Date:   Wed, 16 Mar 2022 10:21:15 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Jui-Tse Huang <juitse.huang@...il.com>
Cc:     Jonathan Corbet <corbet@....net>,
        Valentin Schneider <valentin.schneider@....com>,
        Mauro Carvalho Chehab <mchehab+huawei@...nel.org>,
        Huaixin Chang <changhuaixin@...ux.alibaba.com>,
        Beata Michalska <beata.michalska@....com>,
        Chun-Hung Tseng <henrybear327@...il.com>,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        Ching-Chun Huang <jserv@...s.ncku.edu.tw>,
        Randy Dunlap <rdunlap@...radead.org>,
        Brendan Gregg <bgregg@...flix.com>,
        Yiwei Lin <s921975628@...il.com>
Subject: Re: [PATCH v3] docs/scheduler: Introduce the doc of load average

On Tue, Mar 15, 2022 at 10:08:57PM +0800, Jui-Tse Huang wrote:
> The load average is one of a common as well as easy observed statistic
> provied by Linux, but still not well documented, which makes the numbers

I'm really not sure what the target audience is; and I hate you're
making me read rst garbage. For someone trying to make changes to
load_avg.c this document is pure confusion, for a user trying to make
sense of that number, I would imagine much the same.

> that users observes from the output of top, htop or other system
> monitoring application are only numbers. This patch gives a discussion
> on how Linux calculates the load average as well as what metrics are
> concerned while calculating the load average.
> 
> The discussion flow is divided into several parts:
> 1. The expression used to get the load average.
> 2. Why Linux choose such average method from the other.
> 2. The meaning of each term in the expression.
> 3. The metrics, that is, the type of tasks that will be covered in the
>    calculation.
> 4. A brief explanation over the fixed-point nubmer since the weights
>    defined in the Linux kernel are based on it.
> 
> Signed-off-by: Jui-Tse Huang <juitse.huang@...il.com>
> Signed-off-by: Yiwei Lin <s921975628@...il.com>
> Signed-off-by: Ching-Chun (Jim) Huang <jserv@...s.ncku.edu.tw>
> Co-Developed-by: Yiwei Lin <s921975628@...il.com>
> 
> ---
> 
> v3:
>   - Fix typo (Randy Dunlap)
>   - Add further reading that links to Brendan Gregg's blog
> 
> v2:
>   - Fix typo (Chun-Hung Tseng) 
> 
>  Documentation/scheduler/index.rst        |  1 +
>  Documentation/scheduler/load-average.rst | 82 ++++++++++++++++++++++++
>  2 files changed, 83 insertions(+)
>  create mode 100644 Documentation/scheduler/load-average.rst
> 
> diff --git a/Documentation/scheduler/index.rst b/Documentation/scheduler/index.rst
> index 88900aabdbf7..bdc779b4190f 100644
> --- a/Documentation/scheduler/index.rst
> +++ b/Documentation/scheduler/index.rst
> @@ -17,6 +17,7 @@ Linux Scheduler
>      sched-nice-design
>      sched-rt-group
>      sched-stats
> +    load-average
>  
>      text_files
>  
> diff --git a/Documentation/scheduler/load-average.rst b/Documentation/scheduler/load-average.rst
> new file mode 100644
> index 000000000000..27ce6cbae5f4
> --- /dev/null
> +++ b/Documentation/scheduler/load-average.rst
> @@ -0,0 +1,82 @@
> +============
> +Load Average
> +============
> +
> +The load average, provided by common operating systems, indicates the average
> +number of system load over a period of time.  In Linux, it shows the average
> +number of tasks running and waiting for CPU time. The following expression is
> +used in Linux to update the load average::
> +
> +                / 0                                      , if t = 0
> +    load_{t} = |
> +                \ load_{t - 1} * exp + active * (1 - exp), otherwise


Easier to follow is:

	load_{0} = 0
	load_{t+1} = load_{t} * exp + active * (1 - exp)

> +The expression represents the exponential moving average of the historical
> +loading of the system. There are several reasons that Linux kernel chooses
> +exponential moving average from other similar average equations such as simple
> +moving average or cumulative moving average:
> +
> +#. The exponential moving average consumes fixed memory space, while the simple
> +   moving average has O(n) space complexity where n is the number of timeslices
> +   within a given interval.
> +#. The exponential moving average not only applies a higher weight to the most
> +   recent record but also declines the weight exponentially, which makes the
> +   resulting load average reflect the situation of the current system. Neither
> +   the simple moving average nor cumulative moving average has this feature.
> +
> +In the expression, the load_{t} indicates the calculated load average at the
> +given time t.

> +The active is the most recent recorded system load. In Linux, the system load

That's inaccurate at best, active is not the load, since you just gave a
definition of load. As such, load is a function of time and active.

Also, stop saying in Linux, you're reading the Linux documentation this
is a given.

> +means the number of tasks in the state of TASK_RUNNING or TASK_UNINTERRUPTIBLE
> +of the entire system. Tasks with TASK_UNINTERRUPTIBLE state are usually waiting
> +for disk I/O or holding an uninterruptible lock, which is considered as a part
> +of system resource, thus, Linux kernel covers them while calculating the load
> +average.

This is inacurate, consider TASK_NOLOAD.

> +The exp means the weight applied to the previous report of load average, while
> +(1 - exp) is the weight applied to the most recently recorded system load.

I get really itchy from statements like this; either you can read a
formula or you can't, stuff like this doesn't help much in either case.

> +There are three different weights defined in the Linux kernel, in
> +include/linux/sched/loadavg.h, to perform statistics in various timescales::
> +
> +    // include/linux/sched/loadavg.h
> +    ...
> +    #define EXP_1    1884    /* 1/exp(5sec/1min) as fixed-point */
> +    #define EXP_5    2014    /* 1/exp(5sec/5min) */
> +    #define EXP_15   2037    /* 1/exp(5sec/15min) */
> +    ...
> +
> +According to the expression shown on the top of this page, the weight (exp)
> +controls how much of the last load load_{t - 1} will take place in the
> +calculation of current load, while (1 - exp) is the weight applied to the most
> +recent record of system load active.

What page, this is a non-paginated document. Also, you're repeating
yourself.

> +Due to the security issue, the weights are defined as fixed-point numbers based

This is complete nonsense

> +on the unsigned integer rather than floating-pointing numbers. The introduction
> +of the fixed-point number keeps the FPU away from the calculation process. Since
> +the precision of the fixed-point used in the Linux kernel is 11 bits, a
> +fixed-point can be converted to a floating-point by dividing it by 2048, as in
> +the expressions shown bellow::
> +
> +    EXP_1  = 1884 / 2048 = 0.919922
> +    EXP_5  = 2014 / 2048 = 0.983398
> +    EXP_15 = 2037 / 2048 = 0.994629
> +
> +Which indicates the weights applied to active are::
> +
> +    (1 - EXP_1)  = (1 - 0.919922) = 0.080078
> +    (1 - EXP_5)  = (1 - 0.983398) = 0.016602
> +    (1 - EXP_15) = (1 - 0.994629) = 0.005371

I don't think this is the place to explain fixed point arithmetic. The
consumer of load_avg doesn't need the know, the developer looking at
loadavg.c will have *MUCH* bigger problems.

> +The load average will be updated every 5 seconds. Each time the scheduler_tick()
> +be called, the function calc_global_load_tick() will also be invoked, which
> +makes the active of each CPU core be calculated and be merged globally. Finally,
> +the load average will be updated with that global active.

That's wishful thinking, have you read loadavg.c ?

> +
> +As a user, the load average can be observed via top, htop, or other system
> +monitor application, or more directly, by the following command::
> +
> +    $ cat /proc/loadavg
> +
> +Further Reading
> +---------------
> +The explanation and analysis done by Brendan Gregg on `his blog
> +<https://www.brendangregg.com/blog/2017-08-08/linux-load-averages.html>`_.

That blogpost is actually useful, unlike most of what you've written
here. Why not only link that and leave out the rest?

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