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Fix CPU metrics calculations for containers
## The problem The previous implementation of CPU metrics for containers was inaccurate. It reported trends (more/less CPU usage), but didn't accurately report the CPU metrics. This was the implementation was using the "total CPU usage" as the "total CPU time", which is not the same. The "total CPU usage" is how much the CPU was used during our measurements, but does not account for the CPU idle time, which the "total CPU time" does include. This caused the CPU metrics being reported to combine towards a total of a 100% usage, while the container itself may have only used 1/2% of the CPU at that time. The breakdown between `user` and `system` only applied to the `total usage` and not the "total CPU time". The implementation didn't account for the `idle` time which a CPU has, because the `/sys/fs` virtual file system does not report this value. It reports a total usage (combined `user` group, `system` group and whatever it doesn't expose in separate groups), and a breakdown for the `user` and `system` groups. ## The solution Instead we can use the timeframe in which the measurements were done, 60 seconds, as the "total CPU time". These values can be compare because the `total usage`, `user` and `system` values are also reported in nanoseconds. (Nanoseconds for `total usage` and `seconds / 100` for the `user` and `system` groups.) 60 seconds is the time in which we measure. We will use this as the "total CPU time". The delta values of the values reported for `total usage`, `user` and `system`, the time how long they spent on these groups within those 60 seconds, can then be used to compare the percentages for those values. Using the calculation: `delta value / total CPU time (60 seconds) * 100 = value in %` For example: ``` time frame = 60 seconds = 100% (60 / 60 * 100 = 100) total usage = 30 seconds = 50% (30 / 60 * 100 = 50) user = 24 seconds = 40% (24 / 60 * 100 = 40) system = 6 seconds = 10% ( 6 / 60 * 100 = 10) ``` Where with the previously implementation the result would have been: ``` total usage = 30 seconds = 100% (30 / 30 * 100 = 50) user = 24 seconds = 40% (24 / 30 * 100 = 80) system = 6 seconds = 10% ( 6 / 30 * 100 = 20) ``` In addition we now report the `total_usage` of the container CPU, rather than only the breakdown between the `user` and `system` groups. This implementation differs from the non-container implementation for CPU metrics, where we do not have a `total_usage` metric. ### When a container has multiple CPU cores When a container has multiple CPU cores the measurement time does not change. But the container can use more resources during that time, counting towards a higher reported usage value. This means that a container with 2 CPU cores running both cores to their maximum during the measurement time will report 200% in total usage. This implementation will report the same metrics as the output from `docker stats`. We've decided to keep the reported values the same so there is less confusion about what is the accurate metric. This creates a difference in reporting for CPU usage between container systems and non-container systems. For non-containers all the reported CPU group metrics combine together towards a 100% value regardless of how many cores are available, this is because we know the "total CPU time" there as the idle time and other groups are exposed. See: https://github.com/appsignal/probes-rs/blob/e2ac0412f6357d64423073adf82009a116ae226b/src/cpu.rs#L141-L162 If we would want to always report a 100% value no matter how many CPU cores a container has available, we can divide the reported values by the number of CPU cores. This is what we did here in our test script: https://gist.github.com/tombruijn/5f5e0c34af40cfb3967eca81ad8b5317#file-appsignal_stats-rb-L141 and is what `ctop -scale-cpu` (ctop (https://bcicen.github.io/ctop/) being an alternative to `docker stats`). But please consider the following section as well as that will not fix the problem with multiple containers on one host system. ### About container neighbors While we report the same values as `docker stats`, `docker stats` doesn't actually report the container's CPU usage. Instead it shows the container's CPU usage impact on the host system. This is not immediately visible when using one container on the host system, but when multiple containers are running at the same time this problem becomes more clear. Multiple containers running on the same host system will impact the readings of `docker stats` as the host's resources will be shared among those containers. For example: We have a Docker system on our host machine that has 3 CPU cores available. We then start containers without any specific CPU limits. - In the scenario of one container maxing out its CPU cores it will be reported as 300% CPU usage by `docker stats`. Which is the total CPU time available. - When three containers are maxing out their CPU cores it will be reported by `docker stats` as 100% CPU usage for every container as the resources are shared amongst them evenly. The problem here remains that we do not know the container's actual "total CPU time" it has available, total usage + idle time, isolated to the container itself. To make it more complicated, if all container would max out their CPU, the containers' CPU time gets throttled as well. Which is something we may want to expose in the future as it shows when a container has less resources available at which time. ## Acknowledgements and sources Based on the implementation of this Ruby script written together with Robert Beekman @matsimitsu: https://gist.github.com/tombruijn/5f5e0c34af40cfb3967eca81ad8b5317 Thanks Robert! Based on the docs as described here: https://docs.docker.com/config/containers/runmetrics/ Co-authored-by: Robert Beekman <robert@matsimitsu.nl>
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