ART笔记 —— dvm_lock_sample

看ART代码过程中，看到LogContentionEvent函数，记起logcat events中有大量的dvm_lock_sample信息。

 dvm_lock_sample: [com.banyac.midrive.map,1,main,98,ReportUtil.java,119,-,189,19] 进程名，主线程？线程名，锁等待时间，下个持有者文件名，行号，上个持有者文件名，行号，等待百分比

1. 打印此LOG时，线程即将持有锁。
2. 上一个持有者文件名为“-”时，表示与下个持有者在同一个文件中
3. 发生锁等待时这个LOG有概率打印，等待百分比越大，概率越大。具体请看代码。

art/runtime/monitor.cc

void Monitor::Lock(Thread* self) {  MutexLock mu(self, monitor_lock_);  while (true) {    if (owner_ == nullptr) {  // Unowned.      owner_ = self;      CHECK_EQ(lock_count_, 0);      // When debugging, save the current monitor holder for future      // acquisition failures to use in sampled logging.      if (lock_profiling_threshold_ != 0) {        locking_method_ = self->GetCurrentMethod(&locking_dex_pc_);      }      return;    } else if (owner_ == self) {  // Recursive.      lock_count_++;      return;    }    // Contended.    const bool log_contention = (lock_profiling_threshold_ != 0);    uint64_t wait_start_ms = log_contention ? MilliTime() : 0;    mirror::ArtMethod* owners_method = locking_method_;    uint32_t owners_dex_pc = locking_dex_pc_;    // Do this before releasing the lock so that we don't get deflated.    size_t num_waiters = num_waiters_;    ++num_waiters_;    monitor_lock_.Unlock(self);  // Let go of locks in order.    self->SetMonitorEnterObject(GetObject());    {      ScopedThreadStateChange tsc(self, kBlocked);  // Change to blocked and give up mutator_lock_.      MutexLock mu2(self, monitor_lock_);  // Reacquire monitor_lock_ without mutator_lock_ for Wait.      if (owner_ != NULL) {  // Did the owner_ give the lock up?        monitor_contenders_.Wait(self);  // Still contended so wait.        // Woken from contention.        if (log_contention) {          uint64_t wait_ms = MilliTime() - wait_start_ms;          uint32_t sample_percent;          if (wait_ms >= lock_profiling_threshold_) {            sample_percent = 100;          } else {            sample_percent = 100 * wait_ms / lock_profiling_threshold_;          }          if (sample_percent != 0 && (static_cast<uint32_t>(rand() % 100) < sample_percent)) {            const char* owners_filename;            uint32_t owners_line_number;            TranslateLocation(owners_method, owners_dex_pc, &owners_filename, &owners_line_number);            if (wait_ms > kLongWaitMs && owners_method != nullptr) {              LOG(WARNING) << "Long monitor contention event with owner method="                  << PrettyMethod(owners_method) << " from " << owners_filename << ":"                  << owners_line_number << " waiters=" << num_waiters << " for "                  << PrettyDuration(MsToNs(wait_ms));            }            LogContentionEvent(self, wait_ms, sample_percent, owners_filename, owners_line_number);          }        }      }    }    self->SetMonitorEnterObject(nullptr);    monitor_lock_.Lock(self);  // Reacquire locks in order.    --num_waiters_;  }}