presto-main/src/main/java/io/prestosql/operator/TaskContext.java/TaskContext/start
class TaskContext:    public void start()
    {
        DateTime now = DateTime.now();
        executionStartTime.compareAndSet(null, now);
        startNanos.compareAndSet(0, System.nanoTime());
        startFullGcCount.compareAndSet(-1, gcMonitor.getMajorGcCount());
        startFullGcTimeNanos.compareAndSet(-1, gcMonitor.getMajorGcTime().roundTo(NANOSECONDS));

        // always update last execution start time
        lastExecutionStartTime.set(now);
    }