Get Docker Container CPU Usage as Percentage

Question

Docker provides an interactive stats command, docker stats [cid] which gives up to date information on the CPU usage, like so:

CONTAINER      CPU %          MEM USAGE/LIMIT       MEM %       NET I/O
36e8a65d       0.03%          4.086 MiB/7.798 GiB   0.05%       281.3 MiB/288.3 MiB

I'm trying to get the CPU usage as a percentage in a digestible format to do some analysis.

I've seen the stats in /sys/fs which seem to provide similar values as the Docker Remote API which gives me this JSON blob:

{
    "cpu_usage": {
        "usage_in_usermode": 345230000000, 
        "total_usage": 430576697133, 
        "percpu_usage": [
            112999686856, 
            106377031910, 
            113291361597, 
            97908616770
        ], 
        "usage_in_kernelmode": 80670000000
    }, 
    "system_cpu_usage": 440576670000000, 
    "throttling_data": {
        "throttled_time": 0, 
        "periods": 0, 
        "throttled_periods": 0
    }
}

But I'm unsure how to get an exact CPU Usage as a percentage from that.

Any ideas?

Answer 1

If you are going to use the Stats API call - you can take a look at how the docker client does it: https://github.com/docker/docker/blob/eb131c5383db8cac633919f82abad86c99bffbe5/cli/command/container/stats_helpers.go#L175-L188

func calculateCPUPercent(previousCPU, previousSystem uint64, v *types.StatsJSON) float64 {
    var (
        cpuPercent = 0.0
        // calculate the change for the cpu usage of the container in between readings
        cpuDelta = float64(v.CPUStats.CPUUsage.TotalUsage) - float64(previousCPU)
        // calculate the change for the entire system between readings
        systemDelta = float64(v.CPUStats.SystemUsage) - float64(previousSystem)
    )

    if systemDelta > 0.0 && cpuDelta > 0.0 {
        cpuPercent = (cpuDelta / systemDelta) * float64(len(v.CPUStats.CPUUsage.PercpuUsage)) * 100.0
    }
    return cpuPercent
}

Basically, you take a point of reference, then see the difference in say 10 secs, you can then tell how much of the time was used by the container. Say, we start with 0 SystemCPUUsage and 0 CPUUsage for the container. If after 10 secs, we have 10 SystemCPUUsage and 1 CPUUsage, then we have 10% usage. You are just given the results in nanoseconds, not seconds, in the API. The actual time does not matter, the total SystemCPUUsage change is what matters, then compare CPUUSage to that.