Linux Learning CPU Load Rate calculation method understanding

The concept of Load average originates from UNIX systems, and although the formulas vary from one family to the other, they are used to measure the number of processes that are using the CPU and are waiting for the CPU, in a word, the number of runable processes. So load average can be used as a reference for CPU bottlenecks, and if it is larger than the number of CPUs, the CPU may not be enough.

But there's a bit of a difference on Linux!

The load average on Linux includes the number of processes that uninterruptible sleep, in addition to the number of processes that are using the CPU and the number of processes that are waiting for the CPU. The process is usually in uninterruptible sleep state while waiting for the IO device and waiting for the network. The logic of the Linux designer is that uninterruptible sleep should be ephemeral and will soon resume running, so it is equivalent to runnable. However, uninterruptible sleep even if it is also a short sleep, not to mention the real world uninterruptible sleep may not be very short, a lot of, or a long time uninterruptible Sleep usually means that the IO device is experiencing a bottleneck. As we all know, the process of sleep state does not need the CPU, even if all the CPU is idle, the process of sleep is not running, so the number of sleep process is definitely not suitable for measuring CPU load, Linux put uninterruptible The sleep process's calculation of load average directly overturns the original meaning of load average. So on the Linux system, the load average this indicator basically loses the function, because you do not know what it means, when see load average high, you do not know is runnable process too much or uninterruptible Sleep process too much, you can not determine whether the CPU is not enough or the IO device has a bottleneck.

On the other side, it can also explain why the CPU load is high when the disk is slow (when a large amount of disk is used). Basically, I have a high CPU load situation in two cases: the CPU itself to handle too many tasks, coupled with soft interrupts and context switching too often lead to high load, and then the disk is too slow to cause too much non-disruptive sleep caused high CPU load.

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Linux Learning CPU Load Rate calculation method understanding