - generation 0, 3 subspaces  _gen_counters = new PSGenerationCounters("new", 0, 3, _min_gen_size,                                           _max_gen_size, _virtual_space);  // Compute maximum space sizes for performance counters  ParallelScavengeHeap* heap = ParallelScavengeHeap::heap();	.......  if (UseAdaptiveSizePolicy) {    max_survivor_size = size / MinSurvivorRatio;    // round the survivor space size down to the nearest alignment    // and make sure its size is greater than 0.    max_survivor_size = align_size_down(max_survivor_size, alignment);    max_survivor_size = MAX2(max_survivor_size, alignment);    // set the maximum size of eden to be the size of the young gen    // less two times the minimum survivor size. The minimum survivor    // size for UseAdaptiveSizePolicy is one alignment.    max_eden_size = size - 2 * alignment;  } else {    max_survivor_size = size / InitialSurvivorRatio;    // round the survivor space size down to the nearest alignment    // and make sure its size is greater than 0.    max_survivor_size = align_size_down(max_survivor_size, alignment);    max_survivor_size = MAX2(max_survivor_size, alignment);    // set the maximum size of eden to be the size of the young gen    // less two times the survivor size when the generation is 100%    // committed. The minimum survivor size for -UseAdaptiveSizePolicy    // is dependent on the committed portion (current capacity) of the    // generation - the less space committed, the smaller the survivor    // space, possibly as small as an alignment. However, we are interested    // in the case where the young generation is 100% committed, as this    // is the point where eden reaches its maximum size. At this point,    // the size of a survivor space is max_survivor_size.    max_eden_size = size - 2 * max_survivor_size;  }  _eden_counters = new SpaceCounters("eden", 0, max_eden_size, _eden_space,                                     _gen_counters);  _from_counters = new SpaceCounters("s0", 1, max_survivor_size, _from_space,                                     _gen_counters);  _to_counters = new SpaceCounters("s1", 2, max_survivor_size, _to_space,                                   _gen_counters);}

void PSOldGen::initialize_work(const char* perf_data_name, int level) {  MemRegion limit_reserved((HeapWord*)virtual_space()->low_boundary(),    heap_word_size(_max_gen_size)); 	......  ParallelScavengeHeap* heap = ParallelScavengeHeap::heap(); 	......  _object_space = new MutableSpace(virtual_space()->alignment());		....  object_space()->initialize(cmr,                             SpaceDecorator::Clear,                             SpaceDecorator::Mangle);  _object_mark_sweep = new PSMarkSweepDecorator(_object_space, start_array(), MarkSweepDeadRatio);	...........}

2、VM_ParallelGCFailedAllocation

同样我们来看下VM_CollectForAllocation的子类VM_ParallelGCFailedAllocation

class VM_ParallelGCFailedAllocation : public VM_CollectForAllocation { public:  VM_ParallelGCFailedAllocation(size_t word_size, uint gc_count);  virtual VMOp_Type type() const {    return VMOp_ParallelGCFailedAllocation;  }  virtual void doit();};

​ 我们再来看下其doit()方法的实现：

1）、doit()

oid VM_ParallelGCFailedAllocation::doit() {  SvcGCMarker sgcm(SvcGCMarker::MINOR);  ParallelScavengeHeap* heap = ParallelScavengeHeap::heap();  GCCauseSetter gccs(heap, _gc_cause);  _result = heap->failed_mem_allocate(_word_size);  if (_result == NULL && GCLocker::is_active_and_needs_gc()) {    set_gc_locked();  }}

2）、failed_mem_allocate(_word_size)

HeapWord* ParallelScavengeHeap::failed_mem_allocate(size_t size) {	....  const bool invoked_full_gc = PSScavenge::invoke();  HeapWord* result = young_gen()->allocate(size);  // Second level allocation failure.  //   Mark sweep and allocate in young generation.  if (result == NULL && !invoked_full_gc) {    do_full_collection(false);    result = young_gen()->allocate(size);  }  death_march_check(result, size);  // Third level allocation failure.  //   After mark sweep and young generation allocation failure,  //   allocate in old generation.  if (result == NULL) {    result = old_gen()->allocate(size);  }  // Fourth level allocation failure. We're running out of memory.  //   More complete mark sweep and allocate in young generation.  if (result == NULL) {    do_full_collection(true);    result = young_gen()->allocate(size);  }  // Fifth level allocation failure.  //   After more complete mark sweep, allocate in old generation.  if (result == NULL) {    result = old_gen()->allocate(size);  }  return result;}

​ 这里首先是通过PSScavenge::invoke()进行回收，然后再在young_gen()年轻代申请内存，如果成功就能return了，如果失败了，判断刚才是不是执行的invoked_full_gc，如果还没有执行full_gc，则通过do_full_collection再进行GC，然后再按年轻代-》老年代的顺序申请分配。

bool PSScavenge::invoke() {  ParallelScavengeHeap* const heap = ParallelScavengeHeap::heap();  PSAdaptiveSizePolicy* policy = heap->size_policy();  IsGCActiveMark mark;  const bool scavenge_done = PSScavenge::invoke_no_policy();  const bool need_full_gc = !scavenge_done ||    policy->should_full_GC(heap->old_gen()->free_in_bytes());  bool full_gc_done = false;  if (UsePerfData) {    PSGCAdaptivePolicyCounters* const counters = heap->gc_policy_counters();    const int ffs_val = need_full_gc ? full_follows_scavenge : not_skipped;    counters->update_full_follows_scavenge(ffs_val);  }  if (need_full_gc) {    GCCauseSetter gccs(heap, GCCause::_adaptive_size_policy);    CollectorPolicy* cp = heap->collector_policy();    const bool clear_all_softrefs = cp->should_clear_all_soft_refs();    if (UseParallelOldGC) {      full_gc_done = PSParallelCompact::invoke_no_policy(clear_all_softrefs);    } else {      full_gc_done = PSMarkSweep::invoke_no_policy(clear_all_softrefs);    }  }  return full_gc_done;}

可以看到这里进行GC的时候，会判断UseParallelOldGC，分别选择不同的类型进行GC，分别选择PSParallelCompact或PSMarkSweep。

​ 以上我们就分析了三种主要GC类型的初始化&垃圾收集的初始垃圾处理。然后G1垃圾收集器，大体也是这个过程，就不再具体展开了。

​ 再看还需不需要分析下具体收集过程，这个要明白以及这几种GC收集器为什么要怎样设计，以及内部的不同之处能带来性能上的差异，还是比较困难，这个以后再说吧。下一篇我们就来看下各种不同GC的具体使用，再结合源码来看下其为什么要这样传入参数。

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