(六) GC关键类

GarbageCollector是虚基类，它是ART中垃圾回收器的代表。GC工作就是从Garbage-Collector的Run函数开始的。

class GarbageCollector : public RootVisitor, public IsMarkedVisitor, public MarkObjectVisitor {

}

(1) GC的具体工作则由其子类实现的RunPhases函数来处理。
(2) 计算本次GC所用时间

void GarbageCollector::Run(GcCause gc_cause, bool clear_soft_references) {
  ScopedTrace trace(android::base::StringPrintf("%s %s GC", PrettyCause(gc_cause), GetName()));
  Thread* self = Thread::Current();
  Runtime* runtime = Runtime::Current();
  uint64_t start_time = NanoTime();
  uint64_t thread_cpu_start_time = ThreadCpuNanoTime();
  GetHeap()->CalculatePreGcWeightedAllocatedBytes();
  Iteration* current_iteration = GetCurrentIteration();
  current_iteration->Reset(gc_cause, clear_soft_references);
  // Note transaction mode is single-threaded and there's no asynchronous GC and this flag doesn't
  // change in the middle of a GC.
  is_transaction_active_ = runtime->IsActiveTransaction();
//(1)
  RunPhases();  // Run all the GC phases.
  GetHeap()->CalculatePostGcWeightedAllocatedBytes();
  // Add the current timings to the cumulative timings.
  cumulative_timings_.AddLogger(*GetTimings());
  // Update cumulative statistics with how many bytes the GC iteration freed.
  total_freed_objects_ += current_iteration->GetFreedObjects() +
      current_iteration->GetFreedLargeObjects();
  total_scanned_bytes_ += current_iteration->GetScannedBytes();
  int64_t freed_bytes = current_iteration->GetFreedBytes() +
      current_iteration->GetFreedLargeObjectBytes();
  total_freed_bytes_ += freed_bytes;
  // Rounding negative freed bytes to 0 as we are not interested in such corner cases.
  freed_bytes_histogram_.AddValue(std::max<int64_t>(freed_bytes / KB, 0));
  uint64_t end_time = NanoTime();
  uint64_t thread_cpu_end_time = ThreadCpuNanoTime();
  total_thread_cpu_time_ns_ += thread_cpu_end_time - thread_cpu_start_time;
//(2)
  uint64_t duration_ns = end_time - start_time;
  current_iteration->SetDurationNs(duration_ns);
  if (Locks::mutator_lock_->IsExclusiveHeld(self)) {
    // The entire GC was paused, clear the fake pauses which might be in the pause times and add
    // the whole GC duration.
    current_iteration->pause_times_.clear();
    RegisterPause(duration_ns);
  }
  total_time_ns_ += duration_ns;
  uint64_t total_pause_time = 0;
  for (uint64_t pause_time : current_iteration->GetPauseTimes()) {
    MutexLock mu(self, pause_histogram_lock_);
    pause_histogram_.AdjustAndAddValue(pause_time);
    total_pause_time += pause_time;
  }
  metrics::ArtMetrics* metrics = runtime->GetMetrics();
  // Report STW pause time in microseconds.
  metrics->WorldStopTimeDuringGCAvg()->Add(total_pause_time / 1'000);
  // Report total collection time of all GCs put together.
  metrics->TotalGcCollectionTime()->Add(NsToMs(duration_ns));
  if (are_metrics_initialized_) {
    metrics_gc_count_->Add(1);
    // Report GC time in milliseconds.
    gc_time_histogram_->Add(NsToMs(duration_ns));
    // Throughput in bytes/s. Add 1us to prevent possible division by 0.
    uint64_t throughput = (current_iteration->GetScannedBytes() * 1'000'000)
            / (NsToUs(duration_ns) + 1);
    // Report in MB/s.
    throughput /= MB;
    gc_tracing_throughput_hist_->Add(throughput);
    gc_tracing_throughput_avg_->Add(throughput);

    // Report GC throughput in MB/s.
    throughput = current_iteration->GetEstimatedThroughput() / MB;
    gc_throughput_histogram_->Add(throughput);
    gc_throughput_avg_->Add(throughput);
  }
  is_transaction_active_ = false;
}