what's meaning the container_cpu_cfs_throttled

问题:

cadvisor has two metrics container_cpu_cfs_throttled_seconds_total and container_cpu_cfs_throttled_periods_total

I have confuse what does that means ..

I have found about two explain：

1. container run with cpu limit, when container cpu over limit , the container will be "throttled" and add time to container_cpu_cfs_throttled_seconds_total

   that means ：
    (1). only container cpu over limit, rate(container_cpu_cfs_throttled_seconds_total) > 0. 
    (2). we can use this metrics to alert container cpu over limit ... 
   

2. when host in heavy cpu pressure, it will "throttled" container with POD QoS(Guaranteed > Burstable > Best-Effort) ...

   that means ：
    (1). container_cpu_cfs_throttled_seconds_total will add has no relate with how many cpu container used and cpu limit ..
    (2). this metrics can not to alert container cpu over limit ..