core/src/main/java/com/orientechnologies/orient/core/storage/impl/local/statistic/OPerformanceStatisticManager.java
public void startMonitoring() {
    switchLock.acquireWriteLock();
    try {
      if (!statistics.isEmpty() && !enabled)
        throw new IllegalStateException("Monitoring is already started on thread level and can not be started on system level");

      deadThreadsStatistic = null;
      postMeasurementStatistic = null;

      enabled = true;
    } finally {
      switchLock.releaseWriteLock();
    }
  }
public void startMonitoring() {
    switchLock.acquireWriteLock();
    try {
      if (!statistics.isEmpty() && !enabled)
        throw new IllegalStateException("Monitoring is already started on thread level and can not be started on system level");

      deadThreadsStatistic = null;
      postMeasurementStatistic = null;

      enabled = true;
    } finally {
      switchLock.releaseWriteLock();
    }
  }
Starts performance monitoring only for whole system.
After call of this method you can not start monitoring on thread level till call of {@link #stopMonitoring()} is performed.