内存代管理器TenuredGeneration对垃圾对象的回收

怼烎@ 2022-08-06 11:13 138阅读 0赞

内存代管理器TenuredGeneration作为基于内存分代管理的内存堆管理器GenCollectedHeap默认的旧生代管理器,它对垃圾对象的回收算法要比年青代Gc要复杂的多,但其主体思路就是:标记-清除-压缩.本文主要围绕内存代Gc时的三个过程来详细讲解TenuredGeneration是如何进行垃圾回收的.

## 一.Gc条件 ##

可对 TenuredGeneration管理的旧生代进行Gc的条件主要有4个,满足其中的任何一个即可对旧生代进行垃圾回收:

1.当前是Full Gc
    2.可在当前内存代分配请求的空间
    3.当前内存代空闲空间<10000
    4.当前内存代容量>上一次Gc之前的容量

具体代码可参见如下:

bool TenuredGeneration::should_collect(bool  full, size_t size, bool   is_tlab) {
      // This should be one big conditional or (||), but I want to be able to tell
      // why it returns what it returns (without re-evaluating the conditionals
      // in case they aren't idempotent), so I'm doing it this way.
      // DeMorgan says it's okay.
      bool result = false;
    
      if (!result && full) {
        result = true;
        if (PrintGC && Verbose) {
          gclog_or_tty->print_cr("TenuredGeneration::should_collect: because full");
        }
      }
    
      if (!result && should_allocate(size, is_tlab)) {
        result = true;
        if (PrintGC && Verbose) {
          gclog_or_tty->print_cr("TenuredGeneration::should_collect: because"
                        " should_allocate(" SIZE_FORMAT ")", size);
        }
      }
    
      // If we don't have very much free space.
      // XXX: 10000 should be a percentage of the capacity!!!
      if (!result && free() < 10000) {
        result = true;
        if (PrintGC && Verbose) {
          gclog_or_tty->print_cr("TenuredGeneration::should_collect: because"
                        " free(): " SIZE_FORMAT,
                        free());
        }
      }
    
      // If we had to expand to accomodate promotions from younger generations
      if (!result && _capacity_at_prologue < capacity()) {
        result = true;
        if (PrintGC && Verbose) {
          gclog_or_tty->print_cr("TenuredGeneration::should_collect: because"
                        "_capacity_at_prologue: " SIZE_FORMAT " < capacity(): " SIZE_FORMAT,
                        _capacity_at_prologue, capacity());
        }
      }
    
      return result;
    }

## 二.Gc基本流程 ##

默认的旧生代管理器TenuredGeneration回收垃圾对象的基本思路就是:

第一步: 标记所有的active对象
    
    第二步: 计算所有active对象在其内存代压缩后的偏移位置
    
    第三步: 更新所有active对象的地址映射表
    
    第四步: 移动复制所有的active对象到新的存储位置

### 1.标记所有的active对象 ###

TenuredGeneration标记所有active对象的过程跟年青代Gc相似,都是从根对象开始以深度优先的方式搜索标记所有的active对象,具体过程如下:

/**
     * 从根对象开始递归迭代标记所有活动的对象
     */
    void GenMarkSweep::mark_sweep_phase1(int level,
                                      bool clear_all_softrefs) {
      // Recursively traverse all live objects and mark them
      TraceTime tm("phase 1", PrintGC && Verbose, true, gclog_or_tty);
      trace(" 1");
    
      VALIDATE_MARK_SWEEP_ONLY(reset_live_oop_tracking(false));
    
      GenCollectedHeap* gch = GenCollectedHeap::heap();
    
      // Because follow_root_closure is created statically, cannot
      // use OopsInGenClosure constructor which takes a generation,
      // as the Universe has not been created when the static constructors
      // are run.
      follow_root_closure.set_orig_generation(gch->get_gen(level));
    
      /**
       * 遍历当前所有的根对象并标记,并递归遍历标记它们的引用对象
       */
      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);
    
      //标记所有的软引用对象
      {
        ref_processor()->setup_policy(clear_all_softrefs);
        ref_processor()->process_discovered_references(&is_alive, &keep_alive, &follow_stack_closure, NULL);
      }
    
      // Follow system dictionary roots and unload classes
      bool purged_class = SystemDictionary::do_unloading(&is_alive);
    
      // 清理代码高速缓冲区
      CodeCache::do_unloading(&is_alive, &keep_alive, purged_class);
      follow_stack(); // Flush marking stack
    
      // Update subklass/sibling/implementor links of live klasses
      follow_weak_klass_links();
      assert(_marking_stack.is_empty(), "just drained");
    
      //清理未被标记的软/弱引用对象
      follow_mdo_weak_refs();
      assert(_marking_stack.is_empty(), "just drained");
    
      //清除没有被引用的常量字符串
      StringTable::unlink(&is_alive);
    
      //清除符号表中没有被引用的符号
      SymbolTable::unlink();
    
      assert(_marking_stack.is_empty(), "stack should be empty by now");
    }

### 2.计算所有active对象在其内存代压缩后的偏移位置 ###

以内存代为单位计算各内存代中的active对象压缩后的新存储位置:

/**
     * 通过对象的标记,计算存活的对象在其内存区压缩后的偏移位置
     */
    void GenMarkSweep::mark_sweep_phase2() {
      // Now all live objects are marked, compute the new object addresses.
    
      // It is imperative that we traverse perm_gen LAST. If dead space is
      // allowed a range of dead object may get overwritten by a dead int
      // array. If perm_gen is not traversed last a klassOop may get
      // overwritten. This is fine since it is dead, but if the class has dead
      // instances we have to skip them, and in order to find their size we
      // need the klassOop!
      //
      // It is not required that we traverse spaces in the same order in
      // phase2, phase3 and phase4, but the ValidateMarkSweep live oops
      // tracking expects us to do so. See comment under phase4.
    
      GenCollectedHeap* gch = GenCollectedHeap::heap();
      Generation* pg = gch->perm_gen();
    
      TraceTime tm("phase 2", PrintGC && Verbose, true, gclog_or_tty);
      trace("2");
    
      VALIDATE_MARK_SWEEP_ONLY(reset_live_oop_tracking(false));
    
      //计算年青代+老生代中active对象在其内存代压缩后的偏移位置
      gch->prepare_for_compaction();
    
      VALIDATE_MARK_SWEEP_ONLY(_live_oops_index_at_perm = _live_oops_index);
    
      //计算永久代中active对象在其内存代压缩后的偏移位置
      CompactPoint perm_cp(pg, NULL, NULL);
      pg->prepare_for_compaction(&perm_cp);
    
    }

在内存代内部,又是以内存区为单位来计算的:

/**
     * 计算内存代放入各个内存区中活着的对象在其压缩后的偏移位置
     */
    void Generation::prepare_for_compaction(CompactPoint* cp) {
      // Generic implementation, can be specialized
      CompactibleSpace* space = first_compaction_space();
      while (space != NULL) {
        space->prepare_for_compaction(cp);
        space = space->next_compaction_space();
      }
    }

内存区的计算方法是(对象新的存储地址存放在对象实例的\_makr属性中):

/**
     * 计算内存区中所有活着(被标记)的对象在其被压缩后的偏移位置
     */
    void CompactibleSpace::prepare_for_compaction(CompactPoint* cp) {
      SCAN_AND_FORWARD(cp, end, block_is_obj, block_size);
    }
    
    #define SCAN_AND_FORWARD(cp,scan_limit,block_is_obj,block_size) {            \
      /* Compute the new addresses for the live objects and store it in the mark \
       * Used by universe::mark_sweep_phase2()                                   \
       */                                                                        \
      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());                                              \
                                                                                 \
      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();                               \
      }                                                                          \
                                                                                 \
      /* 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 = (MarkSweepAlwaysCompactCount < 1)                         \
        ||((invocations % MarkSweepAlwaysCompactCount) != 0);                    \
                                                                                 \
      size_t allowed_deadspace = 0;                                              \
      if (skip_dead) {                                                           \
        const size_t ratio = allowed_dead_ratio();                               \
        allowed_deadspace = (capacity() * ratio / 100) / HeapWordSize;           \
      }                                                                          \
                                                                                 \
      /**                                                                        \
       * 当前内存区压缩的开始位置和结束位置                                                                                                                                                   \
       */                                                                        \
      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;   /*当前内存区中最后一个active对象*/                 \
                                                                                 \
      const intx interval = PrefetchScanIntervalInBytes;                         \
                                                                                 \
       /**                                                                       \
        * 开始遍历该内存区分配的所有对象                                                                                                                                                        \
        */         																 \
      while (q < t) {                                                            \
        assert(!block_is_obj(q) ||                                               \
               oop(q)->mark()->is_marked() || oop(q)->mark()->is_unlocked() ||   \
               oop(q)->mark()->has_bias_pattern(),                               \
               "these are the only valid states during a mark sweep");           \
        /**                                                                      \
         * active对象,则确定其新的存储位置                                           \
         */                                                                      \
        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 */\
          /*对象大小*/                                                            \
          size_t size = block_size(q);                                           \
          compact_top = cp->space->forward(oop(q), size, cp, compact_top);       \
          q += size;                                                             \
          end_of_live = q;                                                       \
        } else {                                                                 \
          /* 垃圾对象,则一直遍历到其下一个active对象 */                                \
          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()));\
                                                                                 \
          /* 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;                                                          \
            }                                                                    \
          }                                                                      \
                                                                                 \
          /* otherwise, it really is a free region. */                           \
                                                                                 \
          /* 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);                                               \
                                                                                 \
          /* see if this is the first dead region. */                            \
          if (q < first_dead) {                                                  \
            first_dead = q;                                                      \
          }                                                                      \
                                                                                 \
          /* move on to the next object */                                       \
          q = end;                                                               \
        }                                                                        \
      }                                                                          \
                                                                                 \
      assert(q == t, "just checking");                                           \
      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;                                                  \
                                                                                 \
      /* save the compaction_top of the compaction space. */                     \
      cp->space->set_compaction_top(compact_top);                                \
    }

### 3.更新所有active对象的地址映射表 ###

该过程的实现和标记所有的active对象很相似,就是一个是标记,一个是更新地址指针:

/**
     * 从根对象开始递归迭代更新所有活动对象的地址映射表(调整所有的对象指针)
     */
    void GenMarkSweep::mark_sweep_phase3(int level) {
      GenCollectedHeap* gch = GenCollectedHeap::heap();
      Generation* pg = gch->perm_gen();
    
      // Adjust the pointers to reflect the new locations
      TraceTime tm("phase 3", PrintGC && Verbose, true, gclog_or_tty);
      trace("3");
    
      VALIDATE_MARK_SWEEP_ONLY(reset_live_oop_tracking(false));
    
      // Needs to be done before the system dictionary is adjusted.
      pg->pre_adjust_pointers();
    
      // Because the two closures below are created statically, cannot
      // use OopsInGenClosure constructor which takes a generation,
      // as the Universe has not been created when the static constructors
      // are run.
      adjust_root_pointer_closure.set_orig_generation(gch->get_gen(level));
      adjust_pointer_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_AllClasses,
                                    &adjust_root_pointer_closure,
                                    false, // do not walk code
                                    &adjust_root_pointer_closure);
    
      // 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);
    
      adjust_marks();
      GenAdjustPointersClosure blk;
      gch->generation_iterate(&blk, true);
      pg->adjust_pointers();
    }

这里的对象地址映射表实际上就是前面的博文 [内存堆Gc时公认的根对象 ][Gc_]中提到的实例对象句柄.

/**
     * 调整对象的物理地址指针
     */
    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());	//对象新的地址指针
        assert(new_obj != NULL ||                         // is forwarding ptr?
               obj->mark() == markOopDesc::prototype() || // not gc marked?
               (UseBiasedLocking && obj->mark()->has_bias_pattern()) ||
                                                          // not gc marked?
               obj->is_shared(),                          // never forwarded?
               "should be forwarded");
    
        if (new_obj != NULL) {
          assert(Universe::heap()->is_in_reserved(new_obj),
                 "should be in object space");
    
          //更新对象的物理地址指针
          oopDesc::encode_store_heap_oop_not_null(p, new_obj);
        }
      }
    
      VALIDATE_MARK_SWEEP_ONLY(track_adjusted_pointer(p, isroot));
    }

### 4.移动复制所有的active对象到新的存储位置    ###

该过程跟计算所有active对象在其内存代压缩后的偏移位置的操作流程很相似,这里不在详细赘述.

## 三.Gc后的内存代大小调整 ##

内存堆管理器在对某些内存代进行Gc之后,基本都会调整他们的内存容量,即对内存代进行扩容还是缩容.这个操作主要受控于两个参数:最小空闲率(MinHeapFreeRatio)和最大空闲率MaxHeapFreeRatio.TenuredGeneration根据这两个参数来调整旧生代的容量过程如下:

/**
     * 调整当前内存代的容量(一般发生在一次Gc之后)
     */
    void TenuredGeneration::compute_new_size() {
      assert(_shrink_factor <= 100, "invalid shrink factor");
      size_t current_shrink_factor = _shrink_factor;
      _shrink_factor = 0;
    
      printf("%s[%d] [tid: %lu]: 当前内存代[%s]开始调整其容量大小.\n", __FILE__, __LINE__, pthread_self(), this->name());
    
      // We don't have floating point command-line arguments
      // Note:  argument processing ensures that MinHeapFreeRatio < 100.
      const double minimum_free_percentage = MinHeapFreeRatio / 100.0;	//内存代的最小空闲率
      const double maximum_used_percentage = 1.0 - minimum_free_percentage;
    
      //一次Gc之后，当前内存代的使用量和容量
      const size_t used_after_gc = used();
      const size_t capacity_after_gc = capacity();
    
      //计算当前内存代期望的最小容量
      const double min_tmp = used_after_gc / maximum_used_percentage;
      size_t minimum_desired_capacity = (size_t)MIN2(min_tmp, double(max_uintx));
      // Don't shrink less than the initial generation size
      minimum_desired_capacity = MAX2(minimum_desired_capacity, spec()->init_size());
    
      assert(used_after_gc <= minimum_desired_capacity, "sanity check");
    
      if (PrintGC && Verbose) {
        const size_t free_after_gc = free();
        const double free_percentage = ((double)free_after_gc) / capacity_after_gc;
        gclog_or_tty->print_cr("TenuredGeneration::compute_new_size: ");
        gclog_or_tty->print_cr("  "
                      "  minimum_free_percentage: %6.2f"
                      "  maximum_used_percentage: %6.2f",
                      minimum_free_percentage,
                      maximum_used_percentage);
        gclog_or_tty->print_cr("  "
                      "   free_after_gc   : %6.1fK"
                      "   used_after_gc   : %6.1fK"
                      "   capacity_after_gc   : %6.1fK",
                      free_after_gc / (double) K,
                      used_after_gc / (double) K,
                      capacity_after_gc / (double) K);
        gclog_or_tty->print_cr("  "
                      "   free_percentage: %6.2f",
                      free_percentage);
      }
    
      //当前内存代的实际容量小于期望的容量，则扩展当前内存代的容量
      if (capacity_after_gc < minimum_desired_capacity) {
        // If we have less free space than we want then expand
        size_t expand_bytes = minimum_desired_capacity - capacity_after_gc;
    
        // Don't expand unless it's significant
        if (expand_bytes >= _min_heap_delta_bytes) {
          printf("%s[%d] [tid: %lu]: Gc之后,试图为内存代[%s]的容量扩大 %lu bytes.\n", __FILE__, __LINE__, pthread_self(), this->name(), expand_bytes);
          expand(expand_bytes, 0); // safe if expansion fails
        }
    
        if (PrintGC && Verbose) {
          gclog_or_tty->print_cr("    expanding:"
                        "  minimum_desired_capacity: %6.1fK"
                        "  expand_bytes: %6.1fK"
                        "  _min_heap_delta_bytes: %6.1fK",
                        minimum_desired_capacity / (double) K,
                        expand_bytes / (double) K,
                        _min_heap_delta_bytes / (double) K);
        }
    
        return;
      }
    
      // No expansion, now see if we want to shrink
      size_t shrink_bytes = 0;
      // We would never want to shrink more than this
      size_t max_shrink_bytes = capacity_after_gc - minimum_desired_capacity;
    
      if (MaxHeapFreeRatio < 100) {
    	//计算当前内存代期望的大容量
        const double maximum_free_percentage = MaxHeapFreeRatio / 100.0;
        const double minimum_used_percentage = 1.0 - maximum_free_percentage;
        const double max_tmp = used_after_gc / minimum_used_percentage;
        size_t maximum_desired_capacity = (size_t)MIN2(max_tmp, double(max_uintx));
        maximum_desired_capacity = MAX2(maximum_desired_capacity, spec()->init_size());
    
        if (PrintGC && Verbose) {
          gclog_or_tty->print_cr("  "
                                 "  maximum_free_percentage: %6.2f"
                                 "  minimum_used_percentage: %6.2f",
                                 maximum_free_percentage,
                                 minimum_used_percentage);
          gclog_or_tty->print_cr("  "
                                 "  _capacity_at_prologue: %6.1fK"
                                 "  minimum_desired_capacity: %6.1fK"
                                 "  maximum_desired_capacity: %6.1fK",
                                 _capacity_at_prologue / (double) K,
                                 minimum_desired_capacity / (double) K,
                                 maximum_desired_capacity / (double) K);
        }
        assert(minimum_desired_capacity <= maximum_desired_capacity,
               "sanity check");
    
        //当前内存代Gc之后的容量大于期望的最大容量
        if (capacity_after_gc > maximum_desired_capacity) {
          // Capacity too large, compute shrinking size
          shrink_bytes = capacity_after_gc - maximum_desired_capacity;
          // We don't want shrink all the way back to initSize if people call
          // System.gc(), because some programs do that between "phases" and then
          // we'd just have to grow the heap up again for the next phase.  So we
          // damp the shrinking: 0% on the first call, 10% on the second call, 40%
          // on the third call, and 100% by the fourth call.  But if we recompute
          // size without shrinking, it goes back to 0%.
          shrink_bytes = shrink_bytes / 100 * current_shrink_factor;
          assert(shrink_bytes <= max_shrink_bytes, "invalid shrink size");
    
          if (current_shrink_factor == 0) {
            _shrink_factor = 10;
          } else {
            _shrink_factor = MIN2(current_shrink_factor * 4, (size_t) 100);
          }
    
          if (PrintGC && Verbose) {
            gclog_or_tty->print_cr("  "
                          "  shrinking:"
                          "  initSize: %.1fK"
                          "  maximum_desired_capacity: %.1fK",
                          spec()->init_size() / (double) K,
                          maximum_desired_capacity / (double) K);
            gclog_or_tty->print_cr("  "
                          "  shrink_bytes: %.1fK"
                          "  current_shrink_factor: %d"
                          "  new shrink factor: %d"
                          "  _min_heap_delta_bytes: %.1fK",
                          shrink_bytes / (double) K,
                          current_shrink_factor,
                          _shrink_factor,
                          _min_heap_delta_bytes / (double) K);
          }
        }
      }
    
      if (capacity_after_gc > _capacity_at_prologue) {
        //当前内存代Gc之后的容量大于Gc之前的容量,那么就是在内存代Gc时为了存储升级来的active对象而扩展了内存容量,
    	//现在至少应该缩小到Gc之前的容量大小
        size_t expansion_for_promotion = capacity_after_gc - _capacity_at_prologue;
        expansion_for_promotion = MIN2(expansion_for_promotion, max_shrink_bytes);
        // We have two shrinking computations, take the largest
        shrink_bytes = MAX2(shrink_bytes, expansion_for_promotion);
    
        assert(shrink_bytes <= max_shrink_bytes, "invalid shrink size");
        if (PrintGC && Verbose) {
          gclog_or_tty->print_cr("  "
                                 "  aggressive shrinking:"
                                 "  _capacity_at_prologue: %.1fK"
                                 "  capacity_after_gc: %.1fK"
                                 "  expansion_for_promotion: %.1fK"
                                 "  shrink_bytes: %.1fK",
                                 capacity_after_gc / (double) K,
                                 _capacity_at_prologue / (double) K,
                                 expansion_for_promotion / (double) K,
                                 shrink_bytes / (double) K);
        }
      }
    
      // Don't shrink unless it's significant
      if (shrink_bytes >= _min_heap_delta_bytes) {
    	printf("%s[%d] [tid: %lu]: Gc之后,试图为内存代[%s]的容量缩小 %lu bytes.\n", __FILE__, __LINE__, pthread_self(), this->name(), shrink_bytes);
        shrink(shrink_bytes);
      }
    
      assert(used() == used_after_gc && used_after_gc <= capacity(),
             "sanity check");
    }

[Gc_]: http://blog.csdn.net/xhh198781/article/details/42213847