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# JVM源码分析之G1垃圾收集器分析一

发表于 2019-12-07 更新于 2019-12-20 分类于 Java ， JVM ， GC ， C++ 阅读次数： 阅读次数： Disqus：

对 Java 的 gc 实现比较感兴趣，原先一般都是看周志明的书，但其实并没有讲具体的 gc 源码，而是把整个思路和流程讲解了一下
特别是 G1 的具体实现
一般对 G1 的理解其实就是把原先整块的新生代老年代分成了以 region 为单位的小块内存，简而言之，就是原先对新生代老年代的收集会涉及到整个代的堆内存空间，而G1 把它变成了更细致的小块内存
这带来了一个很明显的好处和一个很明显的坏处，好处是内存收集可以更灵活，耗时会变短，但整个收集的处理复杂度就变高了
目前看了一点点关于 G1 收集的预期时间相关的代码

    HeapWord* G1CollectedHeap::do_collection_pause(size_t word_size,
                                                   uint gc_count_before,
                                                   bool* succeeded,
                                                   GCCause::Cause gc_cause) {
      assert_heap_not_locked_and_not_at_safepoint();
      VM_G1CollectForAllocation op(word_size,
                                   gc_count_before,
                                   gc_cause,
                                   false, /* should_initiate_conc_mark */
                                   g1_policy()->max_pause_time_ms());
      VMThread::execute(&op);

      HeapWord* result = op.result();
      bool ret_succeeded = op.prologue_succeeded() && op.pause_succeeded();
      assert(result == NULL || ret_succeeded,
             "the result should be NULL if the VM did not succeed");
      *succeeded = ret_succeeded;

      assert_heap_not_locked();
      return result;
    }

这里就是收集时需要停顿的，其中`VMThread::execute(&op);`是具体执行的，真正执行的是`VM_G1CollectForAllocation::doit`方法

    void VM_G1CollectForAllocation::doit() {
      G1CollectedHeap* g1h = G1CollectedHeap::heap();
      assert(!_should_initiate_conc_mark || g1h->should_do_concurrent_full_gc(_gc_cause),
          "only a GC locker, a System.gc(), stats update, whitebox, or a hum allocation induced GC should start a cycle");

      if (_word_size > 0) {
        // An allocation has been requested. So, try to do that first.
        _result = g1h->attempt_allocation_at_safepoint(_word_size,
                                                       false /* expect_null_cur_alloc_region */);
        if (_result != NULL) {
          // If we can successfully allocate before we actually do the
          // pause then we will consider this pause successful.
          _pause_succeeded = true;
          return;
        }
      }

      GCCauseSetter x(g1h, _gc_cause);
      if (_should_initiate_conc_mark) {
        // It's safer to read old_marking_cycles_completed() here, given
        // that noone else will be updating it concurrently. Since we'll
        // only need it if we're initiating a marking cycle, no point in
        // setting it earlier.
        _old_marking_cycles_completed_before = g1h->old_marking_cycles_completed();

        // At this point we are supposed to start a concurrent cycle. We
        // will do so if one is not already in progress.
        bool res = g1h->g1_policy()->force_initial_mark_if_outside_cycle(_gc_cause);

        // The above routine returns true if we were able to force the
        // next GC pause to be an initial mark; it returns false if a
        // marking cycle is already in progress.
        //
        // If a marking cycle is already in progress just return and skip the
        // pause below - if the reason for requesting this initial mark pause
        // was due to a System.gc() then the requesting thread should block in
        // doit_epilogue() until the marking cycle is complete.
        //
        // If this initial mark pause was requested as part of a humongous
        // allocation then we know that the marking cycle must just have
        // been started by another thread (possibly also allocating a humongous
        // object) as there was no active marking cycle when the requesting
        // thread checked before calling collect() in
        // attempt_allocation_humongous(). Retrying the GC, in this case,
        // will cause the requesting thread to spin inside collect() until the
        // just started marking cycle is complete - which may be a while. So
        // we do NOT retry the GC.
        if (!res) {
          assert(_word_size == 0, "Concurrent Full GC/Humongous Object IM shouldn't be allocating");
          if (_gc_cause != GCCause::_g1_humongous_allocation) {
            _should_retry_gc = true;
          }
          return;
        }
      }

      // Try a partial collection of some kind.
      _pause_succeeded = g1h->do_collection_pause_at_safepoint(_target_pause_time_ms);

      if (_pause_succeeded) {
        if (_word_size > 0) {
          // An allocation had been requested. Do it, eventually trying a stronger
          // kind of GC.
          _result = g1h->satisfy_failed_allocation(_word_size, &_pause_succeeded);
        } else {
          bool should_upgrade_to_full = !g1h->should_do_concurrent_full_gc(_gc_cause) &&
                                        !g1h->has_regions_left_for_allocation();
          if (should_upgrade_to_full) {
            // There has been a request to perform a GC to free some space. We have no
            // information on how much memory has been asked for. In case there are
            // absolutely no regions left to allocate into, do a maximally compacting full GC.
            log_info(gc, ergo)("Attempting maximally compacting collection");
            _pause_succeeded = g1h->do_full_collection(false, /* explicit gc */
                                                       true   /* clear_all_soft_refs */);
          }
        }
        guarantee(_pause_succeeded, "Elevated collections during the safepoint must always succeed.");
      } else {
        assert(_result == NULL, "invariant");
        // The only reason for the pause to not be successful is that, the GC locker is
        // active (or has become active since the prologue was executed). In this case
        // we should retry the pause after waiting for the GC locker to become inactive.
        _should_retry_gc = true;
      }
    }

这里可以看到核心的是`G1CollectedHeap::do_collection_pause_at_safepoint`这个方法，它带上了目标暂停时间的值

    G1CollectedHeap::do_collection_pause_at_safepoint(double target_pause_time_ms) {
      assert_at_safepoint_on_vm_thread();
      guarantee(!is_gc_active(), "collection is not reentrant");

      if (GCLocker::check_active_before_gc()) {
        return false;
      }

      _gc_timer_stw->register_gc_start();

      GCIdMark gc_id_mark;
      _gc_tracer_stw->report_gc_start(gc_cause(), _gc_timer_stw->gc_start());

      SvcGCMarker sgcm(SvcGCMarker::MINOR);
      ResourceMark rm;

      g1_policy()->note_gc_start();

      wait_for_root_region_scanning();

      print_heap_before_gc();
      print_heap_regions();
      trace_heap_before_gc(_gc_tracer_stw);

      _verifier->verify_region_sets_optional();
      _verifier->verify_dirty_young_regions();

      // We should not be doing initial mark unless the conc mark thread is running
      if (!_cm_thread->should_terminate()) {
        // This call will decide whether this pause is an initial-mark
        // pause. If it is, in_initial_mark_gc() will return true
        // for the duration of this pause.
        g1_policy()->decide_on_conc_mark_initiation();
      }

      // We do not allow initial-mark to be piggy-backed on a mixed GC.
      assert(!collector_state()->in_initial_mark_gc() ||
              collector_state()->in_young_only_phase(), "sanity");

      // We also do not allow mixed GCs during marking.
      assert(!collector_state()->mark_or_rebuild_in_progress() || collector_state()->in_young_only_phase(), "sanity");

      // Record whether this pause is an initial mark. When the current
      // thread has completed its logging output and it's safe to signal
      // the CM thread, the flag's value in the policy has been reset.
      bool should_start_conc_mark = collector_state()->in_initial_mark_gc();

      // Inner scope for scope based logging, timers, and stats collection
      {
        EvacuationInfo evacuation_info;

        if (collector_state()->in_initial_mark_gc()) {
          // We are about to start a marking cycle, so we increment the
          // full collection counter.
          increment_old_marking_cycles_started();
          _cm->gc_tracer_cm()->set_gc_cause(gc_cause());
        }

        _gc_tracer_stw->report_yc_type(collector_state()->yc_type());

        GCTraceCPUTime tcpu;

        G1HeapVerifier::G1VerifyType verify_type;
        FormatBuffer<> gc_string("Pause Young ");
        if (collector_state()->in_initial_mark_gc()) {
          gc_string.append("(Concurrent Start)");
          verify_type = G1HeapVerifier::G1VerifyConcurrentStart;
        } else if (collector_state()->in_young_only_phase()) {
          if (collector_state()->in_young_gc_before_mixed()) {
            gc_string.append("(Prepare Mixed)");
          } else {
            gc_string.append("(Normal)");
          }
          verify_type = G1HeapVerifier::G1VerifyYoungNormal;
        } else {
          gc_string.append("(Mixed)");
          verify_type = G1HeapVerifier::G1VerifyMixed;
        }
        GCTraceTime(Info, gc) tm(gc_string, NULL, gc_cause(), true);

        uint active_workers = AdaptiveSizePolicy::calc_active_workers(workers()->total_workers(),
                                                                      workers()->active_workers(),
                                                                      Threads::number_of_non_daemon_threads());
        active_workers = workers()->update_active_workers(active_workers);
        log_info(gc,task)("Using %u workers of %u for evacuation", active_workers, workers()->total_workers());

        TraceCollectorStats tcs(g1mm()->incremental_collection_counters());
        TraceMemoryManagerStats tms(&_memory_manager, gc_cause(),
                                    collector_state()->yc_type() == Mixed /* allMemoryPoolsAffected */);

        G1HeapTransition heap_transition(this);
        size_t heap_used_bytes_before_gc = used();

        // Don't dynamically change the number of GC threads this early.  A value of
        // 0 is used to indicate serial work.  When parallel work is done,
        // it will be set.

        { // Call to jvmpi::post_class_unload_events must occur outside of active GC
          IsGCActiveMark x;

          gc_prologue(false);

          if (VerifyRememberedSets) {
            log_info(gc, verify)("[Verifying RemSets before GC]");
            VerifyRegionRemSetClosure v_cl;
            heap_region_iterate(&v_cl);
          }

          _verifier->verify_before_gc(verify_type);

          _verifier->check_bitmaps("GC Start");

    #if COMPILER2_OR_JVMCI
          DerivedPointerTable::clear();
    #endif

          // Please see comment in g1CollectedHeap.hpp and
          // G1CollectedHeap::ref_processing_init() to see how
          // reference processing currently works in G1.

          // Enable discovery in the STW reference processor
          _ref_processor_stw->enable_discovery();

          {
            // We want to temporarily turn off discovery by the
            // CM ref processor, if necessary, and turn it back on
            // on again later if we do. Using a scoped
            // NoRefDiscovery object will do this.
            NoRefDiscovery no_cm_discovery(_ref_processor_cm);

            // Forget the current alloc region (we might even choose it to be part
            // of the collection set!).
            _allocator->release_mutator_alloc_region();

            // This timing is only used by the ergonomics to handle our pause target.
            // It is unclear why this should not include the full pause. We will
            // investigate this in CR 7178365.
            //
            // Preserving the old comment here if that helps the investigation:
            //
            // The elapsed time induced by the start time below deliberately elides
            // the possible verification above.
            double sample_start_time_sec = os::elapsedTime();

            g1_policy()->record_collection_pause_start(sample_start_time_sec);

            if (collector_state()->in_initial_mark_gc()) {
              concurrent_mark()->pre_initial_mark();
            }

            g1_policy()->finalize_collection_set(target_pause_time_ms, &_survivor);

            evacuation_info.set_collectionset_regions(collection_set()->region_length());

            // Make sure the remembered sets are up to date. This needs to be
            // done before register_humongous_regions_with_cset(), because the
            // remembered sets are used there to choose eager reclaim candidates.
            // If the remembered sets are not up to date we might miss some
            // entries that need to be handled.
            g1_rem_set()->cleanupHRRS();

            register_humongous_regions_with_cset();

            assert(_verifier->check_cset_fast_test(), "Inconsistency in the InCSetState table.");

            // We call this after finalize_cset() to
            // ensure that the CSet has been finalized.
            _cm->verify_no_cset_oops();

            if (_hr_printer.is_active()) {
              G1PrintCollectionSetClosure cl(&_hr_printer);
              _collection_set.iterate(&cl);
            }

            // Initialize the GC alloc regions.
            _allocator->init_gc_alloc_regions(evacuation_info);

            G1ParScanThreadStateSet per_thread_states(this, workers()->active_workers(), collection_set()->young_region_length());
            pre_evacuate_collection_set();

            // Actually do the work...
            evacuate_collection_set(&per_thread_states);

            post_evacuate_collection_set(evacuation_info, &per_thread_states);

            const size_t* surviving_young_words = per_thread_states.surviving_young_words();
            free_collection_set(&_collection_set, evacuation_info, surviving_young_words);

            eagerly_reclaim_humongous_regions();

            record_obj_copy_mem_stats();
            _survivor_evac_stats.adjust_desired_plab_sz();
            _old_evac_stats.adjust_desired_plab_sz();

            double start = os::elapsedTime();
            start_new_collection_set();
            g1_policy()->phase_times()->record_start_new_cset_time_ms((os::elapsedTime() - start) * 1000.0);

            if (evacuation_failed()) {
              set_used(recalculate_used());
              if (_archive_allocator != NULL) {
                _archive_allocator->clear_used();
              }
              for (uint i = 0; i < ParallelGCThreads; i++) {
                if (_evacuation_failed_info_array[i].has_failed()) {
                  _gc_tracer_stw->report_evacuation_failed(_evacuation_failed_info_array[i]);
                }
              }
            } else {
              // The "used" of the the collection set have already been subtracted
              // when they were freed.  Add in the bytes evacuated.
              increase_used(g1_policy()->bytes_copied_during_gc());
            }

            if (collector_state()->in_initial_mark_gc()) {
              // We have to do this before we notify the CM threads that
              // they can start working to make sure that all the
              // appropriate initialization is done on the CM object.
              concurrent_mark()->post_initial_mark();
              // Note that we don't actually trigger the CM thread at
              // this point. We do that later when we're sure that
              // the current thread has completed its logging output.
            }

            allocate_dummy_regions();

            _allocator->init_mutator_alloc_region();

            {
              size_t expand_bytes = _heap_sizing_policy->expansion_amount();
              if (expand_bytes > 0) {
                size_t bytes_before = capacity();
                // No need for an ergo logging here,
                // expansion_amount() does this when it returns a value > 0.
                double expand_ms;
                if (!expand(expand_bytes, _workers, &expand_ms)) {
                  // We failed to expand the heap. Cannot do anything about it.
                }
                g1_policy()->phase_times()->record_expand_heap_time(expand_ms);
              }
            }

            // We redo the verification but now wrt to the new CSet which
            // has just got initialized after the previous CSet was freed.
            _cm->verify_no_cset_oops();

            // This timing is only used by the ergonomics to handle our pause target.
            // It is unclear why this should not include the full pause. We will
            // investigate this in CR 7178365.
            double sample_end_time_sec = os::elapsedTime();
            double pause_time_ms = (sample_end_time_sec - sample_start_time_sec) * MILLIUNITS;
            size_t total_cards_scanned = g1_policy()->phase_times()->sum_thread_work_items(G1GCPhaseTimes::ScanRS, G1GCPhaseTimes::ScanRSScannedCards);
            g1_policy()->record_collection_pause_end(pause_time_ms, total_cards_scanned, heap_used_bytes_before_gc);

            evacuation_info.set_collectionset_used_before(collection_set()->bytes_used_before());
            evacuation_info.set_bytes_copied(g1_policy()->bytes_copied_during_gc());

            if (VerifyRememberedSets) {
              log_info(gc, verify)("[Verifying RemSets after GC]");
              VerifyRegionRemSetClosure v_cl;
              heap_region_iterate(&v_cl);
            }

            _verifier->verify_after_gc(verify_type);
            _verifier->check_bitmaps("GC End");

            assert(!_ref_processor_stw->discovery_enabled(), "Postcondition");
            _ref_processor_stw->verify_no_references_recorded();

            // CM reference discovery will be re-enabled if necessary.
          }

    #ifdef TRACESPINNING
          ParallelTaskTerminator::print_termination_counts();
    #endif

          gc_epilogue(false);
        }

        // Print the remainder of the GC log output.
        if (evacuation_failed()) {
          log_info(gc)("To-space exhausted");
        }

        g1_policy()->print_phases();
        heap_transition.print();

        // It is not yet to safe to tell the concurrent mark to
        // start as we have some optional output below. We don't want the
        // output from the concurrent mark thread interfering with this
        // logging output either.

        _hrm.verify_optional();
        _verifier->verify_region_sets_optional();

        TASKQUEUE_STATS_ONLY(print_taskqueue_stats());
        TASKQUEUE_STATS_ONLY(reset_taskqueue_stats());

        print_heap_after_gc();
        print_heap_regions();
        trace_heap_after_gc(_gc_tracer_stw);

        // We must call G1MonitoringSupport::update_sizes() in the same scoping level
        // as an active TraceMemoryManagerStats object (i.e. before the destructor for the
        // TraceMemoryManagerStats is called) so that the G1 memory pools are updated
        // before any GC notifications are raised.
        g1mm()->update_sizes();

        _gc_tracer_stw->report_evacuation_info(&evacuation_info);
        _gc_tracer_stw->report_tenuring_threshold(_g1_policy->tenuring_threshold());
        _gc_timer_stw->register_gc_end();
        _gc_tracer_stw->report_gc_end(_gc_timer_stw->gc_end(), _gc_timer_stw->time_partitions());
      }
      // It should now be safe to tell the concurrent mark thread to start
      // without its logging output interfering with the logging output
      // that came from the pause.

      if (should_start_conc_mark) {
        // CAUTION: after the doConcurrentMark() call below,
        // the concurrent marking thread(s) could be running
        // concurrently with us. Make sure that anything after
        // this point does not assume that we are the only GC thread
        // running. Note: of course, the actual marking work will
        // not start until the safepoint itself is released in
        // SuspendibleThreadSet::desynchronize().
        do_concurrent_mark();
      }

      return true;
    }

往下走就是这一步`G1Policy::finalize_collection_set`,去处理新生代和老年代

    void G1Policy::finalize_collection_set(double target_pause_time_ms, G1SurvivorRegions* survivor) {
      double time_remaining_ms = _collection_set->finalize_young_part(target_pause_time_ms, survivor);
      _collection_set->finalize_old_part(time_remaining_ms);
    }

这里分别调用了两个方法，可以看到剩余时间是往下传的，来看一下具体的方法

    double G1CollectionSet::finalize_young_part(double target_pause_time_ms, G1SurvivorRegions* survivors) {
      double young_start_time_sec = os::elapsedTime();

      finalize_incremental_building();

      guarantee(target_pause_time_ms > 0.0,
                "target_pause_time_ms = %1.6lf should be positive", target_pause_time_ms);

      size_t pending_cards = _policy->pending_cards();
      double base_time_ms = _policy->predict_base_elapsed_time_ms(pending_cards);
      double time_remaining_ms = MAX2(target_pause_time_ms - base_time_ms, 0.0);

      log_trace(gc, ergo, cset)("Start choosing CSet. pending cards: " SIZE_FORMAT " predicted base time: %1.2fms remaining time: %1.2fms target pause time: %1.2fms",
                                pending_cards, base_time_ms, time_remaining_ms, target_pause_time_ms);

      // The young list is laid with the survivor regions from the previous
      // pause are appended to the RHS of the young list, i.e.
      //   [Newly Young Regions ++ Survivors from last pause].

      uint survivor_region_length = survivors->length();
      uint eden_region_length = _g1h->eden_regions_count();
      init_region_lengths(eden_region_length, survivor_region_length);

      verify_young_cset_indices();

      // Clear the fields that point to the survivor list - they are all young now.
      survivors->convert_to_eden();

      _bytes_used_before = _inc_bytes_used_before;
      time_remaining_ms = MAX2(time_remaining_ms - _inc_predicted_elapsed_time_ms, 0.0);

      log_trace(gc, ergo, cset)("Add young regions to CSet. eden: %u regions, survivors: %u regions, predicted young region time: %1.2fms, target pause time: %1.2fms",
                                eden_region_length, survivor_region_length, _inc_predicted_elapsed_time_ms, target_pause_time_ms);

      // The number of recorded young regions is the incremental
      // collection set's current size
      set_recorded_rs_lengths(_inc_recorded_rs_lengths);

      double young_end_time_sec = os::elapsedTime();
      phase_times()->record_young_cset_choice_time_ms((young_end_time_sec - young_start_time_sec) * 1000.0);

      return time_remaining_ms;
    }

下面是老年代的部分

    void G1CollectionSet::finalize_old_part(double time_remaining_ms) {
      double non_young_start_time_sec = os::elapsedTime();
      double predicted_old_time_ms = 0.0;

      if (collector_state()->in_mixed_phase()) {
        cset_chooser()->verify();
        const uint min_old_cset_length = _policy->calc_min_old_cset_length();
        const uint max_old_cset_length = _policy->calc_max_old_cset_length();

        uint expensive_region_num = 0;
        bool check_time_remaining = _policy->adaptive_young_list_length();

        HeapRegion* hr = cset_chooser()->peek();
        while (hr != NULL) {
          if (old_region_length() >= max_old_cset_length) {
            // Added maximum number of old regions to the CSet.
            log_debug(gc, ergo, cset)("Finish adding old regions to CSet (old CSet region num reached max). old %u regions, max %u regions",
                                      old_region_length(), max_old_cset_length);
            break;
          }

          // Stop adding regions if the remaining reclaimable space is
          // not above G1HeapWastePercent.
          size_t reclaimable_bytes = cset_chooser()->remaining_reclaimable_bytes();
          double reclaimable_percent = _policy->reclaimable_bytes_percent(reclaimable_bytes);
          double threshold = (double) G1HeapWastePercent;
          if (reclaimable_percent <= threshold) {
            // We've added enough old regions that the amount of uncollected
            // reclaimable space is at or below the waste threshold. Stop
            // adding old regions to the CSet.
            log_debug(gc, ergo, cset)("Finish adding old regions to CSet (reclaimable percentage not over threshold). "
                                      "old %u regions, max %u regions, reclaimable: " SIZE_FORMAT "B (%1.2f%%) threshold: " UINTX_FORMAT "%%",
                                      old_region_length(), max_old_cset_length, reclaimable_bytes, reclaimable_percent, G1HeapWastePercent);
            break;
          }

          double predicted_time_ms = predict_region_elapsed_time_ms(hr);
          if (check_time_remaining) {
            if (predicted_time_ms > time_remaining_ms) {
              // Too expensive for the current CSet.

              if (old_region_length() >= min_old_cset_length) {
                // We have added the minimum number of old regions to the CSet,
                // we are done with this CSet.
                log_debug(gc, ergo, cset)("Finish adding old regions to CSet (predicted time is too high). "
                                          "predicted time: %1.2fms, remaining time: %1.2fms old %u regions, min %u regions",
                                          predicted_time_ms, time_remaining_ms, old_region_length(), min_old_cset_length);
                break;
              }

              // We'll add it anyway given that we haven't reached the
              // minimum number of old regions.
              expensive_region_num += 1;
            }
          } else {
            if (old_region_length() >= min_old_cset_length) {
              // In the non-auto-tuning case, we'll finish adding regions
              // to the CSet if we reach the minimum.

              log_debug(gc, ergo, cset)("Finish adding old regions to CSet (old CSet region num reached min). old %u regions, min %u regions",
                                        old_region_length(), min_old_cset_length);
              break;
            }
          }

          // We will add this region to the CSet.
          time_remaining_ms = MAX2(time_remaining_ms - predicted_time_ms, 0.0);
          predicted_old_time_ms += predicted_time_ms;
          cset_chooser()->pop(); // already have region via peek()
          _g1h->old_set_remove(hr);
          add_old_region(hr);

          hr = cset_chooser()->peek();
        }
        if (hr == NULL) {
          log_debug(gc, ergo, cset)("Finish adding old regions to CSet (candidate old regions not available)");
        }

        if (expensive_region_num > 0) {
          // We print the information once here at the end, predicated on
          // whether we added any apparently expensive regions or not, to
          // avoid generating output per region.
          log_debug(gc, ergo, cset)("Added expensive regions to CSet (old CSet region num not reached min)."
                                    "old: %u regions, expensive: %u regions, min: %u regions, remaining time: %1.2fms",
                                    old_region_length(), expensive_region_num, min_old_cset_length, time_remaining_ms);
        }

        cset_chooser()->verify();
      }

      stop_incremental_building();

      log_debug(gc, ergo, cset)("Finish choosing CSet. old: %u regions, predicted old region time: %1.2fms, time remaining: %1.2f",
                                old_region_length(), predicted_old_time_ms, time_remaining_ms);

      double non_young_end_time_sec = os::elapsedTime();
      phase_times()->record_non_young_cset_choice_time_ms((non_young_end_time_sec - non_young_start_time_sec) * 1000.0);

      QuickSort::sort(_collection_set_regions, _collection_set_cur_length, compare_region_idx, true);
    }

上面第三行是个判断，当前是否是 mixed 回收阶段，如果不是的话其实是没有老年代什么事的，所以可以看到代码基本是从这个 if 判断
`if (collector_state()->in_mixed_phase()) {`开始往下走的
先写到这，偏向于做笔记用，有错轻拍