1.时间单位

秒：second

毫秒：Millisecond

微妙：Microsecond

纳秒：nanosecond

1秒 = 1000毫秒；1毫秒=1000微秒, 1微秒=1000纳秒,

2.获取时间戳

native层

	nsecs_t now = systemTime(SYSTEM_TIME_MONOTONIC);

    txt：这里返回的就是纳秒

java层

long now = SystemClock.uptimeMillis();

kernel层

s64 time = ktime_to_us(ktime_get());