JVM源码分析之老年代TenuredGeneration的垃圾回收算法实现

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接着上文《JVM源码分析之新生代DefNewGeneration的实现》,本文对老年代TenuredGeneration的垃圾回收算法进行分析。

TenuredGeneration

老年代TenuredGeneration使用标记-压缩-清理算法进行垃圾回收,将标记对象移动到堆的另一端,同时更新对象的引用地址,算法的具体实现位于TenuredGeneration::collect()方法

void TenuredGeneration::collect(bool full,
                                bool clear_all_soft_refs,
                                size_t size,
                                bool is_tlab) {
  retire_alloc_buffers_before_full_gc();
  OneContigSpaceCardGeneration::collect(full, clear_all_soft_refs,
                                        size, is_tlab);
}

调用父类的OneContigSpaceCardGenerationcollect()方法,实现如下

void OneContigSpaceCardGeneration::collect(bool full,
                                           bool clear_all_soft_refs,
                                           size_t size,
                                           bool is_tlab) {
  SpecializationStats::clear();
  // Temporarily expand the span of our ref processor, so
  // refs discovery is over the entire heap, not just this generation
  ReferenceProcessorSpanMutator
    x(ref_processor(), GenCollectedHeap::heap()->reserved_region());
  GenMarkSweep::invoke_at_safepoint(_level, ref_processor(), clear_all_soft_refs);
  SpecializationStats::print();
}

其中GenMarkSweep::invoke_at_safepoint()是垃圾回收算法实现的核心,下面对invoke_at_safepoint方法进行分析。

GC前准备

  // 设置引用处理器和引用的处理策略;
  _ref_processor = rp;
  rp->setup_policy(clear_all_softrefs);

  // 设置输出日志;
  TraceTime t1("Full GC", PrintGC && !PrintGCDetails, true, gclog_or_tty);

  // When collecting the permanent generation methodOops may be moving,
  // so we either have to flush all bcp data or convert it into bci.
  CodeCache::gc_prologue();
  Threads::gc_prologue();

  // 增加永久代回收的统计次数
  // Increment the invocation count for the permanent generation, since it is
  // implicitly collected whenever we do a full mark sweep collection.
  gch->perm_gen()->stat_record()->invocations++;

  // 统计GC前的内存堆已使用大小
  // Capture heap size before collection for printing.
  size_t gch_prev_used = gch->used();

  // 保存当前内存代和更低的内存代、以及永久代的已使用区域
  // 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(level, true /* perm */);

  // 初始化遍历栈,用来保存对象和对象头的对应关系
  allocate_stacks();

执行GC

GC使用标记-压缩-清理算法MarkSweepCompact,整个过程一共4阶段,分别对应4个方法的实现:

// Mark live objects
static void mark_sweep_phase1(int level, bool clear_all_softrefs);
// Calculate new addresses
static void mark_sweep_phase2();
// Update pointers
static void mark_sweep_phase3(int level);
// Move objects to new positions
static void mark_sweep_phase4();

一、mark_sweep_phase1: 标记活跃对象

1、标记根对象,这部分实现和新生代类似,只是不扫描Younger gens的对象

follow_root_closure.set_orig_generation(gch->get_gen(level));
gch->gen_process_strong_roots(level,
                                false, // Younger gens are not roots.
                                true,  // activate StrongRootsScope
                                true,  // Collecting permanent generation.
                                SharedHeap::SO_SystemClasses,
                                &follow_root_closure,
                                true,   // walk code active on stacks
                                &follow_root_closure);

其中follow_root_closure负责处理活跃对象,其工作函数如下:

void MarkSweep::FollowRootClosure::do_oop(oop* p)       { follow_root(p); }
void MarkSweep::FollowRootClosure::do_oop(narrowOop* p) { follow_root(p); }

工作函数接着调用follow_root()方法,完成活跃对象的标记工作,实现如下:

template <class T> inline void MarkSweep::follow_root(T* p) {
  // ... 省略一些代码
  T heap_oop = oopDesc::load_heap_oop(p);
  if (!oopDesc::is_null(heap_oop)) {
    oop obj = oopDesc::decode_heap_oop_not_null(heap_oop);
    if (!obj->mark()->is_marked()) {
      mark_object(obj);
      obj->follow_contents();
    }
  }
  follow_stack();
}

如果对象还没有被标记,即obj->mark()->is_marked()返回false,通过调用mark_object()方法标记该对象,接着调用follow_contents()follow_stack()方法处理该对象。

1) mark_object()实现对象的标记过程,如下:

inline void MarkSweep::mark_object(oop obj) {
  // some marks may contain information we need to preserve so we store them away
  // and overwrite the mark.  We'll restore it at the end of markSweep.
  markOop mark = obj->mark();
  obj->set_mark(markOopDesc::prototype()->set_marked());

  if (mark->must_be_preserved(obj)) {
    preserve_mark(obj, mark);
  }
}

设置对象的对象头为被标记状态,有些对象的对象头可能包含一些信息,需要在GC结束之后进行恢复,可以通过调用preserve_mark()方法保存对象和对应的对象头,实现如下:

void MarkSweep::preserve_mark(oop obj, markOop mark) {
  // We try to store preserved marks in the to space of the new generation since
  // this is storage which should be available.  Most of the time this should be
  // sufficient space for the marks we need to preserve but if it isn't we fall
  // back to using Stacks to keep track of the overflow.
  if (_preserved_count < _preserved_count_max) {
    _preserved_marks[_preserved_count++].init(obj, mark);
  } else {
    _preserved_mark_stack.push(mark);
    _preserved_oop_stack.push(obj);
  }
}

2)follow_contents()负责处理活跃对象的引用对象,实现如下:

inline void oopDesc::follow_contents(void) {
  assert (is_gc_marked(), "should be marked");
  blueprint()->oop_follow_contents(this);
}

其中对象实例instanceKlassoop_follow_contents()方法实现如下

void instanceKlass::oop_follow_contents(oop obj) {
  assert(obj != NULL, "can't follow the content of NULL object");
  obj->follow_header();
  InstanceKlass_OOP_MAP_ITERATE( \
    obj, \
    MarkSweep::mark_and_push(p), \
    assert_is_in_closed_subset)
}

inline void oopDesc::follow_header() {
  if (UseCompressedOops) {
    MarkSweep::mark_and_push(compressed_klass_addr());
  } else {
    MarkSweep::mark_and_push(klass_addr());
  }
}

可以发现,oop_follow_contents方法最终调用MarkSweep::mark_and_push方法处理引用对象,标记引用对象并插入到_marking_stack栈中

template <class T> inline void MarkSweep::mark_and_push(T* p) {
//  assert(Universe::heap()->is_in_reserved(p), "should be in object space");
  T heap_oop = oopDesc::load_heap_oop(p);
  if (!oopDesc::is_null(heap_oop)) {
    oop obj = oopDesc::decode_heap_oop_not_null(heap_oop);
    if (!obj->mark()->is_marked()) {
      mark_object(obj);
      _marking_stack.push(obj);
    }
  }
}

3)follow_stack()负责处理_marking_stack栈中的对象,并调用对象的follow_contents方法处理其引用对象,直到栈中的对象为空,实现如下:

void MarkSweep::follow_stack() {
  do {
    while (!_marking_stack.is_empty()) {
      oop obj = _marking_stack.pop();
      assert (obj->is_gc_marked(), "p must be marked");
      obj->follow_contents();
    }
    // Process ObjArrays one at a time to avoid marking stack bloat.
    if (!_objarray_stack.is_empty()) {
      ObjArrayTask task = _objarray_stack.pop();
      objArrayKlass* const k = (objArrayKlass*)task.obj()->blueprint();
      k->oop_follow_contents(task.obj(), task.index());
    }
  } while (!_marking_stack.is_empty() || !_objarray_stack.is_empty());
}

除了_marking_stack栈,还有一个_objarray_stack栈,用于处理数组对象,当数组非常大时,如果数组对象的引用全都放在标记栈中的话,就会出现爆栈的可能。

到此为止,所有的活跃对象都已经被标记。

2、处理在标记过程中发现的引用;

// Process reference objects found during marking
  {
    ref_processor()->setup_policy(clear_all_softrefs);
    ref_processor()->process_discovered_references(
      &is_alive, &keep_alive, &follow_stack_closure, NULL);
  }

3、卸载不再使用的类,并清理CodeCache和标记栈;

// Follow system dictionary roots and unload classes
bool purged_class = SystemDictionary::do_unloading(&is_alive);

// Follow code cache roots
CodeCache::do_unloading(&is_alive, &keep_alive, purged_class);
follow_stack(); // Flush marking stack

4、当有类卸载之后,需要更新存活类的子类、兄弟类、实现类的引用关系,清理未被标记的软引用和弱引用;

follow_weak_klass_links();
assert(_marking_stack.is_empty(), "just drained");

// Visit memoized MDO's and clear any unmarked weak refs
follow_mdo_weak_refs();
assert(_marking_stack.is_empty(), "just drained");

5、清理字符串常量池中没有被标记过的对象;

// Visit interned string tables and delete unmarked oops
StringTable::unlink(&is_alive);

// 实现
void StringTable::unlink(BoolObjectClosure* is_alive) {
  // Readers of the table are unlocked, so we should only be removing
  // entries at a safepoint.
  assert(SafepointSynchronize::is_at_safepoint(), "must be at safepoint");
  for (int i = 0; i < the_table()->table_size(); ++i) {
    for (HashtableEntry<oop>** p = the_table()->bucket_addr(i); *p != NULL; ) {
      HashtableEntry<oop>* entry = *p;
      if (entry->is_shared()) {
        break;
      }
      assert(entry->literal() != NULL, "just checking");
      if (is_alive->do_object_b(entry->literal())) {
        p = entry->next_addr();
      } else {
        *p = entry->next();
        the_table()->free_entry(entry);
      }
    }
  }
}

6、清理符号表中没有被引用的符号

// Clean up unreferenced symbols in symbol table.
SymbolTable::unlink();

// Remove unreferenced symbols from the symbol table
// This is done late during GC.  This doesn't use the hash table unlink because
// it assumes that the literals are oops.
void SymbolTable::unlink() {
  int removed = 0;
  int total = 0;
  size_t memory_total = 0;
  for (int i = 0; i < the_table()->table_size(); ++i) {
    for (HashtableEntry<Symbol*>** p = the_table()->bucket_addr(i); *p != NULL; ) {
      HashtableEntry<Symbol*>* entry = *p;
      if (entry->is_shared()) {
        break;
      }
      Symbol* s = entry->literal();
      memory_total += s->object_size();
      total++;
      assert(s != NULL, "just checking");
      // If reference count is zero, remove.
      if (s->refcount() == 0) {
        delete s;
        removed++;
        *p = entry->next();
        the_table()->free_entry(entry);
      } else {
        p = entry->next_addr();
      }
    }
  }
  symbols_removed += removed;
  symbols_counted += total;
  // Exclude printing for normal PrintGCDetails because people parse
  // this output.
  if (PrintGCDetails && Verbose && WizardMode) {
    gclog_or_tty->print(" [Symbols=%d size=" SIZE_FORMAT "K] ", total,
                        (memory_total*HeapWordSize)/1024);
  }
}

二、mark_sweep_phase2: 计算活跃对象在压缩完成之后的新地址

在第一步中,所有的活跃对象都已经被标记完成,接下来就是遍历所有的对象,把活跃对象移动到内存区域的一端,并重新计算新对象的地址,实现如下:

void GenMarkSweep::mark_sweep_phase2() {
  GenCollectedHeap* gch = GenCollectedHeap::heap();
  Generation* pg = gch->perm_gen();
   // ...
  VALIDATE_MARK_SWEEP_ONLY(reset_live_oop_tracking(false));
  gch->prepare_for_compaction();
  VALIDATE_MARK_SWEEP_ONLY(_live_oops_index_at_perm = _live_oops_index);
  CompactPoint perm_cp(pg, NULL, NULL);
  pg->prepare_for_compaction(&perm_cp);
}

其中prepare_for_compaction()定义在GenCollectedHeap中,实现如下:

void GenCollectedHeap::prepare_for_compaction() {
  Generation* scanning_gen = _gens[_n_gens-1];
  // Start by compacting into same gen.
  CompactPoint cp(scanning_gen, NULL, NULL);
  while (scanning_gen != NULL) {
    scanning_gen->prepare_for_compaction(&cp);
    scanning_gen = prev_gen(scanning_gen);
  }
}

prepare_for_compaction的方法名定义,可以看出这是进行压缩前的前期工作,在老年代中只有一个ContiguousSpace类型的内存区_the_space,它的prepare_for_compaction()方法实现如下:

// Faster object search.
void ContiguousSpace::prepare_for_compaction(CompactPoint* cp) {
  SCAN_AND_FORWARD(cp, top, block_is_always_obj, obj_size);
}

其中SCAN_AND_FORWARD函数的实现位于space.hpp文件中,为活跃对象计算新地址并保存在对象头,分析过程如下:
1、compact_top指针指向压缩目标的内存空间起始地址,在开始之前,指向当前内存区域的起始地址;

HeapWord* compact_top; /* This is where we are currently compacting to. */ 

/* We're sure to be here before any objects are compacted into this        
 * space, so this is a good time to initialize this:                       
 */                                                                        
set_compaction_top(bottom());

2、初始化CompactPoint,并设置当前要执行压缩的区域的指针compact_top,如果CompactPoint所对应的区域space为空,则初始化CompactPointspace为内存代的第一块区域,设置compact_top为区域的起始地址;否则设置compact_topCompactPoint中保存的值,继续该区域的压缩工作;

if (cp->space == NULL) {                                                   
  assert(cp->gen != NULL, "need a generation");                            
  assert(cp->threshold == NULL, "just checking");                          
  assert(cp->gen->first_compaction_space() == this, "just checking");      
  cp->space = cp->gen->first_compaction_space();                           
  compact_top = cp->space->bottom();                                       
  cp->space->set_compaction_top(compact_top);                              
  cp->threshold = cp->space->initialize_threshold();                       
} else {                                                                   
  compact_top = cp->space->compaction_top();                               
}

3、在没有明显的压缩效果之前,我们允许一些垃圾对象移动到内存区域的底部,即开始位置,每进行MarkSweepAlwaysCompactCount(默认4次)FGC时,再进行一次完全压缩,实现如下:

/* We allow some amount of garbage towards the bottom of the space, so     
 * we don't start compacting before there is a significant gain to be made.
 * Occasionally, we want to ensure a full compaction, which is determined  
 * by the MarkSweepAlwaysCompactCount parameter.                           
 */                                                                        
int invocations = SharedHeap::heap()->perm_gen()->stat_record()->invocations;
bool skip_dead = ((invocations % MarkSweepAlwaysCompactCount) != 0);       

size_t allowed_deadspace = 0;                                              
if (skip_dead) {                                                           
  const size_t ratio = allowed_dead_ratio();                               
  allowed_deadspace = (capacity() * ratio / 100) / HeapWordSize;           
}

其中invocations是FGC的总次数,当invocations不是4的倍数时,会在内存区域中留出一块大小为allowed_deadspace的死亡空间,默认为5%,用于后续使用;

4、定义一些基本变量:q为遍历指针,t为扫描边界,end_of_live为最后一个活跃对象的地址,LiveRange保存着死亡对象后面活跃对象的地址区间,first_dead为第一个死亡对象的地址,默认是该区域的末端地址;

HeapWord* q = bottom();                                                    
HeapWord* t = scan_limit();                                                

HeapWord*  end_of_live= q;    /* One byte beyond the last byte of the last 
                                 live object. */                           
HeapWord*  first_dead = end();/* The first dead object. */                 
LiveRange* liveRange  = NULL; /* The current live range, recorded in the   
                                 first header of preceding free area. */   
_first_dead = first_dead;

5、开始遍历区域中的对象
如果指针q所指向位置是一个对象,且被标识过,说明这是一个活跃的对象,则通过cp->space->forward()方法计算该对象压缩后的地址;

while (q < t) {                                                            
    if (block_is_obj(q) && oop(q)->is_gc_marked()) {                         
      /* prefetch beyond q */                                                
      Prefetch::write(q, interval);                                          
      /* size_t size = oop(q)->size();  changing this for cms for perm gen */
      size_t size = block_size(q);                                           
      compact_top = cp->space->forward(oop(q), size, cp, compact_top);       
      q += size;                                                             
      end_of_live = q;                                                       
    }

如果对象在压缩之后位置有变化,则将自己的对象头设置为压缩后地址信息,否则表示该对象不需要移动,设置对象头为默认值,并调用register_live_oop方法把原指针保存在栈_live_oops

// store the forwarding pointer into the mark word
  if ((HeapWord*)q != compact_top) {
    q->forward_to(oop(compact_top));
    assert(q->is_gc_marked(), "encoding the pointer should preserve the mark");
  } else {
    // if the object isn't moving we can just set the mark to the default
    // mark and handle it specially later on.
    q->init_mark();
    assert(q->forwardee() == NULL, "should be forwarded to NULL");
  }
 VALIDATE_MARK_SWEEP_ONLY(MarkSweep::register_live_oop(q, size));
 compact_top += size;

如果指针q所指向位置不是一个对象,或没有被标识过,说明是一个死亡对象,则直接跳过,直到碰到活跃对象为止,实现如下:

/* run over all the contiguous dead objects */                         
HeapWord* end = q;                                                     
do {                                                                   
  /* prefetch beyond end */                                            
  Prefetch::write(end, interval);                                      
  end += block_size(end);                                              
} while (end < t && (!block_is_obj(end) || !oop(end)->is_gc_marked()));

6、如果死亡空间allowed_deadspace可用,则计算死亡对象的大小总和为sz,则调用insert_deadspace()方法尝试插入一个大小为sz的对象,当做活跃对象进行处理,实现如下

/* see if we might want to pretend this object is alive so that        
 * we don't have to compact quite as often.                            
 */                                                                    
if (allowed_deadspace > 0 && q == compact_top) {                       
  size_t sz = pointer_delta(end, q);                                   
  if (insert_deadspace(allowed_deadspace, q, sz)) {                    
    compact_top = cp->space->forward(oop(q), sz, cp, compact_top);     
    q = end;                                                           
    end_of_live = end;                                                 
    continue;                                                          
  }                                                                    
}

bool CompactibleSpace::insert_deadspace(size_t& allowed_deadspace_words,
                                        HeapWord* q, size_t deadlength) {
  if (allowed_deadspace_words >= deadlength) {
    allowed_deadspace_words -= deadlength;
    CollectedHeap::fill_with_object(q, deadlength);
    oop(q)->set_mark(oop(q)->mark()->set_marked());
    assert((int) deadlength == oop(q)->size(), "bad filler object size");
    // Recall that we required "q == compaction_top".
    return true;
  } else {
    allowed_deadspace_words = 0;
    return false;
  }
}

如果死亡空间allowed_deadspace大于等于之前连续死亡对象大小总和,则更新allowed_deadspace值,并生成一个大小为sz且标识过的对象,这时需要更新压缩指针compact_top、遍历指针q和最后的活跃对象end_of_live,因为这里把新对象当成一个活跃对象进行处理,并继续往后遍历对象;

否则忽略这些死亡对象,进行以下步骤:
7、当执行到这一步时,说明跳过了一系列的死亡对象,遇到了活跃对象,如果liveRange不为空,则设置当前的结束位置为遍历指针q,此时q正指向死亡区域的第一个对象;由于在死亡对象后遇到了一个新的活跃对象,需要重新构造一个LiveRange对象来记录下一片活跃对象的地址范围,并设置开始和结束为止为end,这里直接把死亡区域的第一个对象当作LiveRange对象,实现如下

/* for the previous LiveRange, record the end of the live objects. */  
if (liveRange) {                                                       
  liveRange->set_end(q);                                               
}                                                                      

/* record the current LiveRange object.                                
 * liveRange->start() is overlaid on the mark word.                    
 */                                                                    
liveRange = (LiveRange*)q;                                             
liveRange->set_start(end);                                             
liveRange->set_end(end);

8、保存第一个死亡对象的地址,并将遍历指针q指向end的位置继续遍历

/* see if this is the first dead region. */                            
if (q < first_dead) {                                                  
  first_dead = q;                                                      
}                                                                      

/* move on to the next object */                                       
q = end;

9、遍历完成之后,如果当前的liveRange不为空,则设置该liveRange的结束位置为q,设置最后一个活跃对象的位置_end_of_live,根据_end_of_live的值重新设置第一个死亡对象的位置_first_dead

if (liveRange != NULL) { 
  liveRange->set_end(q); 
} 
_end_of_live = end_of_live;                                               
if (end_of_live < first_dead) {
  first_dead = end_of_live;
}
_first_dead = first_dead;

10、记录当前区域的压缩位置

cp->space->set_compaction_top(compact_top);

三、mark_sweep_phase3:更新对象的引用地址

1、调用gen_process_strong_roots()并使用adjust_root_pointer_closure处理函数调整根对象指针的引用地址,adjust_root_pointer_closure的实现如下:

void MarkSweep::AdjustPointerClosure::do_oop(oop* p)       { adjust_pointer(p, _is_root); }
void MarkSweep::AdjustPointerClosure::do_oop(narrowOop* p) { adjust_pointer(p, _is_root); }

其中adjust_pointer()方法定义在markSweep.inline.hpp文件中,通过解析对象的对象头,判断对象头中是否保存着经过压缩后的新地址,实现如下

template <class T> inline void MarkSweep::adjust_pointer(T* p, bool isroot) {
  T heap_oop = oopDesc::load_heap_oop(p);
  if (!oopDesc::is_null(heap_oop)) {
    oop obj     = oopDesc::decode_heap_oop_not_null(heap_oop);
    oop new_obj = oop(obj->mark()->decode_pointer());
    // ....
    if (new_obj != NULL) {
     // ...
      oopDesc::encode_store_heap_oop_not_null(p, new_obj);
    }
  }
  VALIDATE_MARK_SWEEP_ONLY(track_adjusted_pointer(p, isroot));
}

2、adjust_code_pointer_closure()方法调整引用指针的引用地址;

// Now adjust pointers in remaining weak roots.  (All of which should
// have been cleared if they pointed to non-surviving objects.)
CodeBlobToOopClosure adjust_code_pointer_closure(&adjust_pointer_closure,
                                                   /*do_marking=*/ false);
gch->gen_process_weak_roots(&adjust_root_pointer_closure,
                              &adjust_code_pointer_closure,
                              &adjust_pointer_closure);

3、使用GenAdjustPointersClosure遍历各内存代,以更新引用对象的引用地址;

adjust_marks();
GenAdjustPointersClosure blk;
gch->generation_iterate(&blk, true);
pg->adjust_pointers();

四、mark_sweep_phase4:移动所有活跃对象到新地址

1、压缩永久代的对象,只有等永久代的对象压缩后,实例对象才能获取正确的类数据地址;
2、使用GenCompactClosure遍历堆上的对象

GenCompactClosure blk;
gch->generation_iterate(&blk, true);

其中generation_iterate()将调用GenCompactClosuredo_generation()方法遍历各个内存代,实现如下

void GenCollectedHeap::generation_iterate(GenClosure* cl,
                                          bool old_to_young) {
  if (old_to_young) {
    for (int i = _n_gens-1; i >= 0; i--) {
      cl->do_generation(_gens[i]);
    }
  } else {
    for (int i = 0; i < _n_gens; i++) {
      cl->do_generation(_gens[i]);
    }
  }
}

GenCompactClosuredo_generation()方法负责调用各个内存代的compact()进行压缩工作

class GenCompactClosure: public GenCollectedHeap::GenClosure {
public:
  void do_generation(Generation* gen) {
    gen->compact();
  }
};

其中老年代的compact()方法实现如下:

void CompactibleSpace::compact() {
  SCAN_AND_COMPACT(obj_size);
}

调用了SCAN_AND_COMPACT函数进行对象的移动

1、变量q是遍历指针,默认为内存区域的起始地址,t是最后一个活跃对象的位置,至于为什么要记录最后一个活跃对象的位置,主要是为了避免当GC后的活跃对象较少时,进行不必要的遍历

#define SCAN_AND_COMPACT(obj_size) {                                            
  /* Copy all live objects to their new location                                
   * Used by MarkSweep::mark_sweep_phase4() */                                  

  HeapWord*       q = bottom();                                                 
  HeapWord* const t = _end_of_live;

2、移动第一个死亡对象之前的活跃对象到新的位置

if (q < t && _first_dead > q &&  !oop(q)->is_gc_marked()) {    
  HeapWord* const end = _first_dead;    
  while (q < end) {        
    size_t size = obj_size(q); 
    VALIDATE_MARK_SWEEP_ONLY(MarkSweep::live_oop_moved_to(q, size, q));          
    q += size;                                                                
  }

3、当遍历到_first_dead时,即第一个死亡对象的位置,如果_first_dead不等于_end_of_live,说明有连续多个死亡对象,而且在第一个死亡对象的对象头保存着LiveRange,通过LiveRange可以获取下一个活跃对象的地址

  if (_first_dead == t) {                                                     
    q = t;                                                                    
  } else {                                                                    
    /* $$$ Funky */                                                           
    q = (HeapWord*) oop(_first_dead)->mark()->decode_pointer();               
  }

4、从新的活跃对象开始新的遍历
如果是死亡对象,则通过LiveRange获取下一个存活对象的地址

while (q < t) {                                                               
  if (!oop(q)->is_gc_marked()) {                                              
    /* mark is pointer to next marked oop */                                  
    debug_only(prev_q = q);                                                   
    q = (HeapWord*) oop(q)->mark()->decode_pointer();                         
    assert(q > prev_q, "we should be moving forward through memory");         
  }

5、如果是活跃对象,则调用live_oop_moved_to方法将对象移动到压缩后的新地址,并初始化新对象的对象头,实现如下

Prefetch::read(q, scan_interval);                                         

/* size and destination */                                                
size_t size = obj_size(q);                                                
HeapWord* compaction_top = (HeapWord*)oop(q)->forwardee();                

/* prefetch beyond compaction_top */                                      
Prefetch::write(compaction_top, copy_interval);                           

/* copy object and reinit its mark */                                     
VALIDATE_MARK_SWEEP_ONLY(
  MarkSweep::live_oop_moved_to(q, size, compaction_top)
);   
//...
Copy::aligned_conjoint_words(q, compaction_top, size);                    
oop(compaction_top)->init_mark();                                         

q += size;

其中live_oop_moved_to()方法实现如下:

void MarkSweep::live_oop_moved_to(HeapWord* q, size_t size,
                                  HeapWord* compaction_top) {
  assert(oop(q)->forwardee() == NULL || oop(q)->forwardee() == oop(compaction_top),
         "should be moved to forwarded location");
  if (ValidateMarkSweep) {
    MarkSweep::validate_live_oop(oop(q), size);
    _live_oops_moved_to->push(oop(compaction_top));
  }
  if (RecordMarkSweepCompaction) {
    _cur_gc_live_oops->push(q);
    _cur_gc_live_oops_moved_to->push(compaction_top);
    _cur_gc_live_oops_size->push(size);
  }
}

原对象的指针已经被保存在_live_oops栈中,对应的把压缩后的对象指针保存在_live_oops_moved_to



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